Nitrogen sources and fluxes in Western Long Island Sound

Western Long Island Sound (WLIS) continues to experience seasonal hypoxia, despite efforts to reduce nitrogen loading. Understanding of the drivers of hypoxia is complicated by the diversity of nitrogen sources, including local and remote rivers, wastewater treatment plants, the coastal ocean, and the atmosphere. New observations of nitrogen concentrations and water currents provide vital information on nitrogen fluxes through these impacted estuarine waters. These measurements resolve tidal variability and were collected during spring, summer, and winter field surveys along a transect crossing the WLIS and a station at the East River tidal strait, which connects WLIS to New York City’s large nitrogen sources. Results show high nutrient concentrations entering from the East River are quickly attenuated before reaching the WLIS boundary. Mass balances suggest that much of the nitrogen passes through the WLIS, however, the composition shifts among nitrogen species (ammonium, nitrate, nitrite, and organic nitrogen). Fluxes indicate there is strong exchange of nitrogen both into and out of the WLIS through its boundaries. Taken together, results suggest rapid biological utilization of nitrate and shifts in nitrogen species can account for WLIS hypoxia during highly stratified periods. A hydrodynamic model with passive tracers is applied to help link nutrient sources to nutrient levels in the impacted WLIS.