Filtration and absorption of Phaeocystis cf globosa by the mussel Mytilus edulis L

Mussels, Mytilus edulis L., were exposed to natural seawater, supplemented with variable amounts of Phaeocystis cf. globosa solitary cells and colonies and to a reference diet consisting of natural seawater, supplemented with Phaeodactylum tricornutum. Clearance rate showed a linear decrease with the amount of colonies present, expressed as total 'circular equivalent surface area'. Extension of the exposure period for 28 h resulted in a further decreased clearance rate. Recovery of the clearance rate began immediately after exposure to untreated natural seawater. In the absence of colonies, there was a moderate clearance rate reduction. Filtration at reduced clearance rates was not compensated by increased particle concentrations and no adaptative response to the diet was observed. In an earlier experiment, we observed that the absorption efficiency of ingested Phaeocystis diet was not significantly different from the reference diet. It is concluded that the adverse effects of Phaeocystis blooms are mainly determined by the total colony surface area. There is no adaptation over time to diets with colonies. Once ingested, Phaeocystis is absorbed normally and has significance as food. Copyright (C) 1997 Elsevier Science B.V.