Is the activity of benthic suspension feeders a factor controlling water quality in the Bay of Brest?

In the Bay of Brest, France, suspension feeders are broadly dominant in the trophic structure of macrobenthic assemblages in terms of species number, number of individuals and biomass. A succession of species exists on an east-west gradient, which is an ecological continuum from a < 10 m depth polyhalme muddy environment to a > 20 m depth marine environment with coarse and clean sedirnents. The abundance of biogenic substrates (shells, calcareous algae, etc.) on the sediment surface favours a high development of sessile epifauna. Some species, such as Crepicula fornicata, Ascidiella aspersa, Ficulina ficus, Ophiothrix fragilis and Ophiocomina nigra, can locally proliferate, reaching 100 g ODW (organic dry wt) m-2. Fertilization of the bay by streams loaded with nutrients induces a very high primary production (280 g C m-' yr-l) characterized by high annual and seasonal variability of phytoplanktonic blooms. However, so far, eutrophication has not been observed. While water renewal by tidal currents is one important factor in the control of eutrophication, suspension feeder activity is undoubtedly another effective control factor. Benthic suspension feeders can filter 7.18 X 10' m3 daily, which represents ca 30% of the total volume of the bay. The phytoplankton production is easily accessible to the benthos because of the relatively long water residence time and the high tidal current mixing. Spatial distribution of populations was found to be related to the tidal current distribution.