Drifting benthos and long-term research: Why community monitoring must cover a wide spatial scale

Abstract

Development of many macrobenthic animals includes a planktonic larva which is often regarded as a classical means of dispersal. Nevertheless, many juvenile and adult benthic specimens have also been found drifting in the water column. They either actively left the sediment or were passively lifted off the sediment by currents or wave action. In the Sylt-Romo Wadden Sea, traps sunk into the sediment were used to quantify drifting specimens as they re-entered the sediment. The goodness-of-fit between the number of drifting specimens and meteorological data achieved by iterative modelling was used as an indicator of the specific susceptibility to passive erosion. In bivalves this estimate of susceptibility to passive resuspension was independent from the species-specific activity in byssus-drifting. Obviously both modes of water column entry are independent from each other.Scoloplos armiger andArenicola marina (both <10 mm length) were exceptionally susceptible to sediment disturbance, but their susceptibility rapidly decreased as the specimens grew larger. In mud snails(Hydrobia ulvae), on the other hand, there was no indication for passive resuspension occurring at all. Thus, since species widely differ in their susceptibility to resuspension, any event of sediment disturbance may profoundly change community composition. This is exemplified in a shallow water offshore community. As a consequence, a sampling grid covering a few km2 turned out to be unable to separate long-term temporal changes from spatial variability.