Topographically generated fronts, very nearshore oceanography and the distribution of larval invertebrates and holoplankters

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Abstract
Foam lines oriented parallel to shore are common features of rocky shores. At times, the water coloration is different on either side of the foam lines, suggesting they are associated with fronts. We investigated the effect of shore-parallel foam lines and associated fronts on distributions of holo- and meroplankton. We performed CTD transects to describe the fronts and carried out vertical zooplankton tows to describe the distribution of zooplankton relative to the fronts. Fronts were within tens of meters of shore and were apparently generated by the interaction of coastal currents with local topography. We sampled four sites (three coves and one open coastal site), some of which were separated by only a few hundred meters. At each site we found shore-parallel foam lines and associated thermal fronts, but the characteristics of the fronts were different at three sites, suggesting that three different mechanisms were generating the fronts. At two coves, the foam line and front appeared to be due to the interaction of wind-driven currents from the north with coastal topography. At the third cove, the front appeared to be due to the expansion of solar-heated surface waters out of the cove. The foam line and front at the open coastal site appeared to be due to boundary mixing. At the coves, the distributions of holoplankters, meroplankters and phytoplankton were clearly altered by the presence of the fronts. At the open coastal site, the front had less effect on the distribution of zooplankton. The coastal ocean is the source of new recruits to the intertidal zone and an important source of food in the form of phytoplankton for filter feeders. We hypothesize that these very nearshore fronts may play an important role in structuring intertidal communities with which they are associated.

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