Abstract
Anthropogenic nitrogen loading to Long Island Sound (LIS) is the primary cause of its eutrophication and occurrence of summertime hypoxia. Two-layered estuarine circulation in the LIS estuary helps promote this eutrophication which in general is disruptive to coastal marine ecosystems. Regulatory efforts have been made by government and private organizations to reduce nitrogen loading but the role dissolved organic nitrogen (DON), which represents on average ~65% of total dissolved nitrogen in LIS, has been largely ignored until now. This is despite recent studies showed that 12% to 72% of the DON could be bioavailable and its direct or indirect utilization by phytoplankton could contribute to eutrophication. This study focuses not only temporal and spatial variation in DON concentration in LIS but also its stable isotopic composition, which offers a natural means to directly follow and trace DON biogeochemical cycling.In 2015 to 2016, samples were collected monthly from 17 stations in LIS and analyzed for NO₃⁻, NH₄⁺ and TDN concentration and δ¹⁵N when concentrations were sufficiently large. DON concentration and δ¹⁵N-DON were calculated from mass balance. We observed DON consumption during August September in the western and central basin, where the anthropogenic nitrogen loading is the highest. Temporal analysis, spatial analysis and estuarine mixing plots indicated removal of DON from the water column. Removal and addition was observed significantly during the spring season. This addition of DON could be the labile DON from phytoplankton cell exudates. In addition, DON removal observed during the same period could be the rapid utilization of that labile DON by phytoplankton or microbial community. Our study revealed that probably DON is being utilized directly or it is being utilized indirectly after re-mineralizing into an inorganic nitrogen source (NO₃⁻ or NH₄⁺). This study suggests that regulating DON in LIS is as important as regulating nitrate and ammonia for reduction of anthropogenic nitrogen loading.