Journal of Plankton Research Vol.25 no.6 pp.645-667, 2003
© Oxford University Press 2003
Observations on the distribution of meroplankton during an upwelling event
Oregon Institute Of Marine Biology, University Of Oregon, Po Box 5389, Charleston, Or 97420, 1 Scripps Institution Of Oceanography, University Of California, San Diego, Ca 92093-0209 And 2 Virginia Institute Of Marine Science, School Of Marine Science, College Of William And Mary, Gloucester Point, Va 23062, Usa
* Corresponding Author: ashanks{at}oimB.UoregoN.Edu
The distribution of the larvae of benthic invertebrates was investigated relative to hydrographic structures as a test of the hypothesis that larvae behave as if they are passive particles. Observations of larval and oceanographic distributions were made off Duck, North Carolina, USA in August 1994. Conditions were characterized by wind-driven coastal upwelling; flow was generally offshore near the surface and onshore below the pycnocline. Within 5 km of the shore the pycnocline was bent upward by the upwelling and it intersected the surface along most of the transects. In zooplankton samples, 20 taxa of larvae were counted (10 bivalve veligers, nine gastropod veligers and one polychaete larvae). Using cluster analysis, larvae were separated into groups with similar patterns of distribution and similar affinities to water properties. The larvae in Cluster 3 did not display a consistent distribution pattern beyond that they tended to be found in warmer surface waters. An earlier paper described the distribution of larvae in the same location during a downwelling event [A. Shanks et al. (2002) J. Plankton Res., 24, 391–416]. Two of the clusters identified during this previous study were quite similar in composition to Clusters 1 and 2 in this study. In both studies, Cluster 1 larvae were found below the pycnocline, but during the upwelling event they were transported shoreward with the advection of the subpycnocline waters by the upwelling circulation. Within 5 km of the shore, Cluster 1 larvae were found at depths shallower than the base of the pycnocline and were often found in patches of high larval concentration. The patches were located where the waters were upwelling. Cluster 2 larvae were found within 5 km of the shore in both studies and tended to be highly concentrated in convergences or divergences. Larvae in Cluster 1 generally appeared to be dispersing as passive particles, except within the zone of upwelling where they may have been swimming against the upwelling flow leading to higher larval concentrations. Cluster 2 larvae appeared to be consistently concentrated in areas of vertical currents, suggesting that they may be attempting to maintain a preferred depth in the face of the vertical flow which would lead to high larval concentration and nearshore larval distributions despite extensive cross-shelf movement of water. Despite their slow swimming speeds, the larvae in Clusters 1 and 2 were not swept offshore by the upwelling event.
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