Vertical distribution and horizontal transport of planktonic larvae of echinoderms and benthic polychaetes in an open coastal sea
A 1953 investigation of merozooplankton in Kiel Bay, which is subject to estuarine exchange between the Baltic Sea and Danish waters, is reviewed. Larvae of 18 polychaete and 6 echinoderm species were abundant and were spatially and temporally distributed as if they had behaved similarly to passive, neutrally buoyant bodies. On three diel stations larvae occurred in very pronounced layers even with weak hydrographic stratification; furthermore, their depth changed with internal waves but not with the day-night cycle. Four cruises during the summer and a station occupied fortnightly through the fall showed that the depths of general occurrence of the larvae were governed by hydrographic processes rather than active orientation. Larvae were tied to water masses, defined by the T-S relation and of known history, and were subject to vertical eddy diffusion (mixing) as well as horizontal transport. Young larvae did not rise towards the surface. Older larvae generally did not occur closer to the sea bed than younger ones, but instances of a passive deposition of metamorphosing larvae to the sea bed could be inferred. "Animal navigation" in the estuarine circulation was not observed. The results are of general application since hydrographic conditions similar to those in Kiel Bay, especially in the upper layers, are common in neritic seas.
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Document Type: Research Article
Publication date: 1986-09-01
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