Chesapeake Legacies: Implications of Legacy N Accumulation for Water Quality Improvements in the Chesapeake Bay

Abstract:

Global flows of reactive nitrogen (N) have increased significantly over the last century in response to land-use change, agricultural intensification and elevated atmospheric N deposition. Excess N flows from upland regions through overland and surface water pathways to the coasts, where it fuels algal growth, disrupts aquatic ecosystems and negatively impacts coastal economies. Since the 1980s, Chesapeake Bay Program partners have worked to implement a wide range of measures—from the upgrading of wastewater treatment plants to implementation of farm-level best management practices—to reduce nutrient flows to the Bay. Despite widespread implementation of such measures and notable reductions in in N inputs, N concentrations in surface waters are in many cases remaining steady. Such lack of response has been attributed to legacy N stores in subsurface reservoirs contributing to time lags between implementation of conservation measures and improvements in water quality. It has remained unclear, however, what the magnitudes of such stores might be, and how they are partitioned between shallow soil and deeper groundwater reservoirs. In the present work, we have synthesized data to develop a more than 100-year trajectory of N inputs to the land surface of Chesapeake Bay watersheds. Based on this dataset, we have used the ELEMeNT model – which pairs a simulation of soil nutrient dynamics with a travel time-based approach – to reconstruct historic nutrient yields and to estimate the influence of legacy N on current and future water quality. Our results show significant accumulation of groundwater N in Chesapeake watersheds, with legacy N accounting for as much as 50% of annual N loading in some watersheds. The present results suggest that time lags on the order of decades should be anticipated when aiming to reduce N export, and that long-term commitment to changes in agricultural management practices will be necessary to meet such goals.