Nitrogen in the Chesapeake Bay - How are We Doing?

The Chesapeake Bay is the largest estuary in the U.S., with a watershed of 64 thousand square miles contributing water to it. The Susquehanna River is the largest contributor of freshwater to the Bay, with 51% of total freshwater inflow, while the Potomac contributes 19% of annual inflow. Much of the water in the Bay therefore originates in or passes through Pennsylvania, making it a key player in determining the health of the Bay.

The Bay is an important natural ecosystem that sustains many people's livelihoods and generates significant income and employment (for example, in 2011, saltwater recreational fisheries generated $1.6 billion in annual sales and 13,000 jobs). Unfortunately, water quality has been compromised because of excess nitrogen (N), phosphorus (P), and sediment flowing into the Bay. According to the USGS report "Nitrogen in the Chesapeake Bay Watershed – a Century of Change 1950-2050," total N-exports to the Bay (in 2020) were 290M lbs. of N per year, 17% from atmospheric deposition, 60% from agriculture, 15% from wastewater, and 7% from developed areas. The N-contribution of agriculture to the Bay is primarily due to the intensification of agriculture, which has been associated with more N-fertilizer use and greater quantities of manure being applied.

Between 1985 and 2019, the flow-normalized N loads to the Bay (measured at river outlets to the Bay) decreased 19% from almost 550M lbs/yr to 440M lbs/yr. One reason for lower N export to the Bay has been the Clean Air Act, last modified in 1990, which resulted in lower N-deposition from atmospheric sources. Further, since 2010, Total Maximum Daily Loads were assigned to sub-watersheds, and this has led to efforts by governmental agencies to improve management of N through the implementation of Best Management Practices, urban management, and upgrades to water treatment facilities. Improvements to water treatment facilities have reduced their N-discharges by 50% between 1985 and 2017, despite a 35% increase in population in the Bay watershed.

Across the entire Chesapeake Bay watershed, N-loads were estimated to be reduced by 11% or 49 million pounds between 1985 and 2014 due to a combination of land retirement and the adoption of Best Management Practices, such as animal manure management, conservation tillage, and bioretention in ponds and wetlands. However, actual measurements suggest that N-loads from agricultural areas did not change substantially between 1990 and the early 2010s. Part of this is caused by legacy effects—it can take several decades before changes in the landscape have their effect on stream quality, but continued high N-inputs in agriculture also play a role.

The authors of the study used simulation models to estimate N-loads to the bay until 2050 under several agricultural scenarios. The most extreme realistic scenarios are presented in the attached figure. The calculations show that even with a 10% reduction in N-inputs used in the most conservative model, water quality goals set for the Bay would not be met. This report suggests that we can expect a continued emphasis on reductions of nitrogen losses from agriculture to the Bay in the years to come. Without one 'silver bullet' that will solve the N-conundrum, a combination of reductions of N-applications, BMPs to improve N-use efficiency, and practices to reduce losses from farmlands to the streams will be looked upon to meet water quality in the Chesapeake Bay.

Figure 1: Stacked annual nitrogen loads exported to the Chesapeake Bay by source, 1950-2050. Fertilizer and manure are combined into a single agricultural source for the modeled time period after 2010 with two future scenarios: (1) increased intensity of both crop and animal agriculture, and (2) decreased intensity of both crop and animal agriculture. Source: Nitrogen in the Chesapeake Bay Watershed, A Century of Change 1950-2050. USGS Circular 1486, Version 1.1, Dec 2021. National Water-Quality Program, USGS, Reston, VA. Note: 1kg = 2.2 lbs.