Estimating Effects of Climate Change on Nitrogen Flux to Chesapeake Bay

Abstract:

Effective nutrient management requires understanding the potential effects of climate change on the fate and transport of nitrogen in the environment. We used an extrapolation of long-term rainfall trends and statistically downscaled temperature projections along with a previously-calibrated spatially-referenced regression (SPARROW) watershed model to estimate effects of climate change over a 30-year period (1995 – 2025) on nitrogen delivery to Chesapeake Bay and tributary streams. A Total-Maximum-Daily Load (TMDL) established in 2010 mandates management practices to reduce nitrogen flux from the watershed be in place by 2025. Recent increases in temperature and precipitation in the Bay watershed are expected to continue through 2025 and beyond. Spatial trends in stream chemistry in the eastern United States reflected in the SPARROW model suggest that such temperature and precipitation increases may have counteracting effects on nitrogen yields to streams. Incorporating climate projections within the SPARROW watershed model suggests that increases in precipitation and, consequently, runoff and streamflow would increase the delivery of nitrogen from uplands to and within many bay tributary streams. Such potential increases are more than offset in many areas by expected declines in nitrogen delivery from watershed uplands to streams under expected future warmer temperatures, possibly due to increasing rates of terrestrial denitrification. Although future nitrogen trends in Bay tributaries will also depend on changes in sources and other human influences, these results suggest that effects of expected climate change may include a decline in nitrogen delivery to surface waters in many areas. Predictions of the effects of climate change on nutrients can support informed decisions about the design, location, expected effectiveness, and prioritization of different ecosystem management and restoration options over a variety of spatial and temporal scales in the Bay watershed and elsewhere.