Tidal circulation and residence time in a macrotidal estuary: Cobscook Bay, Maine

Cobscook Bay is a macrotidal estuary situated near the entrance to the Bay of Fundy, where the mean semi-diurnal tidal range is 5.7 m. Vigorous tidal currents in the bay maintain cold water temperatures year-round and effecti vely exchange nutrients and other dissolved matter with offshore waters. Pa rtly because of the cold water and tidal exchange, a net-pen salmon aquacul ture industry has rapidly expanded in the last decade, raising questions ab out sustainable levels of production in the environmentally pristine bay. T he present study addresses the question of dispersion and flushing of mater ials in the bay, using a three-dimensional numerical model to simulate the circulation driven by the semi-diurnal tide and runoff from principal river s. With initialization based on May 1995 cruise data, the results show that the tidal-mean flushing times for neutral surface particles vary from less than one day in the main channel near the entrance to more than a week in the extremities of the inner arms of the bay. A bay-wide average flushing t ime is about two days, or four tidal cycles, but the detailed distribution is strongly influenced by a pair of counter-rotating eddies that forms in t he central bay during each flooding tide. The eddy pattern preferentially d irects the initial flood into a southern arm of the bay, where particles an d dissolved materials are sequestered. The effective horizontal mixing coef ficient in the main channel of the central bay is 300-400 m(2) s(-1), leadi ng to rapid dispersal of particles and pollutants in the along-channel dire ction and into the shallow inner arms of the bay where they tend to accumul ate. A map of tidal-mean residence time indicates that most current aquacul ture sites are located in reasonably well flushed regions. (C) 1999 Academi c Press.