Award: OCE-1259187

The production of algae is in many ways the backbone of marine ecosystems and food webs. How fast this algae is being produced is generally dictated by the availability of light and of nutrients. In the North Atlantic Ocean, one of the main nutrients, nitrogen, is present in only very low concentrations during the summer. The algae is thus 'nitrogen-starved' in this particular period, leading to relatively low productivity. However, certain areas of the western North Atlantic are positioned downwind of large industrial and agricultural centers. These are known to be significant sources of nitrogen that is released to the atmosphere and carried eastward by the dominant winds before falling back to the surface of the water (FIGURE-1). This latter process, called "deposition of atmospheric nitrogen", has the potential to "fertilize" regions where nitrogen is limited and thus enhance algal production. The increased production can be beneficial or detrimental depending on location. In oceanic waters, the increase could offset part of the carbon dioxide released by human activities. In shallow near-shore waters the increase can generate low oxygen "dead zones" that could suffocate fish and other aquatic life. The actual importance of atmospheric nitrogen deposition remains poorly known because deposition events are naturally episodic and spatially 'patchy', making it challenging to directly measure them. The present award relied on numerical models of the US east coast to simulate the impacts of atmospheric nitrogen deposition over multiple years and to complement the sparse measurements available. The first part of the award focused on the surface waters of the northwestern Atlantic Ocean and the second part focused on the Chesapeake Bay. A high-resolution dataset of atmospheric nitrogen deposition was obtained from the Environmental Protection Agency and was implemented in numerical models of the US east coast. The dataset confirmed that substantial amounts of nitrogen are transported by the dominant winds and released at the surface of the water all along the US east coast (FIGURE-1). However, the study identified a number of processes that mitigate the impacts of this deposition. First, the deposition reduces the amount of nitrogen being transferred from the deep nutrient-rich water to the surface. Second, the increases in algal concentrations at the surface can 'block' some of the sunlight and thus negatively affect algal production below the surface. Despite these mitigating processes, atmospheric nitrogen deposition yields a substantial increase in algal production and concentrations at the surface during the summer. Chlorophyll concentrations (equivalent to algal concentrations) notably increase by 25-30% inside a 'hotspot' located next to the Gulf Stream (FIGURE-2). The results confirm the view that atmospheric deposition has a substantial impact on the marine ecosystem of this region. The second part of the study focused on the Chesapeake Bay, a region experiencing very low levels of oxygen (a phenomenon called hypoxia) at the bottom of the Bay during the summer period (FIGURE-3, LEFT). The study compared the effects of atmospheric nitrogen deposition with two other sources of nitrogen: the nitrogen carried by rivers, and the nitrogen that flows from the continental shelf into the Bay. All three nitrogen sources can affect hypoxia by influencing how much algae is produced and subsequently decomposed, with decomposition being responsible for consuming the oxygen dissolved in the water. The comparison between the three nitrogen sources reveal that atmospheric deposition has about the same gram for gram impact on Chesapeake Bay hypoxia as the riverine source. The impact of deposition on hypoxia is largest in the shallow near-shore regions, riverine nitrogen has a dominant impact in the largest tributaries and the middle Bay, while the nitrogen originating from the continental shelf is most influential near the mouth of the Bay (FIGURE-3, RIGHT). Broader impacts: The results from the award were disseminated in two scientific articles with one published in 2017 and the other one currently under review for Journal of Geophysical Research. The results from the numerical simulations are also permanently archived at a public repository called W&M Publish. The collaborative project supported and trained graduate students at VIMS (Fei Da, who successfully defended his M.Sc.) and at Old Dominion University (Christine Sookhdeo, also M.Sc.). Finally, the project results were incorporated into class lectures for graduate courses on marine policy and marine biogeochemistry at Penn State University. Last Modified: 04/05/2018 Submitted by: Marjorie A Friedrichs