Tidal Circulation and Residence Time in a Macrotidal Estuary: Cobscook Bay, Maine

Abstract

Cobscook Bay is a macrotidal estuary situated near the entrance to the Bay of Fundy, where the mean semi-diurnal tidal range is 5·7m. Vigorous tidal currents in the bay maintain cold water temperatures year-round and effectively exchange nutrients and other dissolved matter with offshore waters. Partly because of the cold water and tidal exchange, a net-pen salmon aquaculture industry has rapidly expanded in the last decade, raising questions about sustainable levels of production in the environmentally pristine bay. The present study addresses the question of dispersion and flushing of materials in the bay, using a three-dimensional numerical model to simulate the circulation driven by the semi-diurnal tide and runoff from principal rivers. 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 time 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 the central bay during each flooding tide. The eddy pattern preferentially directs the initial flood into a southern arm of the bay, where particles and dissolved materials are sequestered. The effective horizontal mixing coefficient in the main channel of the central bay is 300–400m2s−1, leading to rapid dispersal of particles and pollutants in the along-channel direction and into the shallow inner arms of the bay where they tend to accumulate. A map of tidal-mean residence time indicates that most current aquaculture sites are located in reasonably well flushed regions.