In August, larval settlement to the nearshore adult population of Spisula solidissima (i.e. adults within 5 km of shore) probably occurred during upwelling as subthermocline waters with their entrained larvae were pulled shoreward. Ma (2001) studied the distribution and settlement of S. solidissima larvae at a more northerly site (still in the Mid-Atlantic Bight) and earlier in summer (July). He found larvae distributed above the thermocline and, at this time of year, water temperature above the thermocline were similar to those found below the thermocline in our August measurements; larvae of this species may have a preferred temperature range as has been observed in laboratory behavior studies of other bivalves (Hidu & Haskin 1978, Mann & Wolf 1983). In Ma’s work, nearshore settlement of S. solidissima larvae occurred during downwelling events when water from above the thermocline was pulled downward and contacted the bottom. This observation suggests that larvae were, as the hypothesis predicts, transported shoreward by
downwelling and seaward by upwelling. Consistent with the hypothesis, S. solidissima larvae appear to behave like passive particles in flow, but, depending on their position in the water column, their cross-shelf transport is either consistent with the hypothesis— larvae above the thermocline are swept offshore in upwelling and shoreward in downwelling—or exactly opposite when larvae are found below the thermocline. One cannot predict the effect upwelling and
downwelling will have on the distribution of larvae without knowing the vertical distribution of the larvae in the water column.