A Numerical Model Analysis of the Mean and Seasonal Nitrogen Budget on the Northeast U.S. Shelf

Abstract

The supply of nitrogen is a primary limiting factor for the productivity of the Northeast United States (NEUS) continental shelf. In this study, a 12‐year (1996–2007) retrospective physical‐biogeochemical simulation over the Northwest Atlantic was used to analyze the mean and seasonal NEUS shelf nitrogen budget, including the connections between shelf subregions: the Gulf of Maine/Georges Bank (GoM/GB), and the Mid‐Atlantic Bight (MAB). The model captures the primary mean and seasonal patterns of shelf circulation, nitrate, and plankton dynamics. Results confirm aspects of previous nitrogen budget analyses, including the dominance of offshore nitrogen influxes into the GoM/GB and the prominent role of riverine influxes and sedimentary denitrification in the MAB. However, detailed spatiotemporal analysis of nitrogen fluxes highlights the importance of dispersed inflows of shallow to intermediate depth waters (0–75 m), which can at times exceed the deep nitrogen influx emphasized in previous studies. A seasonal analysis shows a pronounced shift from the net import of nitrogen to the GoM/GB region during late fall and winter, to the net export of nitrogen from the region in the spring and early summer. The MAB, in contrast, consistently exports nitrogen to offshore waters. The prominence of the 0‐75m nitrogen supply has implications for the roles of Labrador Slope Water and Atlantic Temperate Slope Water on the NEUS ecosystems, as Atlantic Temperate Slope Water has greater nitrate concentrations than Labrador Slope Water at depth but often less at the surface. Results suggest the need for further study of shallow to intermediate depth inflows beyond those from the Scotian Shelf, particularly during the fall/winter of net nitrogen inflow.

Winter surface salinity in the northeastern Gulf of Maine from five years of SMAP satellite data

“…Positively correlated T and S shelf anomalies (e.g., Loder et al, 2001;Flagg et al, 2006;Grodsky et al, 2017) are density compensated, which allows for salty anomalies to stay afloat and be detectable from space. Upper ocean anomaly trapping (Figure 4c) is corroborated by model analysis of GoM nitrogen flux (Zhang et al, 2019) highlighting the importance of shallow to intermediate depth inflow (m), at times exceeding the deep inflow (see also, Brickman et al, 2018). Similar to MSLP and wind anomaly composites (Figures 12a,b), SST composites are not symmetrical for salty and fresh GoM events.…”

Remote silicate supply regulates spring phytoplankton bloom magnitude in the Gulf of Maine

“…The decrease of bottom nitrate concentration from the NEC to the central GoM indicates that denitrification is important in nitrate removal (Switzer et al 2020). Quantitative analysis based on a 3D biogeochemical model suggests that denitrification removes $ 14 Gmols yr À1 (Zhang et al 2019). Unlike the depletion of nitrate in the deep water, silicate at depth tends to accumulate from the NEC to the GoM (Christensen et al 1996;Switzer et al 2020).…”

“…Our results also suggest the need to consider adding silicate as a limiting nutrient for the regional ecosystem simulations. Many Nutrient-Phytoplankton-Zooplankton-Detritus models have a sole limiting nutrient (mostly nitrogen) for simulating phytoplankton dynamics (Fennel et al 2006;Song et al 2011;Zhang et al 2019). Such a simplification, however, should be well justified based on the phytoplankton community structure and the objectives of simulations.…”

The importance of oxygen for explaining rapid shifts in a marine fish

“…Oxygen concentration, zooplankton density, and salinity values were obtained from a biogeochemical oceanographic hindcast model for the period 1982–2010 (Kang & Curchitser, 2013; Zhang et al., 2019). This ROMS‐COBALT (Regional Oceanic Modeling System‐Carbon, Ocean, Biogeochemistry and Lower Trophics) model covers most of the northwest Atlantic and has a grid configuration of 7 km × 7 km and 40 vertical layers from the sea surface to the sea bottom.…”