Impacts of Atmospheric Nitrogen Deposition and Coastal Nitrogen Fluxes on Oxygen Concentrations in Chesapeake Bay
Although rivers are the primary source of dissolved inorganic nitrogen (DIN) inputs to the Chesapeake Bay, direct atmospheric DIN deposition and coastal DIN concentrations on the continental shelf can also significantly influence hypoxia; however, the relative impact of these additional sources of DIN on Chesapeake Bay hypoxia has not previously been quantified. In this study, the estuarine carbonbiogeochemistry model embedded in the Regional Ocean Modeling System (ChesROMS ECB) is used to examine the relative impact of these three DIN sources. Model simulations highlight that DIN from the atmosphere has roughly the same impact on hypoxia as the same gram for gram change in riverine DIN loading, although their spatial and temporal distributions are distinct. DIN concentrations on the continental shelf have a similar overall impact on hypoxia as DIN from the atmosphere ~0.2 mg L however, atmospheric DIN impacts dissolved oxygen (DO) primarily via the decomposition of autochthonous organic matter, whereas coastal DIN concentrations primarily impact DO via the decomposition of allochthonous organic matter entering the Bay mouth from the shelf. The impacts of atmospheric DIN deposition and coastal DIN concentrations on hypoxia are greatest in summer and occur farther downstream (southern mesohaline) in wet years than in dry years (northern mesohaline) Integrated analyses of the relative contributions of all three DIN sources on summer bottomDO indicate that impacts of atmospheric deposition are largest in the eastern mesohaline shoals, riverine DIN has dominant impacts in the largest tributaries and the oligohaline Bay, while coastal DIN concentrations are most influential in the polyhaline region. Plain Language Summary Most organisms living in the Chesapeake Bay, like fish, crabs, and oysters, need adequate oxygen concentrations to survive. However, general increases in the supply of nutrients to estuaries always enhance the production of algae, and the decomposition of these algae takes away oxygen from other organisms, resulting in hypoxic (low oxygen) conditions or what is commonly referred to as a "dead zone." Generally, researchers focus on how terrestrial nutrients entering the bay, for example, from fertilizer, wastewater treatment, or sewer runoff, produce the Chesapeake Bay dead zone, since they account for most of the nutrients entering the bay. However, the atmospheric and oceanic nutrients directly impacting the bay are often not accurately considered. In this study the impacts of nutrients from the atmosphere and the open ocean on Chesapeake Bay hypoxia are quantified via the application of a three dimensional ecosystemmodel. Atmospheric deposition of nitrate is found to have the same gram for gram impact on hypoxia as terrestrial nitrate entering via rivers. Overall, these two sources of nutrients have the greatest impact in the summer and have similar impacts on dissolved oxygen, reducing oxygen concentrations by up to 0.2 mg L 1 in the mid Chesapeake Bay region where oxygen concentrations are lowest.