Bay water quality continued to decline in 2018-2020

The water quality of the Chesapeake Bay is measured over a two-year assessment period and is determined by how clear the water is—measured by the abundance of underwater grasses. The health of the water is also defined by the amount of dissolved oxygen present, which is necessary for plants and animals to thrive, and chlorophyll a—a measure of algae growth. Over the period of 2018-2020, the water quality of the Chesapeake Bay fell 3.5% to 29.6%. While this is not the lowest score ever recorded, water quality must improve in 70.4% of the Chesapeake Bay and its tidal tributaries if the estuary is to function as a healthy ecosystem.

The health of the Bay’s waters is heavily influenced by the amount of nitrogen, phosphorus and sediment pollution that flows into it from around its 64,000-square-mile watershed. The amount of pollution flowing into the Bay varies each year depending on the weather. In years of intense rainfall—like 2018 and 2019—heavy river flows brought more pollution into the Bay.

“The high river flow in 2018 and 2019 have caused another short-term decline in the health of the Bay,” noted Peter Tango, monitoring coordinator with the U.S. Geological Survey.

However, from October 2019 to September 2020, more moderate river flows—53 billion gallons per day—brought approximately 246 million pounds of nitrogen, 14.4 million pounds of phosphorus and 14.9 billion pounds of sediment into the Bay. This was a reduction of 49%, 47% and 34% from the previous year, respectively.

Flows entering the Bay are measured at River Input Monitoring (RIM) stations located along nine major tributaries of the Chesapeake. In addition, these stations monitor the amount of nitrogen, phosphorus and sediment that enters the Bay from 80% of the watershed. A recent USGS report noted the following mixed trends for these pollutants over the period of 2011—2020:

• Nitrogen: Four stations indicate improving conditions (James River, Patuxent River, Potomac River and Susquehanna River) and five show degrading conditions (Appomattox River, Choptank River, Mattaponi River, Pamunkey River and Rappahannock River).
• Phosphorus: Four stations indicate improving conditions (James River, Pamunkey River, Patuxent River and Susquehanna River), three stations show degrading conditions (Appomattox River, Choptank River and Mattaponi River) and two reflect no change (Potomac River and Rappahannock River).
• Sediment: Four stations indicate improving conditions (James River, Pamunkey River, Patuxent River and Susquehanna River), four stations show degrading conditions (Appomattox River, Choptank River, Mattaponi River and Pamunkey River) and one reflects no change (Potomac River).

“The conditions in the Bay over the past three years have been affected by the high river flows,” said Scott Phillips, Chesapeake Bay coordinator with the USGS. “To better understand the influence of nitrogen, phosphorus and sediment reduction efforts, the USGS looks at 10-year trends in the major rivers entering the Bay and sites across the watershed.”

These “sites across the watershed” that Phillips mentions are the 123 sites that make up the Nontidal Water Quality Monitoring Program. Located around the watershed, these sites provide an additional understanding of the pollutants in the watershed’s rivers and streams, along with their trends.

“Monitoring water quality across the varied habitats of the Bay is critical to understanding periods of improvement and decline in response to climate events and managements,” Tango explained.

A recent analysis of these non-tidal networks over the period of 2011—2020 showed the following:

• Thirty-seven percent of non-tidal network stations show an improvement in the amount of nitrogen pollution, whereas 40% are degrading and 23% show no change.
• Forty-four percent of non-tidal network stations show an improvement in the amount of phosphorus pollution, whereas 23% are degrading and 33% show no change.
• Eighteen percent of non-tidal network stations show an improvement in the amount of sediment pollution, whereas 46% are degrading and 36% show no change.

A recent report by Bay Program partners strengthened the connection between river flows and trends in nitrogen, phosphorus and sediment pollution. It found that rivers flows are so influential to pollution loads that they can mask reductions made from point sources, which is any source from which pollutants are discharged directly into the water, such as from a pipe or storm drain.

To keep track of the Bay’s health as information becomes available, please visit ChesapeakeProgress.