Nitrogen and phosphorus are building blocks of living things and are accordingly natural components of soil. Both are key ingredients in agricultural and lawn fertilizers.
When rainwater and snowmelt flow over soils that are not held in place by plant roots, the flowing water picks up and carries the soil and the nutrients it contains. This occurs on unpaved roads and logging trails, dormant cropland, farm fields, and stream channels.
Urban stormwater picks up nutrients, including nitrogen and phosphorus, from excess lawn fertilizers, yard waste, and pet waste. On top of that, urban stormwater typically does not have a chance to soak in to the ground. The stormwater travels instead to surface waters on roads, in ditches, and in storm sewers.
This path increases the volume of runoff that picks up nutrient-containing sediment as it travels and increases the force of a stream or river. Greater force leads to greater erosion of streambeds or other soils, which are often rich in nutrients.
Nutrients also enter groundwater in leachate from conventional septic systems, which are not designed to remove nutrients from wastewater.
Septic systems that are close to coastal areas are notable contributors of nonpoint source pollution because of the relatively short distance groundwater has to travel.
Atmospheric deposition of nitrogen (from natural sources and combustion of fossil fuels) also adds to nutrient pollution.
When runoff delivers high levels of nutrients, then plants, algae, and cyanobacteria that would otherwise be kept in check by a lack of nitrogen (in Long Island Sound and Cape Cod) or of phosphorus (in Lake Champlain) are suddenly uninhibited in their reproduction. This is called eutrophication.
Some of the problems associated with eutrophication directly relate to a surge in growth of algae and cyanobacteria. These include the nuisance of the blooms (including, sometimes, a bad smell), the fact that blooms block out light that submerged plants need, and the hazard posed by the toxins that some species produce.
As the blooms die, they can set off a cascade of additional problems. Decomposition of algae and cyanobacteria consumes oxygen, which depletes oxygen dissolved in the water.
Fish and other aquatic organisms need dissolved oxygen to live. Mobile organisms such as fish and crabs can sometimes escape the low-oxygen conditions, but immobile organisms such as shellfish cannot.
In the worst case, low oxygen can kill fish by the thousands.
Anna Meyer is an environmental analyst at NEIWPCC.