Physical mechanisms and temporal and spatial patchiness in bivalve larval supply and settlement on the inner shelf of the mid-Atlantic bight

To test the relationship between wind-driven cross-shelf circulation and larval transport/settlement during the summer upwelling season, bivalve (especially surfclam, Spisula solidissima) larval concentrations and larval settlement were examined in July 1997 and 1998 at Long-term Ecosystem Observatory (LEO-15) sites off Tuckerton, New Jersey. Near-bottom larval concentrations measured every 4 hours at two inshore stations at 12-m depth and one offshore station at 20-m depth showed that pulses of highest larval surfclam concentrations coincided with the arrival of warm water at the bottom sampling stations due to downwelling.; The spatial distributions of bivalve larvae were further explored by taking six sets (in July 1998) of adaptive cross-shelf samples at different depths at three to six stations along a 25-km transect. The highest larval surfclam concentrations occurred at the bottom near Beach Haven Ridge during downwelling, and the larvae were concentrated above the thermocline 3 km further offshore during upwelling. Larval concentration during upwelling is hypothesized to be related to a convergence due to offshore Ekman transport and the vertical shear in an upwelling jet. Other larval bivalves showed different temporal and spatial distributions, which are likely to be the consequence of different larval sources and behaviors.; In four consecutive sets of settlement trays deployed at two inshore stations at 12-m depth and one offshore station at 20-m depth in July 1997, larval surfclam seulement was higher inshore than offshore, and the seulement at two stations was higher during and following downwelling. These seulement patterns were related to near-bottom larval concentrations. At one 12-m station on the inshore side of Beach Haven Ridge, surfclam settlement was not correlated with observed high larval concentrations at the initiation of downwelling, probably due to a stronger upcurrent at this station under storm conditions.