Planktonic gradients along a Mediterranean sea cave

The existence of gradients in the composition of planktonic comnmunities through a 50 m long submarine cave of the Medes Islands (NW Mediterranean) has been verified. In spite of an active water exchange (previously evaluated turnover ranges between 12 and 24 hours), strong gradients, persisting throughout all year, were found in both phyto- and zooplanktonic fractions. Numbers of both individuals and taxa of diatoms, dinotlagellates, copepods, and meroplanktonic stages of benthic invertebrates decrease steeply from the entrance of the cave to the inner parts, thus producing an almost total absence of plankton at the end of the cave. On the other hand, benthic diatoms, fungi and spirorbida - all benthic groups accidentally sampled - neither decrease nor even increase to the inner stations, which suggests a tolerance to the "cave effect". These results are so clear-cut that by themselves they can explain the puzzling decay of suspension-feeders among the benthic fauna from the inner parts of submarine caves in two ways : i) scarcity of benthic larvae for settlement, and ii) scarcity of phyto- and zooplankton for food.