AN ASSESSMENT OF THE ANNUAL MASS-BALANCE OF CARBON, NITROGEN, AND PHOSPHORUS IN NARRAGANSETT-BAY

Narragansett Bay is a relatively well-mixed, high salinity coastal emb ayment and estuary complex in southern New England (USA). Much of the shoreline is urban and the watershed is densely developed. We have com bined our data on C, N, and P inputs to this system, on C, N, and P ac cumulation in the sediments, and on denitrification with extensive wor k by others to develop approximate annual mass balances for these elem ents. The results show that primary production within the bay is the m ajor source of organic carbon (4 times greater than other sources), th at land drainage and upstream sewage and fertilizer are the major sour ces of N, and that landward flowing bottom water from offshore may be a major source of dissolved inorganic phosphorus. Most of the nutrient s entering the bay arrive in dissolved inorganic form, though DON is a significant component of the N carried by the rivers. About 40% of th e DIN in the rivers is in the form of ammonia. Sedimentation rates are low in most of Narragansett Bay, and it appears that less than 20% of the total annual input of each of these elements is retained within t he system. A very small amount of C, N, and P is removed in fisheries landings, denitrification in the sediments removes perhaps 10-25% of t he N input, and most of the carbon fixed in the system is respired wit hin it. Stoichiometric calculations suggest that some 10-20% of the or ganic matter formed in the bay is exported to offshore and that Narrag ansett Bay is an autotrophic system. Most of the N and P that enters t he bay is, however, exported to offshore waters in dissolved inorganic form. This assessment of the overall biogeochemical behavior of C, N, and P in the bay is consistent with more rigorously constrained mass balances obtained using large living models or mesocosms of the bay at the Marine Ecosystem Research Laboratory (MERL).