Importance to Mussels of the Benthic Boundary Layer

We examined whether populations of the blue mussel Mytilus edulis (Linnaeus, 1758) and horse mussel Modiolus modiolus (Linnaeus, 1758) could, by their feeding activities, deplete the sestonic food supply. The experiments were conducted in a 5-m-long by 50-cm-wide flume in which a turbulent boundary layer flow was created. The seston concentration was measured both as ATP content and numbers of bacteria by acridine orange staining technique on separate subsamples of seawater. Sampling locations were at the flume inlet and at a point 3.6 m downstream (outlet). Four of the five replicated experiments with blue mussels had significant ATP–seston depletion but no reduction in bacterial numbers at the outlet sampling location. Of three replicated experiments with horse mussels, two had significant ATP–seston depletion and all three had significantly fewer bacterial numbers at the outlet sampling location. We consider that these results provide direct evidence in support of the hypothesis that seawater hydrodynamic factors may be critical in determining the food supply available to suspension-feeding animals. Our results confirm circumstantial evidence on this point obtained from field observations by previous investigators. Experimental verification of seston depletion is a necessary, but not sufficient, step in establishing that suspension-feeding animals may be food limited as required by the trophic group mutual exclusion hypothesis.