At excess levels, nutrients overstimulate the growth of aquatic plants and algae. Excessive growth of these organisms, in turn, uses up dissolved oxygen as they die and decompose and blocks light to deeper waters. These conditions affect the respiration of fish and aquatic invertebrates, lead to a decrease in animal and plant diversity, and affect our use of the water for fishing, swimming, and boating. Excess algae can also block industrial intake pipes. Sediment and other suspended solids can wash off plowed fields, construction and logging sites, urban areas, strip-mined land, and eroded stream banks when it rains.
Pathogens are microorganisms such as bacteria and viruses that can cause human health problems.
These organisms enter water bodies from sources such as inadequately treated effluent from sewage treatment plants, storm water drains, faulty septic systems, medical waste, runoff from livestock pens, and boats that discharge untreated or poorly treated sewage. When found at unsafe levels in nearshore waters, pathogens can lead to beach and shellfish bed closures. These toxic substances can cause death or reproductive failure in the fish, shellfish, and wildlife that use the habitat. Pesticides and herbicides used on farmlands and lawns can be washed into ground and surface waters by rainfall, snowmelt, and irrigation practices and may, ultimately, find their way to nearshore waters. These contaminants are usually very persistent in the environment and can accumulate in fish, shellfish, and wildlife to levels that pose a risk to human health and the environment.