Nutrients

Nutrients are chemicals that plants and animals need to grow and survive. However, excess amounts of nutrients can be harmful to aquatic environments. Elevated levels of nitrogen and phosphorus, two types of nutrients, are the main cause of the Bay's poor water quality and loss of aquatic habitats.

Where do nutrients come from?

Virtually all individuals and industries in the watershed—and even some beyond the watershed—contribute nutrients that can eventually reach the Bay. In general, nutrients reach the Bay in three ways: pipes, runoff from the land and air pollution deposition.

The vast majority of nutrients that reach the Bay through pipes—specific, identifiable points of entry into the environment—come from wastewater treatment plants, though some come from industrial facilities as well. These sources release treated water—often still containing large amounts of nutrients—into local streams and rivers, and eventually the Bay.

Nutrients that run off the land—including agricultural lands and urban and suburban areas—and into local waterways come from fertilizers, septic systems, boat discharges and farm animal manure, among other sources.

Air pollution from vehicles, industries, gas-powered lawn tools and other emitting sources also contribute nutrients to the Bay and its tributaries. What goes up must come down: like anything else, nitrogen released into the air will eventually fall back down onto the land or water. The area where most emission sources contributing airborne nitrates to the Bay originate—called the Bay's airshed—is about seven times the size of the Bay's watershed.

Nutrients also come from a number of natural sources, including soil, plant material, animal waste and the atmosphere.

How are excess nutrients a pressure on the Bay?

Nutrients have always been in the Bay, but not at the excessive levels that are found today. When the Bay was surrounded by forests and wetlands, most nutrients were absorbed or held in place by natural vegetation, and very little ran off the land into the water. As forests and wetlands have been replaced by farms, cities and suburbs to accommodate a growing population, the amount of nutrients entering the Bay has vastly increased.

Excess nutrients fuel the growth of algae, creating dense algae blooms that rob the Bay's aquatic life of sunlight and dissolved oxygen.

• Algae blooms on the surface of the water block the sun's rays from reaching underwater bay grasses growing at the bottom. Algae can also grow directly on the grasses' leaves, further reducing the amount of sunlight they receive. Without sunlight, bay grasses cannot grow and provide critical food and habitat for blue crabs, waterfowl and juvenile fish.

• “Leftover” algae that are not consumed by the Bay's algae-eating organisms eventually die and sink to the bottom. There, they are decomposed by bacteria in a process that leaves bottom waters with little or no dissolved oxygen for crabs, oysters and other bottom-dwelling species. Just like humans and animals on earth, all aquatic species need oxygen to survive.