bloom collapses due to nutrient depletion in the stratified water column and increased grazing pressure by zooplankton. The most limiting nutrient in the marine environment is typically nitrogen (N). This is because most organisms are unable to fix atmospheric nitrogen into usable forms (i.e. ammonium, nitrite, or nitrate). However, with the exception of coastal waters, it can be argued, that iron (Fe) is the most limiting nutrient because it is required to fix nitrogen, but is only available in small quantities in the marine environment, coming from dust storms and leaching from rocks. Phosphorus can also be limiting, particularly in freshwater environments and tropical coastal regions. During winter, wind-driven turbulence and cooling water temperatures break down the stratified water column formed during the summer. This breakdown allows vertical mixing of the water column and replenishes nutrients from deep water to the surface waters and the rest of the euphotic zone. However, vertical mixing also causes high losses, as phytoplankton are carried below the euphotic zone (so their respiration exceeds primary production). In addition, reduced illumination (intensity and daily duration) during winter limits growth rates. Alternative mechanisms Historically, blooms have been explained by Sverdrup's critical depth hypothesis, which says blooms are caused by shoaling of the mixed layer. Similarly, Winder and Cloern (2010) described spring blooms as a response to increasing temperature and light availability. However, new explanations have been offered recently, including that blooms occur due to: Coupling between phytoplankton growth and zooplankton grazing. The onset of near surface stratification in the spring. Mixing of the water column, rather than stratification Low turbulence Increasing light intensity (in shallow water environments). Northward progression At greater latitudes, spring blooms take place later in the year. This northward progression is because spring occurs later, delaying thermal stratification and increases in illumination that promote blooms. A study by Wolf and Woods (1988) showed evidence that spring blooms follow the northward migration of the 12 °C isotherm, suggesting that blooms may be controlled by temperature limitations, in addition to stratification. At high latitudes, the shorter warm season commonly results in one mid-summer bloom. These blooms tend to be more intense than spring blooms of temperate areas because there is a longer duration of daylight for photosynthesis to take place. Also, grazing pressure tends to be lower because the generally cooler temperatures at higher latitudes slow zooplankton metabolism. Species succession The spring bloom often consists of