Nitrogen flow and the interaction of Boston Harbor with Massachusetts Bay

This paper summarizes evidence that most of the considerable nitrogen loading (~8, 470 mmol total N m^-2 yr^-1) to Boston Harbor (Massachusetts, USA) is expelled to shallow shelf waters of Massachusetts Bay, where it strongly influences ecological dynamics. Examination of nitrogen concentrations in the harbor, compared with loading, indicated that removal processes are active in the harbor. Comparison to other estuarine systems showed that the harbor's nitrogen concentrations are consistent with its loading, if they are corrected for tidal flushing effects on the water residence time. Furthermore, extensive measurements of sediment denitrification confirmed that rates of N₂ gas loss are high in an absolute sense (~600-800 mmol N m^-2 yr^-1) but nonetheless remove only a small portion (<10%) of the annual land-derived nitrogen loading. Burial in sediments apparently removes only about 2% of the N input, implying export to offshore environments as the major removal process (~88-90% of N input). Western Massachusetts Bay receiving waters were examined for a signature of export from the harbor. Data consistently show a gradient of decreasing nitrogen concentrations from the harbor to about 10-20 km into the bay. In many cases, plots of nitrogen concentrations versus salinity show nearly conservative mixing character, which implies virtual export. Seasonally, the data suggest most of the export from the harbor in winter is as dissolved inorganic forms (NH₄⁺, NO₃^-, NO₂^-). In summer, export is dominated by the outflow of organic nitrogen forms. Chlorophyll export is evident as well, suggesting that the nutritional coupling of the harbor and hay in summer involves organic fertilization of the bay's surface water. Finally, high-resolution studies over different stages of the tidal cycle help refine understanding of the advection of chlorophyll and stimulation of in situ chlorophyll growth at the seaward edge of the tidal excursion into the bay.