Biomass changes and bentho-pelagic transfers throughout the benthic boundary layer in the English Channel

Abstract

One hundred and eight suprabenthic hauls were taken from six sites in the English Channel with a modified Macer-GIROQ sledge that permitted the sampling of three kinds of organisms in the benthic boundary layer: mesozooplankton, macrozooplankton and suprabenthos. A complete annual cycle was sampled in the Bay of Saint-Brieuc but only spring and autumn samplings were taken from the other sites. Meso- and macrozooplankton biomasses were usually higher at every site during the daytime than at night; in contrast, suprabenthic biomasses were lower during the day than at night. However, at site 5, on pebble substrates, every faunistic group showed a higher biomass during the day than at night, while at site 2 the opposite occurred. Meso- and macrozooplankton biomasses were at their maximum during spring whereas the highest biomass of suprabenthos was observed from summer to autumn. Daytime exchanges were by mesozooplanktonic organisms and night-time exchanges were by suprabenthic species, especially amphipods, mysids and large decapods. Daily transfers showed the same pattern for every faunistic group, and transfers were higher in autumn than in spring, except at site 1 where it was similar during both seasons. Three groups of sites were identified from their annual transfers: sites 1 and 3, on the Brittany coasts, with lowest annual transfers; site 2, offshore Plymouth, with the highest transfer, and the three eastern sites (4, 5 and 6) showing similar annual transfers. The rate between macrobenthic production and annual transfers was high at coarse sand and pebble sites where the benthic macrofauna was endobenthic and sessile, suggesting a concentration of carbon in the bottom. On the other hand, this rate was low on medium sand substrates where the benthic macrofauna was vagile, suggesting that carbon remained concentrated in the benthic boundary layer where exchanges were most important by the migration of both pelagic and benthic organisms in this compartment.