Award: OCE-1260574

Human activities have greatly impacted the global cycling of elements that serve as nutrients for marine phytoplankton, single-celled organisms that are the base of the ocean?s food web. This is particularly true of nitrogen in the form of nitrate and ammonia, the supply of which regulates the productivity of phytoplankton over large areas of the surface ocean. This project examined the atmospheric deposition of nitrogen (the transfer of nitrogen from the atmosphere to the earth?s surface), which has been greatly influenced by human activities, and its potential impact on phytoplankton in ocean waters off the U.S. eastern seaboard. Specifically, the major objective of this project was to test the hypothesis that atmospheric deposition of nitrogen in the form of rain increases the amount and productivity of phytoplankton in the relatively low-productivity waters off the U.S eastern seaboard, and that this effect exceeds the effects of other processes that occur during deposition events, such as changes in the physical environment resulting from increases in winds and decreases in salinity. The hypothesis was tested using a combination of field observations, field experiments, and computer modeling activities. The field observations included: (1) sampling of rainwater and aerosols at a coastal site (over 1 year) and on a research cruise (over 3 weeks), to establish the magnitude and variability of atmospheric nutrient deposition to our study region, and (2) observations and sampling of the upper ocean during a 3 week research cruise, in an effort to identify oceanic responses to atmospheric deposition events, and to conduct experiments that simulate atmospheric nutrient inputs to native phytoplankton in the study region. These field observations have been augmented by three-dimensional atmospheric and oceanic computer model simulations that provide estimates of the impact of atmospheric nitrogen deposition on phytoplankton productivity across our oceanic study region. Although our cruise observations did not provide direct evidence for the impact of rain events on phytoplankton productivity in the water column, the results of the rain and aerosol sampling combined with the shipboard experiments using the native phytoplankton population provide clear support for our project hypothesis. Specifically, summer rain events significantly increased phytoplankton productivity in these waters by contributing nitrate and ammonia, and, to a lesser extent, phosphorus. These findings have been extended by the computer modeling efforts to estimate the impact of atmospheric nitrogen deposition on phytoplankton productivity in our study region. Specifically, we found that: (1) there is a hot spot of atmospheric nitrogen deposition over the Gulf Stream (an intense current that runs from the southwest to the northeast through the study region), which results from the high levels of precipitation in this region, (2) atmospheric nitrogen deposition increases the concentrations of surface nitrate and chlorophyll (an indicator of phytoplankton abundance) in the Gulf Stream region by 14% during the summer, and (3) as a result of the uptake of nitrate that has been deposited from the atmosphere, phytoplankton production increases by 22% in this region during summer. The third finding supports the view that atmospheric nitrogen deposition significantly enhances the transport of carbon from surface waters to deep waters, a process that helps to regulate atmospheric carbon dioxide, which is an important greenhouse gas. We also took the opportunity during the cruise to determine the feasibility of using satellites to estimate the level of nitrogen dioxide in the lower portion of the atmosphere. We were motivated to do this because nitrogen dioxide is the precursor to the nitrogen that is deposited to the ocean surface. If nitrogen dioxide can be measured accurately from space, then we might be able to estimate nitrogen deposition to the ocean from space, which would be very useful. We found that, while satellites can measure the total amount of nitrogen dioxide in an atmospheric column with reasonably good accuracy (overestimating levels by 16%, on average), they are much less successful at determining the level of nitrogen dioxide in the lower atmosphere. In summary, our research demonstrates the importance of atmospheric nitrogen deposition on coastal ocean waters and on the overall cycling of elements in the sea, include the transport of carbon from surface waters to the deep ocean. The computer modeling methods we developed are currently being used to assess the impacts on nitrogen deposition on highly polluted coastal systems, such as the Chesapeake Bay. Our research will thus likely impact managers and decision makers in the Chesapeake Bay Program and other entities that are responsible for keeping coastal waters healthy. In addition to publishing scientific papers on our research, we reported some of our research activities to the general public via a television station in Harrisburg, Pennsylvania. See: http://abc27.com/2015/05/20/hidden-talents-abc27-news-dan-tomaso/ Last Modified: 06/09/2017 Submitted by: Raymond G Najjar