AN INSITU ESTIMATION OF THE EFFECT OF WIND-DRIVEN RESUSPENSION ON THE GROWTH OF THE MUSSEL MYTILUS-EDULIS-L

Previous studies have suggested that the effect of near-bottom seston depletion on individual growth in dense beds of suspension-feeding mussels Mytilus edulis L. may be alleviated by particle enhancement caused by resuspension. As a test of the relative importance of depletion and resuspension, mussels were grown at two intertidal sites, 0 and 1 m off bottom (St Lawrence Estuary, Quebec), with the same immersion time, but with differing phytoplankton concentrations and resuspended loads. Growth at both sites was measured according to two independent methods in order to distinguish between effects of resuspended particles and vertical phytoplankton gradients. The first method involved the estimation of gain in tissue weight and was resuspension-dependent since mussel growth integrated effects of wind events as well as those of calm weather. In the second method, growth was estimated from scope for growth (SFG) measurements carried out only in calm weather, to yield resuspension-free results. Growth at both sites was compared by computing the ratio of growth (0 m : 1 m) for each method, an approach not dependent on agreement of absolute growth rates between the two methods. Results indicated that the net effect of wind events and resuspension did not have a significant effect oil growth during May-July and May-November study periods, and that 0 m : 1 m growth and SFG ratios were usually < 1. This suggests that decreased mussel growth at the bottom of the water column was primarily caused by phytoplankton depletion near the mussels, a process which could not be ameliorated by resuspended food supplies.