Plants and animals need nutrients to survive. But when too many nutrients enter rivers, streams and the Chesapeake Bay, they fuel the growth of algae blooms and create conditions that are harmful for fish, shellfish and other underwater life. In fact, excess nutrients are the main cause of the Bay’s poor health.
Why are excess nutrients a problem for the Chesapeake Bay?
Excess nutrients fuel the growth of harmful algae blooms, which:
- block sunlight from reaching underwater grasses, and
- during decomposition, create low-oxygen “dead zones” that rob the water of oxygen and suffocate marine life.
How do excess nutrients enter the Chesapeake Bay?
While nutrients are a natural part of the Chesapeake Bay ecosystem, nutrients have never been so abundant in the environment. Before humans built roads, homes and farm fields, most nutrients were trapped and absorbed by forest and wetland plants. As these habitats were removed to accommodate a growing population, nutrient pollution to the Bay increased.
Almost all people and industries in the watershed—and even some outside of the watershed—send nutrients into the Bay and its tributaries. Nitrogen and phosphorous are the two nutrients of concern in the area. In general, these nutrients reach the Bay from three sources: wastewater treatment plants; urban, suburban and agricultural runoff; and air pollution.
- There are hundreds of wastewater treatment plants in the watershed. In 2005, watershed jurisdictions put a new permit process in place to limit the amount of nutrients these plants could send into rivers and streams. Computer simulations of pollution controls put in place between 2009 and 2015 indicate that nitrogen loads to the Bay from wastewater treatment plants and combined sewer overflows have declined 57 percent since 1985, while phosphorous loads from the same sources have declined 75 percent.
- Nutrients that run off of the land and into the water through urban, suburban and agricultural runoff come from a range of sources, including lawn fertilizers, septic systems and livestock manure.
- Air pollution emitted by cars and trucks, industries, gas-powered lawn tools and other sources contributes about one-third of the total nitrogen load entering Chesapeake waterways. This air pollution can come from any location within the Bay’s “airshed,” which measures about 570,000 square miles and stretches to Canada, Ohio and South Carolina.
Nutrients can also come from natural sources, like soil, plant material and wild animal waste.
A case study in nutrients: the Conowingo Dam and Chesapeake Bay water quality
Each year, the Susquehanna River provides the Chesapeake Bay with about 41 percent of its nitrogen loads and 25 percent of its phosphorous loads. For decades, three large reservoirs that sit behind dams located along the lower portion of the river have held back some of the nutrient pollution that would have otherwise entered the Bay. But recent studies have drawn attention to these reservoirs’ changing effectiveness as “pollution gates,” with special attention paid to the reservoir behind the Conowingo Hydroelectric Generating Station, or Conowingo Dam.
In 2012, the U.S. Geological Survey (USGS) reported that the reservoir behind the Conowingo Dam had lost its ability to trap sediment and attached nutrients over the long term.
In 2014, the Lower Susquehanna River Watershed Assessment (LSRWA) team released the results of its evaluation of sediment management options at the Conowingo Dam. It found:
- The reservoir behind the Conowingo Dam is trapping sediment in the short-term. Because the reservoir is essentially full, it is trapping smaller amounts of incoming sediment and, during large storms, sending more silt and attached nutrients over the dam and into the Bay more often.
- The nutrients that enter the river upstream and attach to particles of sediment are a bigger threat to water quality than sediment alone.
- The management and mitigation of nutrients and sediment upstream of the reservoir would be more beneficial to Bay health than attempting to manage sediment at the dam through dredging, bypassing or operational changes.
While the sediment that can scour from behind the dam doesn’t take long to settle to the bottom of waterways, the nutrients that are attached to this sediment are released back up into the water column in dissolved form. Because nutrient pollution has a lingering effect on water quality, lowering both nutrient and sediment pollution upstream of the Conowingo Dam would benefit Bay health.
To learn more, visit Learn the Issues: Conowingo Dam.
To lower nutrient pollution in the Bay watershed, consider reducing the amount of pollution that can run off your property: install a green roof, rain garden or rain barrel to capture and absorb rainfall; use porous surfaces like gravel or pavers in place of asphalt or concrete; and redirect home downspouts onto grass or gravel rather than paved driveways or sidewalks. If you have a lawn to take care of, use fertilizers properly: do not use more than needed, and do not apply to dormant lawns or frozen ground. You can also reduce air pollution by walking, biking or taking public transportation, or using electric or manual lawn mowers and yard tools instead of gas-powered machines.