Macrobenthic metabolism as carbon and nitrogen fluxes in a coastal area exposed to strong tidal currents (Dover Strait, eastern English Channel)

Abstract

In view of recent studies of a Dover Strait macrobenthic community (eastern 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 (phytoplanktonic production= 336 g C m⁻² y⁻¹). In this area typical of coarse sediments which are exposed to strong tidal currents in the English Channel, the macrobenthic compartment is suspected to be responsible for the major part of exchanges at the bottom-boundary layer. The macrobenthic biomass is high (mean annual biomass = 281 g afdw m⁻²) and dominated by three species (98.5%): the ophiurid Ophiothrix fragilis and the two cnidarians Urticina felina and Alcyonium digitatum. Carbon and nitrogen budgets including ingestion, production, respiration, excretion and egestion allow the estimation of annual fluxes from the water column 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 column (Respiration + Egestion = 710 g C m⁻² y⁻¹ and Excretion + Egestion =100 g N m⁻² y⁻¹). The net metabolism, as the difference between primary production and consumption of organic matter, shows that this coastal ecosystem is heterotrophic. Its functioning is based on allochtonous material and advection and it appears to be an important source of carbon dioxide.