Depletion of plankton in a raft culture of Mytilus galloprovincialis in Ría de Vigo, NW Spain. II. Zooplankton

Abstract

Depletion of chl a has previously been observed within cultivation rafts of mussels, whereas the effect of mussel filtration on the zooplankton community is unknown. Therefore we investigated the distributions of both chl a and zooplankton across a cultivation raft of blue mussel Mytilus galloprovincialis during summer in the Ría de Vigo, NW Spain. Our hypothesis was that zoo-plankton is depleted in the immediate region of the raft, but that the degree of depletion varies between prey types and therefore differs from that of chl a. We observed a significant average depletion of 57% for chl a and of 26 to 77% for different zooplankton groups at the lower edge and down-stream of the raft. At the lowest current speeds, removal rates of protozooplankton, nauplii and copepodites were lower than for chl a, indicating that zooplankton escaped filtration. In contrast, removal rates of zooplankton >150 μm increased with current speed and exceeded those of chl a at the highest current speeds. This was due to additional filtration by passive filter feeders such as barnacles and hydroids. The present study demonstrates the effect of the raft epifaunal community on the structure of the zooplankton community in the upper productive layer, and stresses the importance of including heterotrophic plankton in estimates of potential bivalve production. © Inter-Research 2008.