Nutrients are chemicals that plants and animals need to grow and survive. But when too many nutrients enter rivers, streams and the Chesapeake Bay, they can create conditions that are harmful for bay grasses, blue crabs and other underwater life. In fact, excess amounts of the nutrients nitrogen and phosphorus are the main cause of the Bay’s poor health.

Excess nutrients fuel the growth of dense algae blooms that:

  • Block sunlight that underwater bay grasses need to grow. Bay grasses provide food for waterfowl and shelter for blue crabs and juvenile fish.
  • Rob the water of oxygen, which crabs, oysters and other bottom-dwelling species need to survive.

Virtually all people and industries in the Bay watershed—and even some outside of the watershed—contribute nutrients to the Bay and its tributaries. In general, excess nutrients reach the Bay from three major sources: specific, identifiable entry pipes; runoff from the land; and air pollution.

  • Wastewater treatment plants send nutrients into the Bay through specific, identifiable entry pipes. These plants release treated wastewater—often still containing large amounts of nutrients—into rivers and streams that flow into the Bay.
  • Nutrients that run off the land—including farmland and urban and suburban areas—come from a number of sources, including fertilizers, septic systems, boat discharges and livestock manure.
  • Air pollution from vehicles, industries, gas-powered lawn tools and other sources contribute nearly one-third of the total nitrogen entering the Chesapeake’s waterways. Airborne nitrogen can enter the Bay from an enormous 570,000-square-mile airshed that stretches north to Canada, west to Ohio and south to South Carolina.

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

Nutrients have always been a part of the Bay ecosystem, but not at the excessive levels found today. Before the region supported significant human activity, most nutrients were absorbed or held in place by natural forest and wetland vegetation. As forests and wetlands have been replaced by farms, cities and suburbs to accommodate a growing population, nutrient pollution to the Bay has vastly increased.