Quantifying The Increased Resiliency Of Chesapeake Bay Hypoxia To Environmental Conditions: A Benefit Of Nutrient Reductions

Seasonal hypoxia is a characteristic feature of the Chesapeake Bay as a result of anthropogenic eutrophication from agriculture and urban development throughout the watershed. Although in recent years coordinated management efforts have successfully reduced the flux of nutrients into the Bay, the overall goal of sufficient oxygen concentrations below the pycnocline for living resources remains unfulfilled. This was particularly apparent in 2018 and 2019 when the volume of hypoxic water exceeded the long-term (35-year) average due to anomalously high riverine discharge. To quantify the impact of watershed nutrient reductions, conventional statistical methods were employed in concert with a 3-D numerical modeling approach to estimate the enhanced resiliency of hypoxia in the Chesapeake Bay to environmental conditions in recent years. A realistic 3-D numerical model hindcast from 2016-2019 was run, along with sensitivity experiments over the same interval that used organic and inorganic nitrogen concentrations representative of 1985 values. Differences between these sensitivity results and the realistic hindcast suggest that had nitrogen reductions not occurred, annual hypoxic volumes (O2 \u3c 3 mg L-1) would have been ~50-120% greater during the average discharge years of 2016-2017 and ~20-50% greater during the wet years of 2018-2019.The relative effect is even greater for O2 \u3c 1 mg L-1 , where annual volumes would havebeen ~80-280% greater in 2016-2017 and ~30-100% greater in 2018-2019. The exact magnitude of this effect is dependent on the terrestrial inputs used to run the numerical model and is particularly sensitive to assumptions regarding organic nitrogen loading. Numerical model results are supported by statistical analysis of observational data; however, the magnitude of change due to nutrient reductions is greater in the numerical modeling results than the statistical analysis. This discrepancy is largely accounted for by warming in the Bay that has exacerbated hypoxia and offset roughly 6-34% of the improvement from nutrient reductions. Although these results reassure policymakers and stakeholders that their efforts to reduce hypoxia have been worthwhile, they also serve as a reminder that greater reductions are needed to counteract the ever-increasing impacts of climate change