Chesapeake Bay water quality declines by four percentage points

An estimated 38% of the Chesapeake Bay and its tidal tributaries met clean water standards for clarity, oxygen and algae growth between 2016 and 2018. This score is lower than the record high 42% from the previous reporting period, but is still the fifth highest estimate of water quality standards attainment since 1985.

This four percentage point decrease is due in large part to a decline in dissolved oxygen in the open waters of the Bay, those areas beyond the shoreline and shallows. Dissolved oxygen is necessary for the survival of the Bay’s aquatic species, and is a factor in the annual dead zone. However, dissolved oxygen conditions in deep water habitat (depths greater than six feet) and measures of chlorophyll a along the Bay’s surface show improvement from the 2015-2017 assessment period, which may indicate increasing resilience. However, water quality must improve in 62% of the Chesapeake Bay and its tidal tributaries if the estuary is to function as a healthy ecosystem.

“This relatively small decline in water quality standards attainment, during a period that included near record rainfall for much of the Bay watershed, is actually an indication that our collective management actions are working.” said James Martin, co-chair of the Chesapeake Bay Program’s Water Quality Goal Implementation Team. “It also underscores the importance of the decisions made by the Chesapeake Bay Program to estimate and offset the anticipated impacts of climate change on our ability to meet water quality goals in the future.”

Water quality is influenced by nitrogen, phosphorus and sediment loads delivered from the watershed. Those loads vary each year due to weather patterns, river flow and changes in land use. The U.S. Geological Survey (USGS) reports that between October 2017 and September 2018, river flow to the Bay measured an above-average 70.5 billion gallons per day, the highest recorded amount since 2011.

Generally, when the watershed receives more rain and river flows increase, the water carries more nitrogen, phosphorus and sediment pollution than usual, increasing loads received by the Bay. This remained true from October 2017 to September 2018 as approximately 423 million pounds of nitrogen, 42.1 million pounds of phosphorus and 15,689 million pounds of sediment reached the Bay: a 66%, 181% and 262% increase from the previous year, respectively. However, the pollutant loads recorded during this time period were a decrease from those observed in 2011, which held the previous record for high flow rates into the Bay.

Because excess nitrogen, phosphorus and sediment are among the leading causes of the Bay’s poor health, the Chesapeake Bay Program uses several environmental indictors to track these pollutants and assess aquatic health. Monitoring data collected from River Input Monitoring (RIM) stations on the nine largest rivers in the watershed help experts estimate the total nitrogen, phosphorus and sediment entering the Chesapeake Bay and its tidal tributaries each year, while data collected from a comprehensive network of hundreds of stations in non-tidal waters allow experts to assess the aquatic response to efforts to reduce agricultural and urban runoff.

The amount of nitrogen, phosphorus and sediment entering the Bay is dependent on variations in weather patterns each year. To this end, it is important to analyze trends over time to understand whether the health of the Bay and its tidal waters are improving or declining. Findings from a recent USGS analysis of short-term trends (2009 to 2018) in pollution loads monitored at the nine RIM stations report the following:

• Short-term trends in nitrogen indicate improving conditions at three stations (James, Patuxent and Potomac) and degrading conditions at five stations.
• Short-term trends in phosphorus indicate improving conditions at the James and Patuxent stations, degrading conditions at four stations and unchanged conditions at the Pamunkey, Potomac and Susquehanna stations.
• Short-term trends in sediment loads indicate improving conditions at the James and Susquehanna stations, degrading conditions at five and no change at the Patuxent and Potomac stations.

The USGS conducted a similar analysis of nitrogen, phosphorus and sediment loads using monitoring data captured at the upstream non-tidal network stations. Short-term trends (2009 to 2018) show the following:

• Forty-one percent of non-tidal network stations report an improvement in nitrogen loads, while 40% are degrading and the remaining 19% show no change.
• Phosphorus loads are improving at 44% of non-tidal network stations, while one-third are degrading, and the remainder show no change.
• Twenty percent of non-tidal network stations show an improvement in sediment loads, with 42% degrading and the remainder showing no change.

Improvements in water quality will take time, and there are often lags between the implementation of best management practices and the visible effects of those practices on a particular waterway. This spring, the Chesapeake Bay Program released the 2018-19 Bay Barometer: Health and Restoration in the Chesapeake Bay Watershed, which further explains how the entire Chesapeake Bay ecosystem and its watershed are responding to the partnership’s collective protection and restoration efforts.