Evidence for vertical circulation cells in the well-mixed area of Georges Bank and their biological implications

Two surveys were conducted in the well-mixed region of Georges Bank to look for secondary vertical circulation cells, the first in 1996 and the second in 1997. Each survey collected high-frequency acoustic, temperature, and fluorescence data along a 1-n.mile square grid. Concurrent ADCP measurements also were made in the second year. MOCNESS and pump samples from both years caught large amounts of sand and organisms typical of this regions such as copepods and hydroids. However, forward problem calculations suggest that the acoustic scattering was dominated by post-larval bivalves. Sand and copepods also accounted for significant amounts of the estimated backscatter. The acoustic data from both surveys contained near-surface vertical bands of high-volume backscatter. The frequency and intensity of these bands was strongly correlated with the magnitude of the current velocity. Significant upwelling and downwelling were observed in the ADCP records, and the acoustic bands often co-occurred in the downwelling zones. Simulations of particle distributions within idealized circulation cells, consistent with the acoustic and ADCP data, suggest that the acoustic bands are caused by aggregations of positively buoyant or upward-swimming scatterers. The circulation cells proposed could have an important effect on the ecology of the well-mixed region by aggregating upward-swimming fish and zooplankton in near-surface patches.