MACROBENTHIC METABOLISM AS CARBON AND NITROGEN FLUXES IN A COASTAL AREA EXPOSED TO STRONG TIDAL CURRENTS (DOVER STRAIT, EASTERN ENGLISH-CHANNEL)

In view of recent studies of a Dover Strait macrobenthic community (ea stern English Channel), an annual budget of carbon and nitrogen fluxes is proposed and discussed in order to understand the contribution of macrobenthic metabolism in this productive coastal area (phytoplankton ic production = 336 g C m(-2) y(-1)). In this area typical of coarse s ediments which are exposed to strong tidal currents in the English Cha nnel, the macrobenthic compartment is suspected to be responsible for the major part of exchanges at the bottom-boundary layer. The macroben thic biomass is high (mean annual biomass = 281 g afdw m(-2)) and domi nated by three species (98.5%): the ophiurid Ophiothrix fragilis and t he two cnidarians Urticina felina and Alcyonium digitatum. Carbon and nitrogen budgets including ingestion, production, respiration, excreti on and egestion allow the estimation of annual fluxes from the water c olumn to the benthic compartment (Ingestion = 820 g C m(-2) y(-1) and 130 g N m(-2) y(-1)) and from the benthic compartment to the water col umn (Respiration + Egestion = 710 g C m(-2) y(-1) and Excretion + Eges tion = 100 g N m(-2) y(-1)). The net metabolism, as the difference bet ween primary production and consumption of organic matter, shows that this coastal ecosystem is heterotrophic. Its functioning is based on a llochtonous material and advection and it appears to be an important s ource of carbon dioxide.