Invertebrate larval availability during summer upwelling and downwelling on the inner continental shelf off New Jersey

This study examined the effects of wind-driven, cross-shelf circulation on invertebrate larval supply to benthic habitats on the inner continental shelf off Tuckerton, New Jersey, USA. Study sites were located along an inshore/offshore transect in the LEO-15 research area at Beach Haven Ridge, a 12-20 m deep, shore-oblique sand ridge. Meroplankton was sampled using Moored, Automated, Serial Zooplankton Pumps which were programmed to take 250-1 samples, similar to1 m above the bottom, every 4 h. Upwelling and downwelling conditions were characterized using wind data from the nearby meteorological tower at the Rutgers University Marine Field Station, and bottom temperatures from LEO-15 node data and the pump sites. Studies were carried out during July in 1996-1998 because previous recruitment studies showed that July was when larval surfclams, Spisula solidissima, were abundant and their settlement maximal. The study was repeated over three years to sample multiple upwelling/downwelling sequences. Although the study focused primarily on the dominant S. solidissima larvae, other meroplankton were also enumerated. Short-lived pulses (12 h or less) of highest larval surfclam concentrations (up to 3000 sample(-1), sample volume = 250 l) often coincided with the initial arrival of warm water at the bottom at the initiation of downwelling, indicating a patchy larval distribution that is probably related to high larval concentrations near fronts. On one date in July 1998, very high concentrations of surfclam and pholad bivalve larvae coincided with the arrival of warm, relatively low salinity water. This pattern suggested a larval concentrating mechanism near an estuarine front, perhaps from the Hudson River plume. Sampling during multiple upwelling/downwelling events over the three summers revealed a close correlation between peaks in larval concentration of bivalves and gastropods coinciding with the arrival of downwelled warm water at the bottom at inner shelf sites.