Chesapeake legacies: the importance of legacy nitrogen to improving Chesapeake Bay water quality

In the Chesapeake Bay, excess nitrogen (N) from both landscape and atmospheric sources has for decades fueled algal growth, disrupted aquatic ecosystems, and negatively impacted coastal economies. Since the 1980s, Chesapeake Bay Program partners have worked to implement a wide range of measures across the region-from the upgrading of wastewater treatment plants to implementation of farm-level best management practices-to reduce N fluxes to the Bay. Despite widespread implementation of such measures and notable reductions in N inputs, water quality across the region has been slow to improve. Such lack of response has in some cases been attributed to N legacies-accumulations of surplus N in soils and groundwater-that can contribute to time lags between implementation of conservation measures and improvements in water quality. Here, we use the ELEMeNT-N modeling framework to explore the role of legacy N in slowing reductions in N loading to the Bay, and to provide estimates of the time required to meet water quality goals in nine major tributary watersheds. Our results first show that recent improvements in water quality can be attributed to decreases in N surplus magnitudes that began to occur in the 1970s and 1980s, and that such improvements will continue in the coming decades. Future simulations suggest that, even with no additional changes in current management practices, goals to reduce N loads across the region by 25% can nearly be met within the next two decades. The present results also suggest that time lags to achieving water quality may vary considerably in the individual study watersheds, with the longest lag times being found in the highly agricultural Choptank watershed, where N surplus magnitudes remain high and where legacy N remains a major control on water quality.

Automated summary

Efforts to reduce nitrogen inputs in the Chesapeake Bay have shown some progress, with recent improvements in water quality attributed to decreases in nitrogen surplus magnitudes. Despite slow improvement, goals to reduce nitrogen loads by 25% across the region may be met within the next two decades even with no additional changes in current management practices. Time lags in achieving water quality goals may vary across different watersheds, with the longest delays seen in highly agricultural areas where legacy nitrogen continues to impact water quality.