Reefs in close proximity to human populations can be faced with local stresses, including poor water quality from land-based sources of pollution. Copper, a common industrial pollutant has been shown to interfere with the life history and development of coral polyps. In addition to runoff, wind blows material into the ocean. This material may be local or from other regions. For example, dust from the Sahara moves to the Caribbean and Florida. Dust also blows from the Gobi and Taklamakan deserts across Korea, Japan, and the Northern Pacific to the Hawaiian Islands. Since 1970, dust deposits have grown due to drought periods in Africa. Dust transport to the Caribbean and Florida varies from year to year with greater flux during positive phases of the North Atlantic Oscillation. The USGS links dust events to reduced health of coral reefs across the Caribbean and Florida, primarily since the 1970s. Dust from the 1883 eruption of Krakatoa in Indonesia appeared in the annular bands of the reef-building coral Montastraea annularis from the Florida Reeftract. Sediment smothers corals and interferes with their ability to feed and reproduce. Pesticides can interfere with coral reproduction and growth. There are studies that present evidence that chemicals in sunscreens contribute to coral bleaching by lowering the resistance of zooxanthellae to viruses, though these studies showed significant flaws in methodology and did not attempt to replicate the complex environment found in coral reefs. Nutrient pollution Nutrient pollution, particularly nitrogen and phosphorus can cause eutrophication, upsetting the balance of the reef by enhancing algal growth and crowding out corals. This nutrient–rich water can enable blooms of fleshy algae and phytoplankton to thrive off coasts. These blooms can create hypoxic conditions by using all available oxygen. Biologically available nitrogen (nitrate plus ammonia) needs to be below 1.0 micromole per liter (less than 0.014 parts per million of nitrogen), and biologically available phosphorus (orthophosphate plus dissolved organic phosphorus) needs to be below 0.1 micromole per liter (less than 0.003 parts per million of phosphorus). In addition concentrations of chlorophyll (in the microscopic plants called phytoplankton) needs to be below 0.5 parts per billion. Both plants also obscure sunlight, killing both fish and coral. High nitrate levels are specifically toxic to corals, while phosphates slow down skeletal growth. Excess nutrients can intensify existing disease, including potentially doubling the spread of Aspergillosis, a fungal infection that kills soft corals such as sea fans, and