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Oil spills in mangroves
San Miguel Natural Reserve is made up of 422 acres of protected coastal lands and is a mosaic of habitats including near shore coral reefs, more than a mile of beachfront, intertidal areas, wetlands, coastal dry forests, and mangroves. Credit: NOAA
Editor's Note: This article is excerpted directly from NOAA, "Oil Spills in Mangroves: Planning and Response Considerations, " July 2010. It has been edited only to conform to the Encyclopedia's style guidelines.
|Mangrove roots. Credit: NOAA
This article is intended to assist those who work in spill response and planning in regions where mangrove ecosystems are an important part of the coastline. By understanding the basics of the ecology of these forests and learning from past oil spills in mangroves, we can better plan for, protect, and respond to spills that may threaten them. Mangroves often border coastlines where coral reefs live offshore, and these two ecosystems are closely linked. Mangroves filter and trap excess sediment that could harm coral, and coral reefs protect shorelines where mangroves grow from excessive wave energy. Both habitats can be adversely impacted by oil spills, and spill responders must often consider tradeoffs between land-based and offshore resources during a response.
Though mangrove forests are in many ways very adaptable ecosystems, and are inherently able to respond to physical changes in their environment, they are highly vulnerable to oil toxicity and can be further damaged by many types of cleanup activities. Thus, we must approach any type of response or restoration activities in mangroves with knowledge and caution.
Oil Toxicity
Key Points
- Mangroves are highly susceptible to oil exposure; oiling may kill them within a few weeks to several months.
- Lighter oils are more acutely toxic to mangroves than are heavier oils. Increased weathering generally lowers oil toxicity.
- Oil-impacted mangroves may suffer yellowed leaves, defoliation, and tree death. More subtle responses include branching of pneumatophores, germination failure, decreased canopy cover, increased rate of mutation, and increased sensitivity to other stresses.
- Response techniques that reduce oil contact with mangroves, such as chemical dispersants, reduce the resultant toxicity as well. Tradeoffs include potential increased toxicity to adjacent communities, and increased penetration of dispersed oil to mangrove sediments.
- The amount of oil reaching the mangroves and the length of time spilled oil remains near the mangroves are key variables in determining the severity of effect.
- Mangrove-associated invertebrates and plants recover more quickly from oiling than do the mangroves themselves, because of the longer time for mangroves to reach maturity.
|Mangrove roots and new shoots coated with oil. Credit: Gavin Newman/Greenpeace
In many tropical regions, mangrove forests are the defining feature of the coastal environment. Mangrove habitats represent the interface between land and sea and, as such, are one of the principal places where spilled oil and associated impacts converge. The diversity and abundance of the biological communities associated with mangroves are evident with the first visit to a healthy mangrove stand.
Observations from many spill events around the world have shown that mangroves suffer both lethal and sublethal effects from oil exposure. Past experience has also taught us that such forests are particularly difficult to protect and clean up once a spill has occurred because they are physically intricate, relatively hard to access, and inhospitable to humans. Each of these considerations contributes to the overall assessment that mangrove forests are a habitat at risk from oil spills. In the rankings of coastal areas in NOAA's Environmental Sensitivity Indices, commonly used as a tool for spill contingency planning around the world, mangrove forests are ranked as the most sensitive of tropical habitats.
In this section, we discuss the toxicity of oil to the broad class of trees called mangroves. In contrast to other habitats, tropical or otherwise, there is a fairly robust literature on the effects of oil to mangroves. This work includes monitoring of mangrove areas oiled during actual spills, field studies of oil impacts on mangroves, and laboratory studies that attempt to control some of the variables that may otherwise complicate the interpretation of research results. Predictably, the body of results is not unanimous in type of impact or the severity of those documented, but there are some consistencies that can serve as the starting point for spill response guidance.
Mechanisms of Oil Toxicity to Mangroves
|Dead mangrove trees lining a stream in Jones Creek in the Niger delta, Nigeria, Africa. The trees were killed by an oil spill that occurred in March 1998. The oil leaked from an ill- maintained 30 year old pipeline. It was two days before the oil company sent in a crew to contain the spill. Credit: George Steimetz / Science Photo Library
It is clear from spills, and field and laboratory studies, that—at least in many circumstances—oil harms or kills mangroves. What is less obvious is how that harm occurs and the mechanism of toxicity. Although there is some consensus that oil causes physical suffocation and toxicological/physiological impacts, researchers disagree as to the relative contributions of each mechanism, which may vary with type of oil and time since the spill (Proffitt et al. 1997).
One of the universal challenges faced by resource managers and spill responders when dealing with oil impacts is the fact that "oil" is a complex mixture of many kinds of chemicals. The oil spilled in one incident is almost certainly different from that spilled in another. In addition, oils within broad categories like "crude oil" or "diesel" can be vastly different, depending on the geological source of the original material, refining processes, and additives incorporated for transportation in barges or tankers. Even if we could somehow stipulate that all spilled oil was to be of a single fixed chemical formulation, petroleum products released into the environment are subjected to differential processes of weathering that immediately begin altering its original physical and chemical characteristics. As a result, samples of oil from exactly the same source can be very different in composition after being subjected to a differing mix of environmental influences.
Much like "oil," the term "mangrove" is also a broadly encompassing and sometimes vague category that defies strict definition. Mangroves are designed for life on the margin—literally. Because the generic term brings together many plant groups, it is easy to imagine the difficulties in forming generalities about the effects of any contaminant—much less an amorphous one like "oil." Nevertheless, we will try to do so.
Similar to the oil toxicity situation for many other intertidal environments, the mangrove-related biological resources at risk in a spill situation can be affected in at least two principal ways: first, from physical effects; second, the true toxicological effects of the petroleum.
Many oil products are highly viscous. In particular, crude oils and heavy fuel oils can be deposited on shorelines and shoreline resources in thick, sticky layers that may either disrupt or completely prevent normal biological processes of exchange with the environment. Even if a petroleum product is not especially toxic in its own right, when oil physically covers plants and animals, they may die from suffocation, starvation, or other physical interference with normal physiological function.
Mangroves have developed a complex series of physiological mechanisms to enable them to survive in a low-oxygen, high-salinity world. A major point to remember in terms of physical effects of oil spills on mangroves is that many, if not most, of these adaptations depend on unimpeded exchange with either water or air. Pneumatophores and their lenticels tend to be located in the same portions of the intertidal most heavily impacted by stranded oil. While coatings of oil can also interfere with salt exchange, the leaves and submerged roots of the mangrove responsible for mediation of salts are often located away from the tidally influenced (and most likely to be oiled) portions of the plant.
These physical impacts of oil are linked to adaptive physiology of the mangrove plants, but are independent of any inherent chemical toxicity in the oil itself. The additional impact from acute or chronic toxicity of the oil would exacerbate the influence of physical smothering. Although many studies and reviews of mangroves and oil indicate that physical mechanisms are the primary means by which oil adversely affects mangroves, other reviewers and mangrove experts discount this weighting. See, for example, Snedaker et al. (1997). They suggest that at least some species can tolerate or accommodate exposure to moderate amounts of oil on breathing roots.
The lighter, or lower molecular weight, aromatic hydrocarbons that often are major components of oil mixtures are also known to damage the cellular membranes in subsurface roots; this, in turn, could impair salt exclusion in those mangroves that have the root filters described previously in this article. Disruption of ion transport mechanisms in mangrove roots, as indicated by sodium to potassium ion ratios in leaves, was identified as the cause of oil-induced stress to mangroves in the 1973 Zoe Colocotronis spill in Puerto Rico (Page et al. 1985). Mangroves oiled by the 1991 Gulf War spill in Saudi Arabia showed tissue death on pneumatophores and a response by the plants in which new, branched pneumatophores grew from lenticels—an apparently compensatory mechanism to provide gaseous exchange (Böer 1993).
Genetic damage is a more subtle effect of oil exposure, but can cause significant impact at the population level. For example, researchers have linked the presence of polynuclear aromatic hydrocarbons (PAHs) in soil to an increased incidence of a mangrove mutation in which chlorophyll is deficient or absent (mangroves such as Rhizophora mangle are viviparous and can self-fertilize, so they are well-suited for genetic screening studies such as those examining the frequency of mutations under different conditions; Klekowski et al. 1994a, 1994b). The presence or absence of pigmentation allows for easy visual recognition of genotype in the trees. The correlation between sediment PAH concentration and frequency of mutation was a strong one, raising the possibility that a spill can impact the genetic mix of exposed mangroves.
Acute Effects
|Oil surrounding mangroves. Credit: NOAA
The acute toxicity of oil to mangroves has been clearly shown in laboratory and field experiments, as well as observed after actual spills. Seedlings and saplings, in particular, are susceptible to oil exposure: in field studies with Avicennia marina, greater than 96% of seedlings exposed to a weathered crude oil died, compared to no deaths among the unoiled controls (Grant et al. 1993). Other studies found that mangrove seedlings could survive in oiled sediments up to the point where food reserves stored in propagules were exhausted, whereupon the plants died.
The Avicennia study cited above also found that fresh crude oil was more toxic than weathered crude. Based on laboratory and field oiling experiments conducted in Australia, the authors cautioned against readily extrapolating results from the laboratory to what could be expected during an actual spill. Container size and adherence of oil to container walls were thought to be important factors that may have skewed laboratory toxicity results by lowering actual exposure concentrations (Grant et al. 1993).
Another set of Australian studies investigated the toxicity of two oil types, a light crude and a Bunker C, to mature mangroves (Rhizophora stylosa) over a period of two years (Duke et al. 2000). A number of interesting results were obtained from this study, including:
- Unoiled control mortality was low over the two-year study period;
- Plots oiled with Bunker C showed no difference in mangrove mortality relative to unoiled controls;
- Mangroves treated with the light crude oil showed a significantly higher mortality than controls and the Bunker C treatment;
- Addition of chemical dispersant to the crude significantly reduced the toxicity but not to control levels;
- Most tree deaths occurred in the first six months after treatment.
The last observation is consistent with conditions observed at several oil spills in mangrove areas. In fact, obvious signs of mangrove stress often begin occurring within the first two weeks of a spill event, and these can range from chlorosis to defoliation to tree death. In the 1999 Roosevelt Roads Naval Air Station (Puerto Rico) spill of JP-5 jet fuel, an initial damage assessment survey conducted in the first month post-spill determined that 46 percent of mangrove trees, saplings, and seedlings along a transect in the most impacted basin area were stressed (defined as showing yellowed, or chlorotic, leaf color). This compared to 0 percent along the unoiled reference transect (Geo-Marine, Inc. 2000). Color infrared, aerial photography taken at regular intervals through 19 months post-spill confirmed the visual observations. Analysis of the infrared photographs of the affected mangrove area indicated that two weeks after the release, 82 percent of the total mangrove area was classified as "impacted" relative to pre-spill conditions.
Under more controlled conditions, studies using fresh crude oils have suggested that defoliation, when it occurs, should reach a maximum between 4-12 weeks post-spill.
A monitoring study conducted in Australia after the Era spill in 1992 found a consistent set of mangrove responses including leaf staining, chlorosis, leaf death, and complete defoliation. Within three months after the oil washed ashore, extensive defoliation of mangrove trees had begun and many appeared to be dead. The degree to which mangroves were damaged and the extent that they recovered from spill damage were correlated to extent of oiling (Wardrop et al. 1996).
In the 1986 Bahía las Minas (Panama) spill, scientists monitoring the effects of the oil on mangroves recorded a band of dead and dying trees where oil had washed ashore five months previously. A year and a half after the spill, dead mangroves were found along 27 km of the coast. Photographs taken just before the spill showed no evidence of tree mortality (Jackson et al. 1989).
Chronic Effects
|Credit: NOAA
The line between acute and chronic impacts can be a little blurry at times. In the case of mangroves, visible response to oiling may be almost immediate, with leaves curling or yellowing, as at the Era and Bahía las Minas spills. The tree, however, may survive for a time only to succumb weeks or months later. Alternatively, depending on the nature of exposure, it may recover to produce new leaf growth.
At least one researcher has summarized acute and chronic effects of oil to mangroves in tabular form, reproduced below (Lewis 1983). In this case, the line between acute and chronic effect was defined at 30 days; others may shift the border one way or the other.
Mangroves can be chronically impacted by oil in several ways. Stressed mangroves could show differences in growth rates or alter reproductive timing or strategy. They may also develop morphological adaptations to help them survive either the physical or chemical consequences of residual oil contamination. Such modifications may require expending additional energy, which in turn, could reduce the mangroves' ability to withstand other non-spill-related stresses they may encounter.
One consequence of the complex physical structure and habitat created by mangrove trees is that oil spilled into the environment is very difficult to clean up. The challenge and cost of doing so, and the remote locations of many mangrove forests, often results in unrecovered oil in mangrove areas affected by spills. This, in turn, may expose the trees and other components of the mangrove community to chronic releases of petroleum as the oil slowly leaches from the substrate, particularly where organic-rich soils are heavily oiled.
Researchers who have compared oil spill impacts at several different spill sites have found similar types of impacts that differ primarily in the magnitude of effect. The degree of impact appears to be related to the physical factors that control oil persistence on the shoreline and exposure to waves and currents. Interestingly, the presence and density of burrowing animals like crabs also affects the persistence of oil in mangrove areas and can determine whether an exposure is short- or long-term, because of oil penetration via the burrows into an otherwise impermeable sediment.
In many parts of the world, mangrove stands co-occur with industrial facilities and thus may be subjected to chronic contamination from petroleum compounds, other organic chemicals, and heavy metals. As a result, it can be difficult to determine the additional stress imposed by a spill event vs. existing stress. Newer assessment tools, such as molecular biomarkers, can isolate sources of stress more readily than non-specific but commonly used methodologies, and show promise for distinguishing spill impacts from other pollution sources.
- Follow-up studies of mangroves oiled during the 1991 Gulf War spill indicated that oiled pneumatophores that survived tended to develop branched secondary pneumatophores. These were observed two years after the spill in areas that were known to have been oiled, and were interpreted to be a response to impairment of normal respiration (Böer 1993)
- Studies of the 1986 Bahía las Minas (Galeta) oil spill in Panama concluded that its impact was "catastrophic." Five years after the incident, researchers suggested that oil remaining in mangrove sediments adversely affected root survival, canopy condition, and growth rates of mangrove seedlings in oil-deforested gaps. Six years after the spill, surviving forests fringing deforested areas showed continued deterioration of canopy leaf biomass (Burns et al. 1993).
- The follow-up study of the 1992 Era spill in Australia also noted a lack of recovery four years after the initial release—although effects themselves had appeared to have peaked, no strong signs of recovery were recorded in the affected mangrove areas (Wardrop et al. 1996).
- The experimental (i.e., intentional and controlled) 1984 TROPICS spill in Panama confirmed long-term impacts to oiled mangroves, termed "devastating" by the original researchers who returned to the study sites ten years later. They found a total mortality of nearly half of the affected trees and a significant subsidence of the underlying sediment. This was compared to a 17-percent mortality at seven months post-oiling, a level that appeared to be stable after 20 months (Dodge et al. 1995).
These results from the more intensively studied spills that have occurred in the last fifteen years suggest that chronic effects of such events can be measured over long time periods, potentially a decade or decades. They also indicate the difficulties in measuring longer-term impacts due to the time frames involved—and, hence, the value of longer-term monitoring of mangrove status following an oil spill.
Mangrove Community Impacts
|Many rural communities use mangrove forests for two essential benefits: the fruit from the trees, and the use of their trunks as supports for fishing nets during high-tides. This is part of a riverbank mangrove field that was destroyed by an oil spill more than two years ago. The only clean-up provided by the oil company was the adding of a layer of sand, which does nothing to repair the land. Here, CEHRD is experimenting with a new method of rapidly increasing the regrowth of the mangroves, a technique that will be taught to locals so that they may reclaim some of their livelihoods after such spills. Credit: Center for Environment, Human Rights and Development
With the realization that mangrove stands provide key habitat and nursery areas for many plants and animals in the tropical coastal environment, many researchers have included the associated biological communities in their assessments of oil impacts. Of course, this considerably broadens the scope of spill-related studies, but realistically, it would be arbitrary and artificial to consider only the impacts of oil on the mangroves themselves.
Studies of the Bahía las Minas spill in Panama concluded that significant long-term impacts occurred to mangrove communities. Both the habitat itself and the epibiotic community changed in oiled areas. After five years, the length of shoreline fringed by mangroves had decreased in oiled areas relative to unoiled areas, and this translated to a decrease in available surface area ranging from 33 to 74 percent, depending on habitat type. In addition, defoliation increased the amount of light reaching the lower portions of the mangrove forest (Burns et al. 1993).
In the Bahía las Minas spill, a massive die-off of plants and animals attached to the mangrove roots followed the initial release. Five years after the spill, the cover of epibiotic bivalves was reduced in oiled areas relative to unoiled reference areas. Open-coast study sites recovered more quickly, although differences in cover of sessile invertebrates remained significant through four years.
More controlled experimental oiling experiments have been less conclusive. One such study in New South Wales, Australia found that invertebrate populations were highly variable with differences attributable to oiling treatment difficult to discern. Though snails were less dense shortly after oiling treatments, they recovered by the end of the study period several months later (McGuinness 1990).
Another experiment in Australia focused on the effect of one toxic component of oil, naphthalene, on a gastropod snail common in the mangroves of eastern Australia. The sublethal endpoint used for impact assessment was the crawling rate of the snails. Two responses were elicited in short- and long-term exposures to naphthalene. An increased level of activity in the short-term exposure was interpreted as an avoidance response, while the decreased crawling rate induced by the longer-term exposure suggested a physiological consequence of the toxicant. The measurable differences in response attributed to the hydrocarbon implied that normal behavior patterns of the snails would be significantly disrupted by oil exposure (Mackey and Hodgkinson 1996).
The TROPICS experimental spill follow-up found no short- or long-term effects to three species of mangrove oysters studied in the experiment. In fact, populations at oiled sites showed the most substantial increases over time that was speculatively attributed to breakdown and mobilization of petroleum hydrocarbons as additional food sources (Dodge et al. 1995).
One area of focus in interpreting mangrove community impacts in the context of oil spill response has been comparing the toxicity of undispersed and dispersed oil to the mangroves themselves and to the associated invertebrate community. The limited findings are somewhat equivocal: one study found that dispersing oil appears to reduce the inherent toxicity of the oil to mangroves, but increases the impacts to exposed invertebrates (Lai 1986). Another assessment concluded no difference in toxicity to crustaceans from dispersed and undispersed crude oil (Duke et al. 2000). However, the same study also evaluated toxicity of Bunker C fuel oil and found that the crude oil was significantly more acutely toxic than the Bunker. The authors attributed this to the physical and chemical differences between the oil types.
The TROPICS study in Panama found a notable lack of mortality to mangrove trees at the oil/dispersant-treated site, in contrast to a measurable and seemingly increasing mortality at the oil-only treatment site.
Australian researchers studying the effects of the 1992 Era spill on fish populations around oiled mangroves found no measurable assemblage differences between groups inside and outside oiled zones, although juveniles of several species were significantly smaller in oiled creeks than in unoiled creeks (Connolly and Jones 1996).
Indirect Impacts
As is the case with most, if not all, spill-affected resources, some indirect impacts on mangroves have been identified. For example, residual oil remaining on the surface of mangrove sediments oiled during the Gulf War spill in Saudi Arabia increased the ambient soil temperatures to the point where germination and growth of intertidal plants was adversely affected (Böer 1993).
In Panama, the breakdown of protective structure provided by roots of dead mangroves caused a secondary impact from the oil spill at Bahía las Minas. For five years post-spill, the tree remnants had protected young seedlings, but when the roots finally gave way, drift logs crushed the recovering mangrove stand and essentially destroyed that part of the mangrove fringe (Duke et al. 1993).
Decomposition of the mangrove root mass following large-scale mortality causes significant erosion and even subsidence of the land where the forest was located. In the experimental TROPICS oiling, approximately 8 cm of surface elevation loss was noted by researchers who returned to the study site 10 years after the oiling (Dodge et al. 1995).
Prolonged flooding of diked mangrove areas due to cleanup operations is a possible indirect spill impact that would be limited to those areas where hydrologic conditions are easily controlled. This was suggested as a factor in the 1999 jet fuel spill at Naval Station Roosevelt Roads in Puerto Rico. In that spill, culverts providing water exchange with coastal waters were closed both to facilitate oil recovery and to prevent the spread of oil to other areas. However, in doing so, the water levels in some basin mangrove forests were held at much higher levels (> 1 meter) than the norm for periods of more than a week. It has been suggested that this action either contributed to or was a major source of mortality to mangroves in the weeks that followed (Wilkinson et al. 2000).
Even though a sublethal exposure to oil may not kill a mangrove stand outright, several post-spill, follow-up studies have suggested that oil can significantly weaken mangroves to the point where they may succumb to other natural stresses they ordinarily would survive. Examples of these stresses include cold weather and hypersalinity (Snedaker et al. 1997).
Summary and Response Implications
|A young heron sits dying amidst oil splattering underneath mangrove on an island impacted by oil from the Deepwater Horizon oil spill in Barataria Bay, along the the coast of Louisiana on Sunday, May 23, 2010. Credit: Department of Ecology, State of Washington.
The body of literature available for the toxicity of oil to mangroves presents a range of results from which we can extract some points for spill response guidance.
- Mangroves are highly susceptible to oil exposure. Acute effects of oil (mortality) occur within six months of exposure and usually within a much shorter time frame (a few weeks). Commonly observed mangrove responses to oil include yellowing of leaves, defoliation, and tree death. More subtle responses include branching of pneumatophores, germination failure, decreased canopy cover, increased rate of mutation, and increased sensitivity to other stresses.
- Different oil types confer different toxicity effects. While this is a universal truth in spill response, for mangroves the lighter oils are more acutely toxic than heavier oils (for example, light crude oil is more toxic than a Bunker-type fuel oil). Similarly, less- weathered oil is more toxic to mangroves than the same oil that has been subjected to longer or more intense weathering.
- The physical effects of oiling (e.g., covering or blocking of specialized tissues for respiration or salt management) can be as damaging to mangroves as the inherent toxicity of the oil. Although some studies indicate that mangroves can tolerate some coating without apparent damage, many others identify physical effects of oiling as the most serious.
- Response techniques that reduce oil contact with mangroves reduce the resultant toxicity as well. For example, chemical dispersants seem to reduce oil toxicity to mangroves. In this case, the tradeoff is the possibility of increased toxicity to adjacent and associated communities, such as offshore coral reefs, and increased penetration of dispersed oil that may reach mangrove sediments.
- Comparing spill impacts at several mangrove sites indicates that variable effects are related to geomorphology and hydrologic kinetics of the mangrove ecosystem that, in turn, control whether oil persists in the mangrove habitat. Oiled mangrove forests that are sheltered from wave and current exposure are likely to be more severely affected than well-exposed, "outer fringe" mangrove areas. A physico-biological consideration that also can be significant is the density of burrows from associated organisms such as crabs, which can increase the penetration and persistence of oil with depth into sediments. Berms can protect inner areas or concentrate oil in front of them.
- Mangrove communities are complex and, as might be expected, the impacts of oil to the associated plants and animals vary. The available information suggests that, while oil spills undoubtedly affect such communities, they appear to recover more quickly than the mangroves themselves. Because of this, longer-term effects are likely to be related to death of the mangroves and loss of the habitat that supports and protects the community.
As we have noted, the toxicity implications from an oil spill in a mangrove area depend on a wide variety of different factors. Generally, the amount of oil reaching the mangroves and the length of time spilled oil remains near the mangroves are key variables in determining the severity of effect. Although it is stating the obvious to a spill responder that prevention is the best tool for minimizing the environmental impacts of an incident, for mangroves this is especially true. Reducing the amount of oil reaching the mangroves not only reduces the short- and long-term toxicological effects but also reduces cleanup impacts and the potential for chronic contamination. In a response, these considerations may translate into increased protection for mangroves at risk from exposure and possible use of response measures that reduce that exposure (e.g., open-water countermeasures such as burning or dispersants, shoreline countermeasures such as chemical cleaners or flushing). The long-term character of many of the mangrove impacts that have been observed argues for serious consideration of such strategies.
Oil Spill Response
Key Points
- Mangroves are highly sensitive to oil and often are priority areas for protection.
- Winds and tides carry spilled oil into mangrove forests, where oil coats the soil surface, aerial roots, and propagules.
- Dispersing or burning oil offshore can prevent or lessen impacts to mangroves.
- Spill containment and cleanup techniques should minimize any additional impacts to mangroves and other natural resources at risk.
|In the village of Kalabilema, Bayelsa, a felled mangrove forest shows the damage of a fire which killed four people in March 2004. The cause of the fire was an old oil spill from leaking pipelines. Credit: Ed Kashi/Corbis
As detailed in the previous section, mangroves are particularly sensitive to oil and, where they are native, often are priority areas for protection. The objective of spill response in mangroves, as in any habitat, is to minimize the damage caused by the accident and released oil. Spill containment and cleanup techniques should minimize any additional impacts to mangroves. Mangrove forests are a biogenically structured habitat—the trees themselves create the habitat. Death of the trees, the structuring organism, causes loss of habitat, with corresponding impact on the suite of associated species dependent upon them, including offshore resources such as coral reefs. Potential response strategies should be evaluated to determine whether the ultimate benefits from the response action outweigh any environmental costs to the mangrove forests and associated sensitive habitats at risk.
Variables such as oil type, weather, location, and availability of response equipment will determine initial spill response options. In the best-case scenario, oil is prevented from moving into and contaminating mangrove areas. Promising, on-water response techniques that can help prevent oil from reaching mangrove forests include chemical dispersion and in-situ burning.
On-Water Response Options to Prevent Mangrove Oiling
Mechanical Recovery Offshore
Mechanical containment and collection of spilled oil on water using equipment such as booms and skimmers are primary initial cleanup methods used at many spills. Experience has shown, though, that mechanical recovery alone usually cannot adequately deal with very large spills offshore. Weather and sea conditions, the nature of the oil, and other factors may limit the effectiveness of mechanical recovery. In such cases, alternative open-water response techniques, such as dispersant application or in-situ burning of oil on water, may significantly reduce the risk that oil will reach shore and impact mangroves and other sensitive intertidal and shoreline habitats.
Offshore Dispersant Application
Chemical dispersants are products applied to oil on the water surface to enhance formation of fine oil droplets, which mix into the water column and are dispersed by currents. Most oils physically disperse naturally to some degree due to agitation created by wave action and ocean turbulence. Chemical dispersants enhance and speed up this natural dispersion process. Dispersing oil soon after release minimizes impacts to wildlife at the water surface (e.g., birds and marine mammals) and reduces the amount of floating oil that reaches sensitive nearshore and shoreline habitats. If applied appropriately offshore, chemical dispersants can be an effective tool for protecting mangrove forests and the habitat they provide. Tradeoffs among other resources at risk, such as potential effects of temporarily higher concentrations of oil in the water column on pelagic organisms and coral reefs, should be considered before dispersant use. When applied appropriately in sufficiently deep water, impacts to corals are expected to be minimal.
Offshore In-situ Burning
In-situ burning is a response technique in which spilled oil is burned in-place. When used appropriately, in-situ burning can remove large quantities of oil quickly and efficiently with minimal logistical support. Like dispersants, in-situ burning can help minimize impacts to wildlife at the water surface and reduce the amount of oil that reaches sensitive nearshore and shoreline habitats, including mangroves. A potential disadvantage of open-water in-situ burning is that a small percentage of the original oil volume may remain as a taffy-like residue after the burn. Floating residue can be collected but residues that sink or escape collection and move inshore could potentially contaminate mangroves.
It is important to note that, in contrast to open-water burning, in-situ burning should not be conducted within mangrove forests, as explained later under "Response Techniques Inappropriate for Mangroves."
Oil Behavior in Mangroves
|Mangrove leaves covered by oil. Credit: Gavin Newman/Greenpeace
Mangroves grow in low-energy depositional areas, which also tend to be the sites where oil accumulates. Spilled oil is carried into mangrove forests by winds and tidal currents. Oil slicks generally move into mangrove forests when the tide is high, depositing on the soil surface and on aerial roots and propagules when the tide recedes. The resulting distribution of deposited oil is typically patchy due to the variability in tidal heights within the forest. If there is a berm or shoreline, oil tends to concentrate and penetrate into the berm or accumulated detrital wrack. The oil can penetrate into the soil, particularly through crustacean burrows and other voids like those formed by dead mangrove roots. Lighter oils tend to penetrate more deeply into mangrove forests than heavier and more weathered oils, but will not persist unless they mix into the soil. However, crude oils and heavier refined products can pool onto sediment surfaces and are highly persistent. These heavy oils and emulsified oil can be trapped in thickets of red mangrove prop roots and black mangrove pneumatophores and are likely to adhere to and coat these surfaces, as well as other organic materials, such as seagrass wrack. Re-oiling from resuspended oil, particularly as tides rise and fall, may further injure plants over time. Where oil persists, sheens may be generated for months or years.
Assessing the extent and distribution of stranded oil can be difficult, particularly in dense forests, because the forest interior sometimes can be oiled even if the mangrove fringe is not, due to its lower tidal height. Access to interior areas of forests usually must be limited in order to minimize damage. Also, the tree canopy may hide oil on the ground during oil-observation overflights. Affected areas may become more apparent from the air as trees die or defoliate. Oiled trees may start to show evidence of effects, such as leaf-yellowing, within weeks after oiling. Trees may take months to die, especially with heavy oils.
Cleanup of oiled interior mangroves can be particularly difficult because some mangrove forests are nearly impenetrable. Intrusive cleanup operations may significantly damage roots and seedlings, and also trample oil deeper into sediments, where it is slower to break down. Consequently, access to interior areas of mangrove forests should be limited and highly supervised. During later, less-supervised stages of mangrove cleanup on Eleanor Island at the 1993 Bouchard B-155 Bunker oil spill in Tampa, Florida, cleanup workers reportedly spread oil from the mangrove fringe to the roots of previously unoiled mangrove plants in the mangrove interior as they moved back and forth removing surface sediment contamination. In spills of relatively fresh, lighter oil, such as diesel or crude, sediment penetration and toxic damage can occur very rapidly and the oil can break down relatively quickly. In such cases, cleanup operations are not expected to save many mangrove trees or effectively remove much oil, and any benefits are probably outweighed by the potential additional damage from access for cleanup.
Natural processes will eventually remove remaining oil. Tidal action and precipitation can help physically flush stranded oil out of contaminated mangrove areas. Weathering processes degrade the oil, gradually reducing quantity and toxicity. Oiled substrate may not be able to support mangrove growth while toxicity levels remain high. Oil can degrade quickly in warm tropical environments, but more slowly if degradation is inhibited by anaerobic soil conditions. Oil may persist for very long periods in the peaty or muddy sediment where mangroves are most often found. Heavier oils can persist in mangrove sediment for decades after a spill.
Cleanup Options for Oiled Mangroves
If mangrove forest shorelines are oiled, extreme caution must be exercised in selecting cleanup activities. Potential benefits of oil removal must be weighed against the risks of potential additional harmful impacts from the cleanup technique.
No Action/Natural Recovery
There are several circumstances under which it is appropriate to do nothing. The foremost of these situations is when cleanup would cause more harm than benefit to mangroves or other associated habitats, or when shorelines are inaccessible. When no cleanup is conducted, oil will slowly degrade and be removed naturally, assisted by natural and storm-generated flushing.
Spills of light oils, which will naturally evaporate and break down very rapidly, do not require cleanup. Such light oils are usually gone within days. Furthermore, light fuel oils such as gasoline and jet fuels typically impart their toxic impacts immediately, and cleanup can do little to reduce the damage. The only light refined product that might warrant some cleanup is diesel (No. 2 fuel oil) if sediment could be contaminated. It is important to recognize, though, that even where no cleanup is advisable, light oils can cause significant injury and contaminated mangrove habitats may require many years to recover.
Cleanup also is not recommended for small accumulations of oil, regardless of product type. Impacts caused by light accumulations generally do not warrant the tradeoffs associated with cleanup activity. Even for major spills, there may be cases for which it is best to take no action, depending on the nature of the oiling and the characteristics of the mangrove forest affected. Generally, cleanup should not be conducted in interior areas of mangrove forests because of the risk of damaging mangrove roots and seedlings, trampling oil into the sediment where it will degrade much more slowly, and spreading oil into previously unoiled areas. Exceptions may be made if access is possible from upland areas or if vegetation is sparse enough to permit access without injury to pneumatophores and prop roots. If cleanup is attempted in interior mangroves, experienced personnel must constantly oversee cleanup crews to prevent further injury.
In any case, attempts should be made to control the movement and spread of any mobile oil within the mangroves to prevent contamination of adjacent areas. Several response techniques described below, including barriers, passive collection, and flushing can be used to help control and contain mobile oil.
Barrier Methods
Several forms of barriers can deflect or contain oil, including booms, sediment berms, dams, and filter fences. Barriers can be used along mangrove shorelines and inlets to prevent oil entry. Proper strategic boom deployment in sheltered lagoon areas may be highly effective in trapping large quantities of mobile oil and reducing oil impact to interior mangroves. To be effective, barriers must be deployed immediately after a spill before oil moves into mangrove areas. This means that appropriate types and sufficient amounts of barrier materials must be stockpiled and available at the time of the spill, and that strategies for boom placement and deployment have already been established and tested.
Because of the soft substrate and sensitivity of prop roots and pneumatophores, barrier methods should be deployed carefully and maintained vigilantly to prevent physical damage during installation and removal. Untended boom that breaks loose can become entangled in the mangrove fringe, breaking off pneumatophores, prop roots, and juvenile plants. Boom deployed under inappropriate conditions or improperly deployed can cause additional harm, so caution must be exercised in planning where, when, and how boom will be used.
There are some shorelines where barriers will be ineffective due to physical characteristics, such as current strength and water depth. Where barrier methods are not an option, mangrove forests will remain vulnerable to contamination. For example, booms generally cannot be deployed successfully along mangrove shorelines with strong currents or along sections of mangrove shorelines behind shallow flats. Also, boom usually is not effective with light oils because they can readily mix into the water column and pass under floating boom. Heavier oils are more likely to remain at the water surface and so are more easily controlled with booms, although very heavy oils can sometimes become negatively buoyant and pass under boom.
Manual Oil Removal
Manual removal, using hand tools and manual labor, is often conducted to remove bulk oiling by heavier oils, such as crude oil or Bunker C oil, stranded in mangroves. Manual removal can help prevent other areas from becoming contaminated as the oil moves around, and helps limit long-term sediment contamination. Consideration should be given, however, to the trade-off between these benefits of manual removal and the mechanical damage to the mangroves that often accompanies manual cleanup. It is nearly impossible to reach the tangle of prop roots and pneumatophores of most mangroves without causing physical damage. Trampling of oil deeper into the sediment from foot traffic can be another harmful consequence of manual cleanup. Garrity and Levings (1996) observed that black mangrove pneumatophores along paths used by cleanup workers were significantly more likely to be killed than those in areas accessed by one or a few workers. Where pneumatophores had been dense at the time of the spill, paths often were bare substrate by 15 months post-spill as broken pneumatophores died and rotted away.
If manual removal is conducted in mangroves, and particularly in interior areas, consideration should be given to ways to minimize foot traffic and other impacts. Conducting activities from boats, when possible, is advisable. Close supervision of cleanup crews is essential.
Passive Collection with Sorbents
Sorbent boom or other sorbent materials can be placed at the fringe of oiled mangrove forests to passively recover any mobile oil, including sheens. Sorbents are oleophilic and either absorb or adsorb oil. They can be composed of either synthetic or natural materials, and they come in a variety of forms, including sausage boom, "pom-pom" or snare boom, sheets, rolls, pellets, and loose particulates. Sorbents vary in their effectiveness depending upon oil type, degree of oil weathering, and sorbent absorption or adsorption capacity. Sorbent materials must be placed and removed carefully to minimize disturbance of sediments and injury to mangrove roots. Sorbent materials must be closely monitored to ensure they do not move and damage mangrove roots, and must be removed when they become saturated or are no longer needed.
Sorbents have been used to wipe heavy oil coating from mangrove surfaces. Before using sorbents in this way, consideration should be given to associated physical damage. This activity is best conducted under close supervision and only in areas where substrate is firm enough to prevent oil mixing into it.
Vacuuming can remove pooled oil or thick oil accumulations from the sediment surface, depressions, and channels. Vacuum equipment ranges from small units to large suction devices mounted on dredges, usually used outside vegetated areas. Generally, vacuuming should be conducted only at the outer fringe of mangrove forests; it is most feasible and least damaging where vegetation is not very dense, enabling easy access. Vacuuming can be used effectively on heavier and medium oils, providing they are still reasonably fluid. Lighter, more flammable petroleum products such as jet fuel and diesel generally should not be vacuumed.
Vacuuming was used effectively to remove thick mats of Bunker C oil that stranded in mangroves during the 1993 Tampa Bay oil spill response (see the section titled "Case Studies" for more details). Vacuuming worked particularly well where oil stranded on sand substrate at the mangrove fringe. The technique was less effective over fine sediment and oyster beds. In order to minimize cleanup damage, care was taken to place the vacuum barge over firm sand substrate, where there were no seagrass beds.
Ambient Water Flooding (Deluge) and Low-Pressure Ambient Water Flushing
|Mangal in Thailand. Credit: CoastalWiki.org
Low-pressure flushing with ambient seawater can wash fluid, loosely adhered oil from the sediment surface and mangrove vegetation into areas where it can be collected, as long as it can be done without resulting in significant physical disturbance of the sediment. Generally, flushing is most feasible at the outer fringe, but can sometimes be used to remove oil trapped within the mangrove forest. Flushing at water levels high enough to submerge sediments may help minimize impact to the substrate. If substrate mixing is likely or unavoidable, responders should allow the oil to weather naturally. Flushing is not effective with heavy oils, such as Bunker C, or highly weathered oils. Oil should be flushed only during ebbing tides to move it out where it can be collected.
Flushing can be a useful technique to help control the movement and spread of mobile oil in mangrove areas to prevent contamination of adjacent areas. When flushing free-floating oil, care should be taken to minimize emulsification.
Chemical Shoreline Cleaners
Chemical shoreline cleaners are products sprayed on oil-coated surfaces to "loosen" the oil so that it can be flushed off with ambient water. Tidal waters or water sprays alone cannot effectively wash away heavy oil. Shoreline cleaning products vary in their toxicity and recoverability of the treated, mobilized oil. Chemical shoreline cleaners loosen or dissolve heavy oil deposited over the lenticels on coated prop roots or pneumatophores so the residue can be washed away and lenticel functioning restored. Functioning of the lenticels, which enable delivery of oxygen to the subsurface roots, is critical to survival of the trees.
Some experimental studies (Teas et al. 1987, 1993) have reported promising results using chemical shoreline cleaners on mangrove trees coated with oil. A shoreline cleaner (Corexit 9580) applied to oiled red mangroves coated with Bunker C oil and then washed with seawater (within 7 days of oiling) reportedly effectively reduced oil adhesion and exposed the lenticels, restoring their air permeability. The study concluded that mangrove trees can be saved with shoreline cleaners if the interval between oiling and cleaning is no longer than about a week. Another study (Quilici et al. 1995) reported harmful effects on mangrove trees treated with shoreline cleaner without flushing. Results likely depend on the particular product used and application technique. Further testing and more experience with the effectiveness and effects of using shoreline cleaners on mangroves are needed to determine whether their use is advisable.
Nutrient Addition/Bioremediation
Nutrient addition can enhance biodegradation of oil under nutrient-limited conditions. Microbes and essential nutrients for oil degradation generally are not limited in mangrove habitats, so nutrient enrichment may not offer much benefit. Studies conducted by Teas et al. (1991) and Quilici et al. (1995) concluded that adding fertilizer does not significantly enhance biodegradation of oil in mangrove sediment. Another study (Scherrer and Mille 1989) reported that oleophilic fertilizer enhanced the oil biodegradation process in peaty mangrove sediment, though the fertilizer in this experiment was added to the oil before the mangrove vegetation was contaminated. In any case, applied nutrients would be difficult to keep in place as tides flood through mangrove forests. There is also some risk that nutrient application might cause localized eutrophication and acute toxicity, particularly from ammonia, due to low mixing rates and shallow waters.
Burns et al. (1999) concluded that aeration of contaminated sediments may be effective in enhancing biodegradation of oil in mangrove sediments, since mangrove sediments are usually anaerobic below surface layers. The researchers suggest a bioremediation strategy that employs selective aeration to promote the survival of the trees vital to maintaining the structural integrity of the mangrove forest. The trees also provide the habitat necessary for the return of burrowing animals to impacted sediments. Burns et al. (1999) point out that aeration is not necessarily a strategy to be used over large areas. Reports on trial experiments to test this strategy are not yet available. More testing of this potential response technique is needed.
Removal of Oiled Wrack and Debris
Heavily oiled wrack and debris should be removed if it can be done without significantly damaging prop roots, pneumatophores, and seedlings or trampling oil into the sediment. However, oiled wrack should not be removed until the threat of oiling has passed, since wrack and leaf litter can act as a sort of natural barrier sorbent and actually protect the trees from direct oil contact. Unoiled and lightly oiled wrack and leaf litter should not be removed because they provide habitat and contribute to the ecosystem.
|Credit: NOAA '
Response Techniques Inappropriate for Mangroves
Under no circumstances should live mangrove vegetation be cut or burned. Both techniques will destroy trees and mangrove habitat. Mangrove trees are slow-growing and take decades to be replaced by mature vegetation. The loss of a large number of trees may compromise the forest structure, making it unlikely to recover naturally. Other cleanup techniques used at some oil spills but inappropriate in mangroves include mechanical oil removal, high-pressure or hot-water flushing, steam-cleaning, slurry sand blasting, trenching, and sediment reworking, tilling, or removal. All these methods would severely damage or destroy mangrove forests and associated organisms and habitats. Techniques such as pressure washing and sand blasting risk causing severe erosion.
Mangrove Recovery and Restoration
Key Points
- Mangroves can take more than 30 years to recover from severe oil spill impacts.
- Adequate tidal exchange is critical to restoration success.
- Mangrove seedling and tree density and health are the only widely measured recovery indicators at many spills.
- Restoration that works with natural recovery processes to reestablish mangrove habitat is the best course of action over the long term.
|Credit: CoastalCare.org
Mangrove ecosystems around the world suffer degradation from logging, coastal development, spraying of herbicides, conversion to fish ponds, and from oil spills and other pollutants. The continued loss of mangrove forests worldwide underscores the importance of projects focusing on restoration of forest structure and functions.
Since mangroves take 20–30+ years to recover from severe oil spill impacts, restoration projects attempt to speed up this recovery process. Adequate tidal exchange is most critical to restoration success. Mangrove restoration projects in Florida and the Caribbean often involve re-establishing natural hydrologic and tidal regimes, planting mangrove propagules, and/or planting marsh plants to provide a "nurse" habitat that can be colonized more easily than bare areas by mangrove trees.
An oil spill alone rarely changes the basic geophysical appearance and shape of the mangrove ecosystem; this is left for hurricanes, clear-cutting, and development. For this reason, restoration after an oil spill may be easier than after an event that substantially changed tidal elevation or hydrology or decimated mangrove trees. However, an oil spill may come as an additional impact on a mangrove ecosystem already degraded by human and industrial development, such as near refineries (Bahía las Minas), ports, or airfields (Roosevelt Roads). Cumulative or chronic impacts may decrease the resiliency of the mangrove ecosystem and increase the time it takes the system to recover or make it more difficult for the system to recover at all.
As with other marsh ecosystems adversely impacted by oil spills, we have learned valuable lessons from past mangrove restoration projects, including those that failed. Restoration projects need a clear goal from the outset that is based on understanding the mangrove ecosystem's natural ability to recover. The most effective role for restoration projects is to correct or assist when natural recruitment mechanisms are impeded or no longer functioning.
|More than one in six mangrove species worldwide are in danger of extinction due to coastal development and other factors. As a result, 11 out of 70 mangrove species will be placed on the IUCN Red List. Credit: CoastalCare.org/IUCN
Recovery of any impacted ecosystem following a perturbation such as an oil spill is interpreted by many to mean a return to the system in place at the time of the spill. Mangroves' specialized niche is in a unique, changeable zone, subject to sediment flow that accretes and erodes, varying amounts of fresh water, impacts from storms and hurricanes, invasion by foreign species, and predation. Thus, even if we had a precise description of ecosystem conditions just before the spill, we still might not be able to return it to its pre-spill state.
A more practical way to measure recovery is to compare the impacted system with an unimpacted one (hopefully, nearby), using metrics such as tree height, density, canopy cover, above-ground biomass, and abundance and diversity of associated invertebrates, fish, and plants. Since compromised ecosystems can be more vulnerable to stresses such as disease or predation, the recovering habitat must also show the resilience of a functioning ecosystem.
Sadly, it is rare to find long-term, follow-up studies on mangroves beyond 1-2 years post-spill. It is even rarer to find studies that measure associated communities of invertebrates or other components of the mangal (mangrove forest habitat) besides the mangrove trees themselves. Even when mangrove trees appear to have recovered, restored mangal may differ from unimpacted mangal in its functioning and ecosystem complexity. Even with its limitations, mangrove tree density and health are the only widely measured recovery indicators at many spills. Keep in mind that the recovery times indicated would probably be even longer if more comprehensive and ecological recovery measures were used.
Mangroves in the Bahía las Minas region of Panama were oiled by the Witwater spill in 1968 and again in 1986 by a refinery spill. Mangroves at Roosevelt Roads Naval Air Station in southeastern Puerto Rico were impacted by spills in 1986 and again in 1999, though different sections of mangroves were oiled at each spill. Because of the short duration of the follow-up studies, no cases were able to document recovery, except for fringe mangroves at the Witwater spill. In most of these studies, mangroves were regrowing in the oil-impacted areas but tree height, percent area of open canopy, and other parameters remained different from controls.
Da Silva et al. (1997) diagrammed generalized mangrove impact and recovery from an oil spill in four stages. These timeframes are approximate and will likely vary in different systems.
- Initial impact ~ 1 year : propagules and young plants are most likely to die during this time
- Structural damage ~ 2 1/2 years : trees begin to die
- Stabilization ~ 5 or more years : deterioration of mangroves ceases, but no improvement noticeable
- Recovery ~ timeframe unknown : system improves via colonization, increased density, etc.
Additional impacts such as from hurricanes, or other natural or human-caused disturbances could significantly delay these recovery processes.
|Credit: NOAA
Mangrove Restoration
|New satellite imagery from the US Geological Survey and NASA show there are 12.3% fewer mangroves than previously believed. Credit: Tim Keegan/TreeHugger
Restoration success has rarely been studied quantitatively, but we know restored mangrove ecosystems often do not equate with natural ones. Shirley (1992) found that plant diversity was similar in restored and natural forests one year after restoration, but that environmental conditions were different and a number of fish and invertebrate species were absent from the restored site. McKee and Faulkner (2000) found that development of structure and biogeochemical functions differed in two restored mangrove stands because of different hydrological and soil conditions. Tree production and stand development was less where tidal exchange was restricted, and some waterlogging occurred due to uneven topography. Other assessments of restoration success, in terms of initial survival and percent cover after one or several years, have been mixed. Cintron (1992) reviewed a number of these projects.
These experiences emphasize the need for developing clear restoration goals that incorporate the mangrove ecosystem and its functions, as well as the growth and health of the trees themselves. Once the goal is defined, the project is designed and implemented, followed by monitoring to ensure that restoration is proceeding as anticipated. Projects should be monitored for 10 or more years to adequately assess long-term survival, resiliency, and complexity of the restored system (Field 1998). Depending on the type of impact and the state of the impacted mangal, restoration may take several approaches:
- Replant mangroves
- Remediate soils
- Encourage natural regeneration through improved site conditions
- Restore an alternate site to provide similar habitat (in-kind restoration)
Replant Mangroves
|Credit: OutskirtOutreach.org
There is an extensive body of technical information on replanting mangroves. Specific details on elevation, use of fertilizer, planting density, species selection, etc. can be found in Snedaker and Biber (1996) and Field (1996, 1998). Today, restoration projects have moved away from broad use of planting except in those cases where natural processes are inadequate to naturally repopulate the area with recruits from surviving trees or more distant sources. Examples include mangrove forests where hydrology has been substantially altered, or where physical barriers such as dead trees, debris, or berms restrict circulation such that propagules have no access to denuded areas.
If planting is chosen as the best course, seedlings will survive best when they are planted in a sheltered location and at appropriate tidal elevation levels for each species. Planted seedlings are lost primarily because of erosion, predation, death from natural causes, planting at incorrect elevations, and residual oil toxicity (Getter et al. 1984). Planting one- to three-year old trees (usually supplied from nurseries) costs more but results in much better survival rates, especially in locations exposed to higher wave energy. Seedlings and propagules can survive even when planted in soils with residual oil contamination, though generally only after oil has weathered for 9-12 months. Red mangrove seedlings (R. mangle) survived when planted in areas with one-year old residual oil at Bahía las Minas. A restoration planting project at St. Croix in the U.S. Virgin Islands planted seedlings 8 years after heavy oiling from the Santa Augusta spill, with 40% survival after two years (Lewis 1989).
Planting is still used to establish new mangrove forests in areas where they have not previously existed (such as in newly accreted shorelines or along human-built structures), or to replant in forests that have been logged. Survival of planted mangroves ranges from 0% to as high as 80% after one year. Lowest rates are often in areas with high wave energy where propagules are simply washed away. A planting technique that successfully increases survival rates of planted mangroves in exposed areas is called the Riley encasement method. Seedlings are planted inside PVC tubes (bamboo can also be used) to anchor and protect the seedlings until they become established (Rothenberger 1999).
Survival rates drop as the time after planting increases (e.g., one to two years or more). Even when plantings survive and grow, densities of planted trees may be lower than those naturally recruited, as found at the Bahía las Minas spill. Five years post-spill, replanted R. mangle survived well (especially in sheltered areas), but trees were less dense than in areas that recolonized naturally (Duke 1996). Restoration that enhances natural recovery processes to reestablish mangrove habitat has proven to be the best course of action over the long term.
Remediate Soils
|Credit: Consortium for Ocean Leadership
Residual oil that has contaminated soils in mangrove forests degrades very slowly, since these soils are anaerobic below the top 1-2 mm (Burns et al. 2000). Experiments and field studies examining the possibility of accelerating oil degradation through addition of nutrients or increased aeration have shown little advantage to these methods. During the first year after a spill, biodegradation occurs at very low levels, and the main routes of oil removal are dissolution and evaporation. Thus, it is critical during spill response to attempt to keep oil from penetrating into sediments. Some restoration-planting projects surround seedlings with clean, fertilizer-augmented soil so the new trees can establish themselves and develop root structures in uncontaminated soils, before having to contend with possible toxic effects from residual oil.
Erosion of soils in mangrove forests following a disturbance can impede future re-establishment of new trees, since mangroves thrive only at specific tidal elevations. Since mangrove root mass comprises 40-60% of the total forest biomass, any substantial die-off of adult trees, as may occur after an oil spill, could cause subsidence of soils and erosion as a secondary impact. In such cases, augmenting soils, or assisting processes of sediment accretion may be a necessary part of restoration activities.
Encourage Natural Regeneration
Restore hydrology
Adequate hydrology is tagged as the most important parameter for mangrove recruitment (Lewis and Streever 2000). When tidal connections have been cut off or altered, as is common along developed coasts, re-establishing these connections can promote natural recruitment and improve the overall health and functioning of the mangrove ecosystem. Roosevelt Roads NAS is an example where impounded mangroves were impacted by a jet fuel spill in 1999. These mangroves suffered both from toxic fuel impacts and from extended submersion of roots when tidal conduits were closed to contain the spill during response. Facilitating or increasing tidal exchange to these impounded mangrove forests could be a promising restoration activity. In-kind restoration conducted after the Tampa Bay spill involved, in part, restoring tidal circulation at a previous dredge disposal site where mangroves had been impounded by dikes.
Plant "nurse" habitat
Since mangrove propagules and seedlings grow best in sheltered conditions, one strategy for more exposed areas is to plant indigenous marsh plants such as Spartina alterniflora to create a nurse habitat. These plants grow quickly (one to two years), trap and hold sediments (which decreases erosion), and create a more sheltered habitat where young mangroves can establish themselves. This staged approach is modeled after natural successional patterns and boosts natural recruitment of mangroves (Mauseth et al. 2001).
Propagules may be available only during certain times of the year or may not distribute far from the parent tree due to poor circulation or blocking by debris. Removing floating debris that may block channels enables propagules to reach and recolonize denuded areas naturally.
Restore in-kind resources
Increasingly, in-kind restoration is used for projects in the United States, especially for resource damage settlements after oil spills. In-kind restoration restores habitat in a different location in the same ecosystem and is meant to contribute to the overall habitat function of the region.
A recent example of in-kind restoration is Tampa Bay, Florida, where several mangrove islets were heavily oiled during a spill in 1993. Restoration efforts purchased a former dredge disposal site within Tampa Bay that included degraded mangrove forest. Tidal connections were restored, marsh grasses were planted along the shoreline, and the land was deeded to the County to function as wildlife habitat and provide water filtering functions for the waters of Tampa Bay (see Case Studies for more detail).
Mangrove Case Studies
Mangroves around the world have been exposed to oil both from individual spills and from chronic pollution from refinery and storage tank discharges. Well-documented oil spills in mangrove areas provide us with a good idea of some of the complexities and variability of the impacts and response options. We have highlighted techniques (learned from field trials, toxicology, and laboratory studies) to measure the health of mangroves. With help from NOAA's IncidentNews.gov database and from colleagues around the world we searched for case studies of oil spills impacting—or potentially impacting—mangroves. We kept our focus on individual incidents and did not include cases involving long-term pollution. However, we know that some spills occurred at sites that had been impacted by spills in the past (Bahía las Minas, Panama and Roosevelt Roads, Puerto Rico). We also focused more on the direct and indirect effects of oiling and cleanup on the mangroves themselves, less on associated fauna and flora. The incidents include a wide range of documentation and a wide range of oil types. From these we identified several case histories that provided information about the incident, response methods, and long-term impacts and recovery. These are briefly reviewed below in chronological order.
One lesson that is quite clear from even a few of the cases is that the full extent of damage to mangroves is not apparent for many months or years after an incident, regardless of the fuel type and extent of response (other than full protection). Many questions remain about most studies. The most important is, How long does recovery actually take? Although a number of post-spill studies were conducted for as long as 10 to 20 years, we were able to find only a few reports where monitoring continued long enough to confirm full recovery.
Zoe Colocotronis, La Parguera, Puerto Rico, 1973
|Mangroves offshore in LaParguera, Puerto Rico. Credit: Panoramio.com
On March 18, 1973, the Zoe Colocotronis ran aground on a reef 3.5 miles off the La Parguera tourist area on the southwest coast of Puerto Rico. The master intentionally released 37,579 barrels (1.58 million gallons) of Venezuelan (Tijuana) crude oil. An estimated 24,000 barrels (1.01 million gallons) stranded on the beaches of Cabo Rojo. Three separate pools of black oil 6-8 inches thick oiled the shore of Cabo Rojo on the Bahía Sucia side. On March 21, a large number of sea cucumbers, conchs, prawns, sea urchins, and polychaete annelids washed ashore. Organisms died in the Thalassia seagrass beds and oil moved into mangrove forests composed of white, red, and black mangrove trees (Nadeau and Berquist 1977).
Cleanup efforts were conducted outside the mangrove areas and involved booming, digging sumps, and pumping the collected oil into tank trucks. On March 23, before the oil in the mangroves could be recovered, an unexpected wind shift drove patches of oil out of the mangroves and into other areas and beaches. By March 24, 604,000 gallons of nearly pure oil had been removed from other areas using sumps, skimmers, and vacuum trucks. Steam cleaning was not used because there was no accessible source of fresh water. No cleanup was conducted in the mangroves.
EPA scientists surveyed the mangrove areas for a week beginning 24 hours after the spill. Detailed surveys were conducted of all oiled areas during the second week after the spill and again during the thirteenth week. Additional EPA site visits were made in January 1974 (10 months later) and January 1976 (34 months later) providing some idea of long-term effects. In one well-studied area, one hectare of red and black mangrove trees was defoliated and died during the three years following the spill. However, the EPA scientists also noted that much of the associated invertebrate life had recovered (Nadeau and Bergquist 1977).
In November 1973, eight months following the spill, oil chemists from Bowdoin College in Maine visited several oiled sites and noted a re-emergence of young trees. Although sediment oil concentrations remained high, the oil was heavily weathered and degraded. These observations suggested that the toxic components were gone in about half a year. This team had also visited oiled black mangrove sites four times between April 1979 and April 1981, 6 to 8 years after the spill. The scientists measured ratios of sodium and potassium in some plants, supporting the idea that oil injured the trees by disrupting salt and water balance and that such disruption might have been alleviated by directed cleanup. However, they made no comment on the visible health of the mangroves at that time (Page et al. 1979; Gilfillan et al. 1981).
Eleven years after the spill other chemists took sediment cores from several previously oiled mangrove sites and found concentrations ranging from 10,000 to 100,000 ppm (dry weight, total unresolved hydrocarbons) in a layer 6 cm below the relatively clean surface sediments. In addition, they found oil, possibly from the 1962 Argea Prima spill, 14-16 cm below the surface. These last researchers did not report the status of the mangrove trees themselves (Corredor et al. 1990).
Peck Slip, Eastern Puerto Rico, 1978
|Oiled spilled by barge Peck Slip after returning to port facility, eastern Puerto Rico. December 1978. Credit: M.O. Hayes/Oil-Spill-Info.com
On December 19, 1978 the Peck Slip released between 440,000 and 450,000 gallons of Bunker C oil into open waters offshore of eastern Puerto Rico. Within two days oil had stranded in segments along 26 km of eastern Puerto Rico shorelines, mostly sand beach. However, some oil entered outer and inner fringing mangroves in three areas, and inner basin mangroves in one of these areas.
No cleanup actions were undertaken although observers noted floating absorbent pads at one site. Surveys of mangroves were conducted shortly after the spill (December-early January 1979; Robinson 1979), about three months later (Gundlach et al. 1979), 10 months later, and 18 months later (Getter et al. 1981).
Mangroves on a small island (Isla de Ramos) were lightly impacted (prop roots had a 15-cm band of oil 50 to 60 cm above the substrate) and apparently did not suffer long-term injury. Near Punta Medio Mundo, about 2.6 acres of inner fringe and inner basin mangrove roots were heavily oiled (prop roots with up to a one-meter band of oil) and two acres moderately oiled (0.3 to 0.45-m band of oil; Robinson, 1979). An estimated 3.5 tons of oil coated the mangrove roots. Algae growing on the prop roots absorbed the oil. Another two acres of mangroves at Pasaje Medio Mundo were moderately oiled with an estimated 1.3 tons of oil (prop roots oiled by a 0.2-meter band on oil).
Within two to three months the heavily oiled inner fringing and basin mangroves at the Punta Medio Mundo forest were defoliated. Prop-root oiling had widened to a band of over two vertical meters, possibly from oiled climbing crabs. Later site visits confirmed that mangroves with the most heavily oiled prop roots remained defoliated 10 and 18 months later (Getter et al. 1981).
This was one of five sites studied by Getter et al. (1981). From these studies the authors urged that inner fringing and inner basin mangroves receive highest priority for protection from oil spills.
No restoration activities were undertaken at this spill.
JP-5 Jet Fuel Spills, Roosevelt Roads, Puerto Rico (1968 and 1999)
|On Oct. 20, 1999 an apparent overfill at a tank farm resulted in a spill of JP5 (Navy formulation of jet fuel) at the US Navy Base at Roosevelt Roads, Puerto Rico. Estimated release was 100,000 gallons. The oil traveled through a buried drainage structure to a 29 acre mangrove forest ("mangrove A"). It continued to flow through a culvert to a second mangrove forest ("mangrove C") that fringes the bay known as Ensenada Honda ("deep bay"). Credit: IncidentNews.gov
In 1986 and again in 1999, Roosevelt Roads Naval Air Station storage tanks released JP5 jet fuel into a cove in eastern Puerto Rico. Before the 1986 and 1999 JP-5 spills, the area had been contaminated by oils from several past spills: a Bunker C spill in 1958 and a diesel spill in 1978, both from onshore storage tanks, and a 210,000-gallon diesel spill in 1981 from a tanker. All of these spills contaminated mangrove areas but effects of the earlier spills are unknown. In both recent cases, mangrove forests were contaminated, though response strategies differed markedly. Effects on mangroves were monitored at both spills.
On November 27, 1986, 59,000 gallons of JP-5 fuel washed down a catchment stream (tidal creek) and into Ensenada Honda. Two mangrove forest areas were contaminated, one in the tidal creek and the other at the head of the saltwater bay.
On October 20, 1999, 112,000 gallons of JP-5 fuel spilled from a day-tank at the U.S. Navy Base. The oil flowed into an underground drainage pipe, which runs under a runway and several roads for several hundred yards. The pipe empties into an open drainage ditch, which drains to a 12-hectare mangrove forest. This forest drains through a culvert into Ensenada Honda Bay.
No cleanup actions were mentioned in reports dealing with the 1986 incident, presumably because of the high evaporation rate of JP-5 jet fuel in open conditions.
In the 1999 incident the Navy's primary environmental concern was the bay. In the face of an approaching hurricane, USN Construction Battalion (Sea Bees) personnel constructed a dam to plug the culvert between the first impacted mangrove (later named "mangrove A") and the mangrove adjacent to the bay (later named "mangrove C"). This dam trapped the water in mangrove area A. The final reports should be consulted for specifics as there were many details to the flow diversion response. Fuel was recovered, where practical, using under flow dams, skimmers, vacuum trucks, and sorbent materials. Attempts to manually remove oil with sorbents proved both ineffective and a human health risk for responders from inhalation of jet fuel fumes. It was estimated that 15 to 20% of the product was recovered, over 70 percent evaporated, and some 10 to 15% (approximately 11,200 - 16,800 gallons) remains unaccounted for; presumably stranded in the mangroves or in the sediments near the spill site.
The fuel flowed through the mangroves and some portion of the oil changed color from almost clear with a slight yellow tint to brown/black, similar to a light crude oil. It is unknown as to whether this was as a result of tannins from the mangroves dissolving into the oil or the JP-5, liberating heavier product remaining from previous spills.
1986 Spill. In the 1986 incident two mangrove areas were contaminated by JP-5 fuel: (1) the northernmost red mangroves drained by the tidal creek, and (2) the mixed species mangroves adjacent to the Coast Guard pier in Ensenada Honda. Local responders noted visible effects on adult trees within 10 days of oiling. Follow-on surveys were conducted in the second area 17 months later and again 23 months later. During these surveys 10 x 10-meter grids along transects documented tree height, canopy, tree death, percent open canopy, seedling counts, and invertebrate biota. There were three transects in oiled areas plus two in unoiled areas. In June 1987, false-color aerial photos were taken of the impacted forest.
Detailed surveys five months later found most adult trees in the oiled areas dead and/or defoliated. However, there were live seedlings with highest densities along the forest front. Furthermore, sediment oil concentrations were extremely low (less than 1 ppm) and similar to concentrations in unoiled areas. Because of the low impact on seedlings and the near-absence of fuel oil six months later, researchers concluded that there was no smothering effect from the jet fuel. Adult tree defoliation and mortality was likely caused by initial direct toxicity of the fuel to root structures.
Apparently these mangroves recovered sufficiently from the 1986 JP-5 spill to merit no comment from personnel responding to the 1999 spill, other than that they were protected by the response itself. Given the location of the 1999 contamination (tidal creek mangroves), very little cleanup was possible. However, the series of water diversion activities resulted in preventing oiling of the mangrove (C) in Ensenada Honda.
1999 Spill. Tidal creek mangroves (areas A and B) were clearly damaged from the 1999 incident, due either to fuel toxicity or extended flooding, or both. Follow-up studies through October 2001 indicated that there was some recovery in the flooded area A two years after the incident, with new propagules and new shoots on injured trees. However, there were no signs of recovery in area B. Of a total of 50 acres of injured mangrove forest, about 30 acres showed no signs of recovery two years later (Csulak 2001).
Vesta Bella Oiling and Cleanup of U.S.Virgin Islands Mangroves, 1991
|Cleanup worker scrubbing down red mangrove prop roots with an oil snare at Haulover following the Vesta Bella spill. Credit: NOAA
On March 6, 1991, the barge Vesta Bella sank southeast of Trinidad, releasing an unknown amount of high aromatic No. 6 fuel oil. The barge continued to leak for more than 20 days. Some oil moved north, eventually stranding on several beaches on the north side of St. John, in the U.S. Virgin Islands. Beach surveys began there on March 23. Red mangrove oiling was not extensive: one-meter prop roots of individual or small groups of mangrove trees were oiled 30 to 35 cm above the substrate. However, the short (15 cm) prop roots of supratidal white mangroves were heavily coated. These trees were also stressed before the spill due to beach erosion.
A modest level of cleaning was attempted with a planned revisit to the site a year later. Roots were carefully wiped by a select group of workers, and then snare boom was strung and allowed to scrub roots with the rise and fall of the tide. Snare boom was removed after 24 hours. One year after the spill the mangroves were revisited and measured for a variety of plant health indicators.
The white mangroves at one site were heavily defoliated but also showed extensive new growth on both oiled and unoiled trees, growth that apparently began six to twelve months post spill. There was some sign of chlorosis and no signs of oil on roots. Close inspection of formerly oiled fringing red mangroves indicated these trees were healthy—fully foliated, with no signs of chlorosis. Only one tree was severely oiled and cleaned at the time of the spill: measurements indicated this tree was in good health.
Unfortunately, no oiled mangroves were left uncleaned, to serve as a reference, so it is difficult to ascribe the good condition of the trees one year later, to the cleaning. However, it is clear that this level of cleaning did not cause any mortality to the trees. The authors caution that this cleanup method was done in areas with a firm substrate. Finally, they confirmed that there was very little contamination of the substrate.
No restoration activities were undertaken at this spill.
T/V Era, Spencer Gulf, South Australia, 1992
|Mangroves of Spencer Gulf, Australia. Credit: Government of South Australia
On August 30, 1992, the tanker Era released an estimated 296 tonnes (974,000 gallons) of heavy Bunker oil (a blend of diesel and heavy residual) at a jetty near the head of Spencer Gulf, South Australia. On the night of September 1-2, an estimated 20 tonnes (5,500 gallons) stranded along 10-15 km of mangrove (Avicennia) forest south of Port Pirie, S.A. However, subsequent surveys estimated that the actual quantity stranded in the mangroves was 57 tonnes (15,600 gallons).
Within two to three hours of the release, the oil slick was treated from vessels spraying dispersants Corexit 9527 and 7667; the following day, aircraft also sprayed slicks with Ardrox dispersant. Responders were advised that cleanup within the mangrove forest was not feasible and would likely increase damage to adjacent, unimpacted areas. Thus, all subsequent activity in the mangrove forest was restricted to detailed and long-term monitoring.
Oiled mangroves were monitored for four years after the spill. This is perhaps one of the most well documented accounts available of the fate and effects of oil in a mangrove forest. Only a brief, highly simplified account can be given here and the reader is advised to consult the report for important details and qualifications (Wardrop et al. 1997).
Due to an extremely high tide, oil penetrated far into the mangrove forest (50 m) coating leaves as well as stems, trunks, and sediment. Oil concentrations and visible damage to mangrove trees were recorded over four years. About 75-100 hectares were oiled: 4.2 heavily, 7.3 moderately, and 38.0 lightly. In 1992 heavy oiling of canopy and extensive mats of oiled seagrass debris characterized heavily oiled areas. By November 1992 mangroves over a total area of 2.3 hectares suffered extensive defoliation; the area expanded slightly to 3.2 hectares by 1995 and then stopped increasing. Trees that were totally defoliated did not recover during the four-year period. Defoliation and degree of sediment oiling were correlated: heavily oiled areas were completely defoliated and moderately oiled areas were "severely" defoliated. In lightly oiled areas trees had less leaf damage and recovered rapidly. "Overall the extent of damage in each of the studied locations, and the speed with which it occurred, has correlated to the oiling classification assigned in the first survey" (Wardrop et al. 1997). Finally, the veracity of the original recommendation of "no cleanup" was supported: injury to mangrove trees was restricted to those initially impacted by moderate to heavy oiling.
Witwater and Texaco Storage Tank Spills, Bahía Las Minas, Panama, 1968 and 1986
|Texaco storage tank spill, Bahia Las Minas. Credit: Smithsonian Tropical Research Institute
Two large oil spills, 18 years apart, resulted in long-term injury and recovery to a portion of the 1,200 ha of mangroves of the Bahía Las Minas area of Panama.
Witwater. On December 13, 1968, the oil tanker Witwater broke up in heavy seas off the Atlantic coast of Panama, spilling 14,000 barrels (588,000 gallons) of Bunker C and diesel oil into the water 5 miles from Galeta Island. Strong seasonal winds pushed the slick towards the island, oiling sand beaches, rocky coasts, and mangroves.
Texaco Storage Tank. On April 27, 1986, a Texaco storage tank at a refinery on Isla Payardi, Panama, ruptured, releasing approximately 240,000 barrels (10.1 million gallons) of medium-weight crude oil. Approximately 140,000 barrels (5.9 million gallons) of oil flooded through a dike and overflowed separators and a retaining lagoon and flowed into Bahía Cativá, an arm of Bahía las Minas.
Witwater. Several thousand barrels were pumped from the waters surrounding Galeta Island, and approximately 5,000 barrels (210,000 gallons) were ignited and burned along shorelines in the bay. By December 17, pumping and shoreline burning cleaned up approximately half of the spilled oil.
Texaco Storage Tank. Refinery personnel reported that 60,000 barrels (2.52 million gallons) of oil were recovered. It is not known how much of this recovered oil was from the sea. Dispersants were applied in Bahía Cativá, Islas Naranjos, offshore of Bahía Las Minas, near Portobelo, and along the northern breakwater at the mouth of the Panama Canal. Although dispersants appeared to be ineffective due to the weathered state of the oil and the calm seas, skimmers recovered some floating oil. Vacuum trucks were used as part of the shore-based cleanup effort. Several channels were dug through the mangroves to drain the oil. These channels appeared, instead, to have helped move the oil inshore. Increased disturbance due to the construction of the channels may have also contributed to subsequent erosion. Oiled rocks and debris were manually removed along the more accessible shorelines. Seawater was sprayed on some sandy areas to aid oil removal. Pumping to recover floating oil appeared to be the most effective oil recovery method. The shallow waters and mangroves rendered many oil spill cleanup techniques impractical.
|Texaco storage tank spill, Bahia Las Minas. Credit: Panama-guide.com
Archived aerial photographs (1966, 1973, 1979, and 1990) and ground surveys were keys to understanding the effects of these two spills on mangrove forests.
Witwater. Despite the cleanup, both red and black mangrove trees were severely oiled, and the majority of the red mangrove seedlings were killed. Oil also damaged many of the mangrove forest inhabitants. Initial reports did not indicate that adult trees had suffered. Aerial survey photos from 1966 and 1973 were used to assess deforestation, oil gaps, and open canopy. About 49 hectares of mangrove forest (representing 4 percent of the total mangrove forest) had been completely deforested in 1973 (five years after the spill). Most deforested areas had new recruits by 1979 (eleven years after the spill) but 3 ha were lost to sea-margin encroachment. Observable differences (oil gaps, and canopy height and structure) and oiled sediment persisted into 1992, 23 years after the Witwater spill.
Texaco Storage Tank Spill. The distribution of oil was surveyed from aircraft for two months following the release. A total of 51 miles of shoreline was heavily oiled, including some mangroves recovering from the Witwater spill. In a central embayment (Bahía Cativá), approximately half the surrounding forested area (and halfway up the intertidal zone) was killed. Oiled habitats within this distance included extensive mangroves, intertidal reef flats, seagrass beds, and subtidal coral reefs. Re-oiling of the shoreline and mangroves was a continuing problem. Oil slicks were regularly observed within Bahía las Minas for at least four years following the spill with oil coming predominantly from areas of fringing mangroves. As the oiled red mangrove trees decayed, it was believed that eroding, underlying sediments released trapped oil.
An affected reef flat habitat was the site of an ongoing study at the Smithsonian Tropical Research Institute's field station at Punta Galeta. A detailed study of mangrove trees revealed that one- to two-year-old seedlings appeared to survive whereas the surrounding adults died. It was believed that, somehow, young seedling structure (perhaps lack of prop roots) enabled the young trees to tolerate periods of oil immersion. It was suggested that the disruption of the substrate before replanting may remove such survivors, hampering forest recovery. Oil persisted in the mangroves through May 1989. Initial oiling of the trees produced measurable amounts of oil on 100% of all the roots that were sampled. Through May 1989, the mangrove roots in the open coast and channel areas showed 70% oiling, while the oiled proportion in the stream mangroves remained 100% oiled. The decrease in oil coverage resulted from weathering, microbial degradation, and loss of oiled bark or encrusting organisms. Root mortality was greater in oiled areas.
Subsequent aerial and ground surveys indicated "recovery of the 1986 spill was well-advanced by 1992" (Duke et al. 1997) due, in part, to extensive restoration. However, about 5 hectares of fringing forest were lost to sea-margin encroachment and there remained important differences between sheltered and exposed areas.
Although ten times more oil was spilled in 1986 than in 1968, this did not result in ten times more damage to mangroves. Calm winds, lower tides, different oil type, and longer weathering time before impact may have resulted in less toxicity.
Because of extensive mangrove mortality, several replanting projects were conducted at Bahía las Minas, in hopes of speeding mangrove forest recovery, which was at the time estimated to take 20 years or longer (Teas et al. 1989).
Experiments to determine whether propagules could survive if planted directly in oiled sediment found 100% mortality up until six months post spill. By nine months post-spill, propagules survived at rates similar to those at unoiled sites. Beginning 12 months after oiling, red mangrove seedlings that had been raised in a separate nursery area were planted (with added fertilizer) in areas of the damaged mangrove forest. A total of 42,000 nursery plants and 44,000 propagules were planted.
Studies conducted in 1989 (33 months post-spill) looked at the effectiveness of the plantings conducted in 1987, by comparing mangrove densities in areas that had recruited naturally with those that were replanted. Though planted seedlings had survived in all areas studied, naturally recruited plants were most dense. Thus, natural recruitment was more effective at recolonizing oil-damaged areas and, over time, natural recruits out-competed planted seedlings. Researchers also noted detrimental collateral impacts from planting, including cutting and removing dead timber for boat access (which removed shelter for seedlings), trampling sediments, digging holes (which accelerated erosion), and damaging existing seedlings (Duke 1996). Overall, planting did not result in a net benefit to the mangrove forest. However, since recolonization of mangroves was lowest in exposed areas, Duke suggests that an effective restoration activity could be to protect very exposed areas until mangrove trees are well established.
Bouchard Barge B-155, Tampa Bay, August 1993
|Barge BOUCHARD B155 incident, Tampa Bay, Florida, August 1993. An overflight of an island and oil in Tampa Bay, Florida. Credit: IncidentNews.gov
On August 10, 1993, the freighter Balsa 37, the barge Ocean 255, and the barge Bouchard 155 collided in the shipping channel west of the Skyway Sunshine Bridge and south of Mullet Key in Tampa Bay, Florida. The collision caused three separate emergencies: (1) the Balsa 37 was listing, threatening to spill phosphate rock; (2) the jet fuel, gasoline, and diesel caught fire on the Ocean 255; and (3) the Bouchard 155 was holed at the port bow, spilling approximately 8,000 barrels (338,000 gallons) of No. 6 fuel oil into Tampa Bay. By August 15 most of the floating fuel oil had come ashore and heavily coated sand beaches, several mangrove islands, and seawalls within Boca Ciega Bay. By August 16 very little floating oil was seen offshore. In the shallow, low-energy areas along the mangrove islands inside Johns Pass and at a few locations in the surf zone, oil had mixed with beach sand and shallow sediments to form underwater tarmats, some of which came ashore on the mangrove keys.
The No. 6 fuel from the barge is the only material known to have been released from this incident. Countermeasures used during this spill were mechanical or manual. Skimming operations were used to collect free-floating oil. Efficiency and effectiveness of skimming operations were extremely high. Oil in and around mangrove islands was removed by vacuuming. Areas were left oiled when it was felt that cleanup methods would cause greater impact than leaving the oil in place. Some of the submerged oil in very shallow areas was removed using buckets and shovels. Oiled seagrass beds were cleaned by gently lifting oil out of them by hand. "How clean is clean" inspections for mangroves, seagrass beds, and other sensitive areas were judged on a case-by-case basis by the inspection committee.
|On August 10, 1993, three ships collided in Tampa Bay, Florida: the BOUCHARD B155 barge, the freighter BALSA 37, and the barge OCEAN 255. The BOUCHARD B155 spilled an estimated 336,000 gallons of No. 6 fuel oil into Tampa Bay. Credit: IncidentNews.gov
Tarmats formed when sediment was mixed with oil along the shallow flats surrounding the islands. Large, thick mats coated mangrove roots, oyster and seagrass beds, and tidal mud flats. Much of this oil was vacuumed out using vacuum transfer units on grounded barges staged around the islands and shallow areas.
Scientists visited oiled and unoiled mangrove keys quarterly between November 1994 and April 1996. Individual trees, pneumatophores, and prop roots were tagged to enumerate trends in defoliation, leaf health, shoot number and length, and mortality of juvenile and adult plants or their structures. Visual oiling trends were documented through late 1995 and sediment samples for wet chemistry collected in 1996. Adult red mangrove trees at the most heavily oiled site (outer Eleanor Island) deteriorated over this time period, with moderate to heavy defoliation and soft, rotting prop roots. "Of marked trees, 20% were totally defoliated and appeared dead by June 1994" (Levings and Garrity 1995). Nine-month mortality of juvenile red and black mangrove plants was 5% at unoiled reference sites, 35% in heavily oiled areas on the protected side of the island and 50% in heavily oiled areas on the exposed side of Eleanor Island. It was predicted additional mortality would continue to occur.
The researchers also measured for signs of sublethal stress in adult trees: one to two years after the spill and cleanup, surviving red mangroves experienced graded negative responses in four measures of shoot growth and production, suggesting that sublethal long-term effects may be common in oiled mangroves. Sediments around trees experiencing these responses contained greater than 500 ppm total hydrocarbons (dry weight).
More follow-up observations are needed at these sites, but we are not aware of any extending beyond three years after the spill and cleanup.
Trustees from state and Federal agencies and the responsible party developed a restoration plan for mangroves and associated habitats damaged in the spill. A compensatory plan provided mangrove and associated wetland habitat for fish, birds, and epibenthic communities at a site in the same watershed but not necessarily actually impacted by the spill.
The responsible party purchased a former dredge disposal site in Boca Ciega Bay and deeded it into public ownership. This site contained degraded mangrove forest that was restored through increased tidal exchanges and removal of exotic plants and debris. On the bayward edge of the mangrove forest, smooth cordgrass (Spartina alterniflora) was planted to create a fringing saltmarsh buffer that could eventually provide habitat for mangrove seedlings. A monitoring program was established with specific "success" criteria outlined, including vegetative cover and height of mangroves, absence of exotic species, and functional tidal exchanges.
Further Reading
Oil Toxicity References
- Böer, B. 1993. Anomalous pneumatophores and adventitious roots of Avicennia marina (Forssk.) Vierh. Mangroves two years after the 1991 Gulf War oil spill in Saudi Arabia. Marine Pollution Bulletin 27:207-211.
- Burns, K.A., S.D. Garrity, and S.C. Levings. 1993. How many years until mangrove ecosystems recover from catastrophic spills? Marine Pollution Bulletin 26(5):239-248.
- Connolly, R.M. and G.K. Jones. 1996. Determining effects of an oil spill on fish communities in a mangrove-seagrass ecosystem in southern Australia. Australasian Journal of Ecotoxicology 2:3-15.
- Dodge, R.E., B.J. Baca, A.H. Knap, S.C. Snedaker, and T.D. Sleeter. 1995. The effects of oil and chemically dispersed oil in tropical ecosystems: 10 years of monitoring experimental sites. MSRC Technical Report Series 95-104. Washington, D.C.: Marine Spill Response Corporation. 82 pp. + appendices.
- Duke, N.C., Z.S. Pinzón, and M.C. Prada. 1993. Mangrove forests recovering from two large oil spills in Bahía Las Minas, Panama, in 1992. In Long-Term Assessment of the 1986 Oil Spill at Bahía Las Minas, Panama. MSRC Technical Report Series 93-019. Washington, D.C.: Marine Spill Response Corporation. pp. 39-87.
- Duke, N.C., K.A. Burns, R.P.J. Swannell, O. Dalhaus, and R.J. Rupp. 2000. Dispersant use and a bioremediation strategy as alternate means of reducing impacts of large oil spills on mangroves: The Gladstone field trials. Marine Pollution Bulletin 41(7-12):403-412.
- Geo-Marine, Inc. 2000. Natural resource damage assessment for a JP-5 fuel spill at Naval Station Roosevelt Roads, Puerto Rico. Draft final report. Norfolk: Commander, Atlantic Division, Naval Facilities Engineering Command. 58 pp.
- Getter, C.D., G.I. Scott, and J. Michel. 1981. The effects of oil spills on mangrove forests: A comparison of five oil spill sites in the Gulf of Mexico and the Caribbean Sea. In Proceedings of the International Oil Spill Conference, pp. 535-540.
- Grant, D.L., P.J. Clarke and W.G. Allaway. 1993. The response of grey mangrove (Avicennia marina (Forsk.) Vierh.) seedlings to spills of crude oil. Journal of Experimental Marine Biology and Ecology 171:273-295.
- Jackson, J., J. Cubit, B. Keller, V. Batista, K. Burns, H. Caffey, R. Caldwell, S. Garrity, C. Getter, C. Gonzalez, H. Guzmán, K. Kaufmann, A. Knap, S. Levings, M. Marshall, R. Steger, R. Thompson, and E. Weil. 1989. Ecological effects of a major oil spill on Panamanian coastal marine communities. Science 243:37-44.
- Klekowski, E.J. Jr., J.E. Corredor, J.M. Morell, and C.A. del Castillo. 1994a. Petroleum pollution and mutation in mangroves. Marine Pollution Bulletin 28(3):166-169.
- Klekowski, E.J. Jr., J.E. Corredor, R. Lowenfeld, E.H. Klekowski, and J.M. Morell. 1994b. Using mangroves to screen for mutagens in tropical marine environments. Marine Pollution Bulletin 28(6):346-350.
- Lai, H.C. 1986. Effects of oil on mangrove organisms. In: Maclean, J.L., L.B. Dizon, and L.V. Hosillos (eds.). Proceedings of the First Asian Fisheries Forum, pp. 285-288.
- Lai, H.C., H.J. Teas, F. Pannier, and J.M. Baker. 1993. Biological impacts of oil pollution: Mangroves. IPIECA Report Series, Volume Four. London: International Petroleum Industry Environmental Conservation Association. 20 pp.
- Lewis, R.R. III. 1983. Impact of oil spills on mangrove forests. In: Tasks for Vegetation Science, Vol. 8 (Biology and Ecology of Mangroves), H.J. Teas, ed. The Hague: Dr W. Junk Publishers. pp. 171-183.
- Mackey, A.P. and M. Hodgkinson. 1996. Assessment of the impact of naphthalene contamination on mangrove fauna using behavioral bioassays. Bulletin of Environmental Contamination and Toxicology 56:279-286.
- McGuinness, K.A. 1990. Effects of oil spills on macro-invertebrates of saltmarshes and mangrove forests in Botany Bay, New South Wales, Australia. Journal of Experimental Marine Biology and Ecology 142:121-135.
- Page, D.S., E.S. Gilfillan, J.C. Foster, J.R. Hotham, and L. Gonzalez. 1985. Mangrove leaf tissue sodium and potassium ion concentrations as sublethal indicators of oil stress in mangrove trees. In Proceedings of the 1985 International Oil Spill Conference, pp. 391-393.
- Proffitt, C.E. (ed.). 1997. Managing oil spills in mangrove ecosystems: effects, remediation, restoration, and modeling. OCS Study MMS 97-0003. New Orleans: U.S. Department of the Interior, Minerals Management Service, Gulf of Mexico OCS Region. 76 pp.
- Snedaker, S. C., P. D. Biber, and R. J. Aravjo. 1997. Oil spills and mangroves: an overview. In: Proffitt, C.E. (ed.). Managing oil spills in mangrove ecosystems: effects, remediation, restoration, and modeling. OCS Study MMS 97-0003. New Orleans: U.S. Department of the Interior, Minerals Management Service, Gulf of Mexico OCS Region. pp. 1-18.
- Wardrop, J.A., B. Wagstaff, P. Pfennig, J. Leeder, and R. Connolly. 1996. The distribution, persistence and effects of petroleum hydrocarbons in mangroves impacted by the "Era" oil spill (September, 1992): Final Phase One report. Adelaide: Office of the Environment Protection Authority, South Australian Department of Environment and Natural Resources.
- Wilkinson, D.L., C. Moore, M. Lopez, and M. Figueroa. 2001. Natural resource damage assessment for a JP-5 fuel spill at Naval Station Roosevelt Roads, Puerto Rico. Pre-final report. Norfolk: Atlantic Division, Naval Facilities Engineering Command.
Oil Spill Response References
- Allen, A. A. and R.J. Ferek. 1993. Advantages and disadvantages of burning spilled oil. In: Proceedings of the 1993 International Oil Spill Conference, pp. 765-772.
- Ballou, T.G., S.C. Hess, R.E. Dodge, A.H. Knap, and T.D. Sleeter. 1989. Effects of untreated and chemically dispersed oil on tropical marine communities: A long-term field experiment. 1989. In: Proceedings of the 1989 International Oil Spill Conference, pp. 447-454.
- Burns, K.A., S.D. Garrity and S.C. Levings. 1993. How many years until mangrove ecosystems recover from catastrophic oil spills? Marine Pollution Bulletin 26(5): 239-248.
- Burns, K.A, S.D. Garrity, D. Jorrisen, J. MacPherson, M. Stoelting, J, Tierney and L. Yelle-Simmons. 1994. The Galeta oil spill. II. Unexpected persistence of oil trapped in mangrove sediments. Estuarine, Coastal and Shelf Science 38:349-364.
- Burns, K.A., S. Codi, C. Pratt, R.J.P Swannell, and N.C. Duke. 1999. Assessing the oil degradation potential of endogenous micro-organisms in tropical marine wetlands. Mangroves and Salt Marshes 3: 67-83.
- Burns, K.A., S. Codi, C. Pratt, and N.C. Duke. 1999. Weathering of hydrocarbons in mangrove sediments: testing the effects of using dispersants to treat oil spills. Organic Geochemistry 30: 1273-1286.
- Chaw, LH, H.J. Teas, F. Pannier, and J.M. Baker. 1993. Biological impacts of oil pollution: mangroves. International Petroleum Industry Environmental Conservation Association (IPIECA) Report Series Volume 4. 20 pp.
- Duke, N.C., Burns, K.A, S. Codi, O. Dalhaus, J.C. Ellison, C. Pratt and R.J. Rupp. 1999. Fate and effects of oil and dispersed oil on mangrove ecosystems in Australia. Final Report to the Australian Petroleum Production Exploration Association. June 12, 1999. Queensland: Australian Institute of Marine Science and CRC Reef Research Centre.
- Cintron-Molero. 1992. Restoring mangrove ecosystems. In G. Thayer (ed.), Restoring The Nation's Marine Environment, College Park, Maryland: Maryland Sea Grant College. pp. 223-277.
- Corredor, J.E., J.M. Morell, and C.E. Del Castillo. 1990. Persistence of spilled crude oil in a tropical intertidal environment. Marine Pollution Bulletin 2(8):385-388.
- Daykin, M., G. Sergy, D. Aurand, G. Shigenaka, Z. Wang, and A. Tang. Aquatic toxicity resulting from in-situ burning of oil-on-water. In: Proceedings of the Seventeenth Arctic and Marine Oil Spill Program (AMOP) Technical Seminar 2:1165-1193.
- Fingas, M.F., F. Ackerman, K. Li, P. Lambert, Z. Wang, M.C. Bissonnette, P.R. Campagna, P. Boileau, N. Laroche, P. Jokuty, R. Nelson, R. D. Turpin, M.J. Trespalacios, G. Halley, J. Belanger, J. Pare, N. Vanderkooy, E.J. Tennyson, D. Aurand, and R. Hiltabrand. 1994a. The Newfoundland Offshore Burn Experiment - NOBE: Preliminary results of emissions measurement. In: Proceedings of the Seventeenth Arctic and Marine Oil Spill Program (AMOP) Technical Seminar 2:1099-1164.
- Fingas, M.F., G. Halley, F. Ackerman, N. Vanderkooy, R. Nelson, M.C. Bissonnette, N. Laroche, P. Lambert, P. Jokuty, K. Li, G. Halley, G. Warbanski, P.R. Campagna, R. D. Turpin, M.J. Trespalacios, D. Dickens, E.J. Tennyson, D. Aurand, and R. Hiltabrand. 1994b. The Newfoundland Offshore Burn Experiment - NOBE: Experimental design and overview. In Proceedings of the Seventeenth Arctic and Marine Oil Spill Program (AMOP) Technical Seminar 2:1053-1163.
- Garrity, S.D. and S.C. Levings. 1993. Chronic oiling and long-term effects of the 1986 Galeta spill on fringing mangroves. In: Proceedings of the 1993 International Oil Spill Conference, pp. 319-324.
- Garrity, S.D. and S.C. Levings. 1993. Effects of an oil spill on some organisms living on mangrove (Rhizophora mangle L.) roots in low wave-energy habitats in Caribbean Panama. Marine Environmental Research 35: 251-271.
- Garrity, S.D. and S.C. Levings. 1994. The 10 August 1993 Tampa Bay oil spill: Injury assessment for the mangrove keys inside John's Pass. Final Report, Findings through June 1994. 140 pp. Silver Spring, Maryland: Damage Assessment Center, National Oceanic and Atmospheric Administration.
- Garrity, S.D., S.C. Levings, and K.A. Burns. 1994. The Galeta oil spill. I. long-term effects on the physical structure of the mangrove fringe. Estuarine, Coastal and Shelf Science 38: 327-348.
- Getter, C.D., T.G. Ballou, and C.B. Koons. 1985. Effects of dispersed oil on mangroves: synthesis of a seven-year study. Marine Pollution Bulletin 16:318-324.
- Getter, C.D., T.G. Ballou, and J.A. Dahlin. 1983. Preliminary results of laboratory testing of oil and dispersants on mangroves. In: Proceedings of the 1983 International Oil Spill Conference, pp. 533-538.
- Ibáñez, M. 1995. Mangrove restoration: Cartagena, Colombia, coastal oil spill case study. In: Proceedings of the 1995 International Oil Spill Conference, pp. 990-991.
- Jackson, J.B., J.D. Cubit, B.D. Keller, V. Batista, K. Burns, M. Caffey, R.L. Caldwell, S.D. Garrity, D.C. Getter, C. Gonzalez, H.M Guzmman, K.W. Kaufmann, A.H. Knap, S.C. Levings, M.J. Marshall, R. Steger, R.C. Thompson, and E. Weil. 1989. Ecological effects of a major oil spill on Panamanian coastal marine communities. Science 243: 37-44.
- Levings, S.C., S.D. Garrity, and K.A. Burns. 1994. The Galeta Oil Spill. III. Chronic reoiling, long-term toxicity of hydrocarbon residues and effects on epibiota in the mangrove fringe. Estuarine, Coastal and Shelf Science 38:365-395.
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- Scientific and Environmental Associates, Inc. 1995. Workshop Proceedings: The Use of Chemical Countermeasures Product Data for Oil Spill Planning and Response, Volume I, April 4-6, 1995. Leesburg, Virginia: SEA. 83 pp.
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Mangrove Recovery and Restoration References
- Ballou, T. G. and R. R. Lewis III. 1989. Environmental assessment and restoration recommendations for a mangrove forest affected by jet fuel. In: Proceedings of the 1989 International Oil Spill Conference, pp. 407-412.
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- Field, C. 1998. Rehabilitation of mangrove ecosystems: an overview. Marine Pollution Bulletin 37:383-392.
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- Gilfilan, E.S., D.S. Page, R.P. Gerber, S. Hansen, J. Cooley and J. Hothman. 1981. Fate of the Zoe Colocotronis oil spill and its effects on infaunal communities associated with mangroves. In: Proceedings of the 1981 International Oil Spill Conference, pp. 360.
- Levings, S.C., S.D. Garrity, E.S. VanVleet, and D.L. Wetzel. 1997. Sublethal injury to red mangroves two years after oiling. In Proceedings of the 1997 International Oil Spill Conference, pp. 1040-41
- Levings, S.C. and S.D. Garrity. 1995. Oiling of mangrove keys in the 1993 Tampa Bay oil spill. In: Proceedings of the 1995 International Oil Spill Conference, pp. 421-428.
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- Lewis, R. R. 1990. Creation and restoration of coastal plain wetlands in Florida. P. 73-101. In: J. A. Kusler and M. E. Kentula (eds.), Wetland Creation and Restoration. Washington, D.C.: Island Press.
- Lewis, R. R. 1979. Large scale mangrove restoration on St. Croix, U.S. Virgin Islands. In: Proceedings of the Sixth Annual Conference on the Restoration and Creation of Wetlands, pp. 231-241.
- Mauseth, G. S., J.S. Urquhart-Donnelly, and R. R. Lewis. 2001. Compensatory restoration of mangrove habitat following the Tampa Bay oil spill. In: Proceedings of the 2001 International Oil Spill Conference, pp.761-767.
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- Nadeau, R.J. and E.T. Bergquist. 1977. Effects of the March 18, 1973 oil spill near Cabo Rojo, Puerto Rico on tropical marine communities. In: Proceedings of the 1977 International Oil Spill Conference, pp. 535-538.
- Rothenberger, P. 1999. Utilization of encasement technology in restoration of mangrove forest on St. Croix, U.S. Virgin Islands. Reef Research 9 (3). Great Barrier Reef Marine Park Authority, Townsville, Australia. 2pp. Available: http://www.gbrmpa.gov.au/corp_site/info_services/publications/reef_research/
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Mangrove Case Studies References
- Nadeau, R.J. and E.T. Bergquist. 1977. Effects of the March 18, 1973 oil spill near Cabo Rojo, Puerto Rico on tropical marine communities. In: Proceedings of the 1977 International Oil Spill Conference, pp. 535-538.
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- Gilfillan, E.S., D.S. Page, R.P. Gerber, S. Hansen, J. Cooley, and J. Hothman. 1981. Fate of the Zoe Colocotronis oil spill and its effects on infaunal communities associated with mangroves. In: Proceedings of the 1981 Interna¬tional Oil Spill Conference, p. 360.
- Page, D.S., E.S, Gilfilan, C.C. Foster, J.R. Hotham, and L. Gonzales. 1985. Mangrove leaf tissue sodium and potassium ion concentrations as sublethal indicators of oil stress in mangroves. In: Proceedings of the 1985 Interna¬tional Oil Spill Conference, pp. 391-393.
- Corredor, J.E., J.M. Morell, and C.E. Del Castillo. 1990. Persistence of spilled crude oil in a tropical intertidal environment. Marine Pollution Bulletin 21:385-388.
- Robinson, J.H. (ed). 1979. The Peck Slip oil spill: a preliminary scientific report. Boulder: Office of Marine Pollution Assessment, National Oceanic and Atmospheric Administration. Unpublished report.
- Gundlach, E.R., J. Michel, G.I. Scott, M.O. Hayes, C.D. Getter, and W.P. Davis. 1979. Ecological assessment of the Peck Slip (19 December 1978) oil spill in eastern Puerto Rico. In: Proceedings, Ecological Damage Assessment Conference, Society of Petroleum Industry Biologists, pp. 303-317.
- Getter, C.D., G.I. Scott, and J. Michel. 1981. The effects of oil spills on mangrove forests: A comparison of five oil spill sites in the Gulf of Mexico and the Caribbean Sea. In: Proceedings of the 1981 Oil Spill Conference, pp. 535-540.
- Ballou, T.G. and R.R. Lewis III. 1989. Environmental assessment and restoration recommendations for a mangrove forest affected by jet fuel. 2 In: Proceedings of the 1989 International Oil Spill Conference, pp. 407-412.
- Lehman, S., F. Lopez, and F. Csulak. 2001. Case study: spill of JP5 fuel at Roosevelt Roads Naval Air Station, Puerto Rico, into a basin mangrove. In: Proceedings of the 2001 International Oil Spill Conference, pp. 197-201.
- Dahlin, J.A., J. Michel, and C. Henry. 1994. Recovery of mangrove habitats at the Vesta Bella oil spill site. HAZMAT Report 95-3. Seattle: Hazardous Materials Response and Assessment Division, National Oceanic and Atmospheric Administration. 30 pp.
- Wardrop, J.A., B. Wagstaff, P. Pfennig, J. Leeder, and R. Connolly. 1997. The distribution, persistence and effects of petroleum hydrocarbons in mangroves impacted by the "Era" oil spill (September, 1992). Final Phase One report (1996). Report ERAREP/96. Adelaide, South Australia: Office of the Environmental Protection Authority, S.A. Department of Environment and Natural Resources.
- Duke, N. 1996. Mangrove reforestation in Panama, an evaluation of planting in areas deforested by a large oil spill. In: C. Field (ed.). Restoration of Mangrove Ecosystems. Okinawa: The International Society for Mangrove Ecosystems. pp. 209-232.
- Duke, N.C., Z.S. Pinzon, and M.C. Prada T. 1997. Large-scale damage to mangrove forests following two large oil spills in Panama. Biotropica 29:2-14.
- Garrity, S.D., S.C. Levings, and K.A. Burns. 1994. The Galeta oil spill: I. Long-term effects on the physical structure of the mangrove fringe. Estuarine, Coastal and Shelf Science 38:327-348.
- Jackson, J.B.C., J.D. Cubit, B.D. Keller, V. Batista, K. Burns, H.M. Caffey, R.L. Caldwell, S.D. Garrity, C.D. Getter, C. Gonzalez, H.M. Guzman, K.W. Kaufmann, A.H. Knap, S.C. Levings, M.J. Marshall, R. Steger, R.C. Thompson, and E. Weil. 1989. Ecological effects of a major oil spill on Panamanian coastal marine communities. Science 243:37-44.
- Teas, H. J., Lasday, A. H., Luque L., Elias, Morales, R. A. De Diego, M. E. and J. M. Baker. 1989. Mangrove restoration after the 1986 refineria Panama oil spill. In: Proceedings of the 1989 International Oil Spill Conference, San Antonio, February 13-16, 1989, pp. 433-437.
- Levings, S.C. and S.D. Garrity. 1995. Oiling of mangrove keys in the 1993 Tampa Bay oil spill. In: Proceedings of the 1995 International Oil Spill Conference, pp. 421-428.
- Levings, S.C., S.D. Garrity, E.S. VanVleet, and D.L. Wetzel. 1997. Sublethal injury to red mangroves two years after oiling. In: Proceedings of the 1997 International Oil Spill Conference, pp. 1040-41.
- Mauseth, G. S., J.S. Urquhart-Donnelly, and R. R. Lewis. 2001. Compensatory restoration of mangrove habitat following the Tampa Bay oil spill. In: Proceedings of the 2001 International Oil Spill Conference, pp.761-767.