FOOD LIMITATION IN COCKLES (CERASTODERMA-EDULE (L)) - INFLUENCES OF LOCATION ON TIDAL FLAT AND OF NEARBY PRESENCE OF MUSSEL BEDS

Dilution of phytoplankton concentrations by suspension-feeding organis ms, and local reduction in current velocity, may lower food supply for other suspension feeders, and consequently affect their growth. Influ ences of food supply on stomach contents and growth of the suspension- feeding bivalve Cerastoderma edule were studied in two situations: (a) inside and outside an experimental mussel bed in a basin with a natur al seawater supply, and (b) along tidal creeks in the field. Cockles s howed a lower increase in shell length, body weight and condition insi de the experimental mussel bed than outside. In the field, phytoplankt on concentrations in the water were lower at upper than at lower tidal -creek sites. At these upper creek sites, cockles were smaller, as wer e their growth rates and stomach contents, compared to the lower creek sites. Largest reductions of phytoplankton concentration in the water were observed at sites near a mussel bed. It is concluded that phytop lankton uptake by suspension feeders (particularly in mussel beds), as well as relatively low current velocities, negatively affects food su pply, stomach contents and growth of C. edule.