Interannual Variability of Coastal Nutrient Fluxes Along the U.S. Eastern Continental Shelf (USECoS): Results from a Land-Ocean-Biogeochemical Modeling System

Abstract:

Understanding of nitrogen cycling on continental shelves, a critical component of global nutrient cycling, is often hampered by limited observations compared to the strong variability apparent on relatively small time and space scales. Numerical models are able to partially alleviate this issue by filling temporal and spatial data gaps and hence resolving annual area-integrated nutrient fluxes. Here a land-ocean-biogeochemical modeling system was implemented to simulate the nitrogen budget during 2004-2007 on the U.S. eastern continental shelf. Quantitative model skill assessment was conducted via extensive comparisons to satellite observations, and in situ data where available. Model simulations demonstrate that denitrification and burial fluxes have pronounced seasonal variation but change little from year to year. In contrast, annual net community production (NCP) varies significantly between years, but overall is positive indicating a net autotrophic system. Whereas the advective influx of nitrogen from the bays and rivers on the western boundary of the shelf changes only slightly among the four years examined, the advective fluxes across the eastern and northern boundaries change dramatically. These changes are associated with changes in the positions of the relatively high-nutrient Labrador Sea waters advecting into the shelf from the north, and the relatively low-nutrient Gulf Stream waters advecting into the region from the southeast. Tests varying the initial conditions (reduced spin-up) and simplifying the modeled plankton structure showed distinct impacts on these nitrogen fluxes: the former strongly affected the advective fluxes, but had little impact on denitrification, burial or NCP, whereas the latter significantly reduced denitrification, burial, and NCP but did not significantly impact the advective fluxes.