Atmospheric nitrogen deposition (AND) influences marine biogeochemical processes by providing reactive (fixed) nitrogen species that enhance primary production in nitrogen-limited regions. Although it is generally assumed that AND in precipitation contributes substantially to anthropogenic nitrogen loading in many coastal marine systems, the biological impact of such AND remains poorly understood.
Our interdisciplinary team will carry out a tightly linked program of process-oriented field measurements and biogeochemical modeling in oligotrophic coastal waters of the eastern U.S.—a region that currently receives high levels of AND—to test the following hypothesis:
Wet AND events stimulate primary productivity and accumulation of algal biomass in coastal waters following summer storms; this effect exceeds the associated biogeochemical responses to wind-induced mixing and increased stratification caused by surface freshening.
Using a summer Lagrangian field experiment in coastal waters of the eastern U.S. between the Delaware Bay and the coastal Carolinas, we will monitor the response of surface-layer biogeochemistry and biology to precipitation events, which will be identified and intercepted using radar and satellite data. During this cruise, we will: (1) estimate wet and dry deposition of a variety of reactive nitrogen species; (2) document the evolution of hydrography, primary productivity, and nitrogen biogeochemistry (including in situ nM concentration and delta 15N of nitrate) in coastal surface waters before, during, and after rainfall events; and (3) conduct shipboard rainwater- and nutrient-addition incubation experiments. Postcruise, we will calibrate a 1-D dynamical and biogeochemical upper-ocean model of the Lagrangian study domain by assimilating the field data using the adjoint method. Sensitivity studies with the calibrated model, in combination with the in situ data and the results from the shipboard experiments, will allow us to test our main hypothesis.
To provide regional and historical context for the field measurements and the associated 1-D modeling, linked regional atmospheric-oceanic biogeochemical modeling will be conducted. Using our cruise measurements, data from a land-based measurement program in coastal Virginia, and historical data from the National Atmospheric Deposition Program database, we will evaluate nitrogen deposition output from an atmospheric chemistry transport model. Calibrated output from the atmospheric model will be used as input to a 3-D coastal ocean biogeochemical model in order to assess regional-scale impacts of AND in waters that are qualitatively similar to those sampled during the cruise.
Intellectual merit: This will be the first comprehensive study to assess biogeochemical and biological impacts of AND on a coastal ocean system. Although previous studies have provided estimates of AND to coastal systems, there have been no targeted studies of the effects of AND on coastal zones. Further, prior modeling efforts of AND impacts used coarse resolution, and thus were unable to realistically capture the response of coastal waters, which are typically characterized by strong gradients and lateral heterogeneity.
Broader impacts: While nutrient management is a major thrust of coastal restoration, non-point sources, such as AND, are poorly quantified. The results from the proposed study will improve our understanding of the impacts of AND on biogeochemical processes that sustain coastal ecosystems. The proposed research will support training of three graduate students and one postdoctoral researcher, and the data from this project will be used in graduate courses on marine policy and marine biogeochemistry.