Abstract

The French continental shelf of the Bay of Biscay is occupied by a large body of silty sediments called “la Grande Vasière”. In the northern part of the Bay of Biscay, the living benthic foraminifera, of the 63–150 μm and >150 μm size fractions, inhabiting this outer shelf silt belt have been studied at four stations (100–130 m deep), together with some environmental parameters (grain size, carbon content, pore water oxygen and nitrate + nitrite content). The faunas of “La Grande Vasière” are characterized by a high foraminiferal density in the 63–150 μm size fraction, which is often dominated by Nonionella iridea. Although the ecology of this species is not well known, its appearance seems to be related to pulsed organic supplies. In our study area, this species is dominant at three stations, but almost absent at a fourth one, where the organic carbon content is minimal. The foraminiferal microhabitat is, for most taxa, restricted to the top four centimeters: only Stainforthia fusiformis has a distinctly deeper microhabitat (mean average living depth = 4.5 cm). Although free oxygen is limited to the uppermost centimeter, a major part of the foraminifera occurs between 1 and 4 cm deep, in an interval with maximum pore water nitrate + nitrite values. The nitrate + nitrite profiles allow us to distinguish between stations A and B, both at a depth of about 100 m. In the days prior to sampling, the organic matter flux seems to have been less intensive at A and B than at stations C and D (depth of 130 m), where the nitrate + nitrite maxima are positioned just below the sediment-water interface. The distinction between these two pairs of stations is confirmed by the faunal composition, with much lower densities of supposedly opportunistic taxa (such as N. iridea) at stations A and B, and by the faunal penetration depth, with much deeper penetration at stations C and D. Our results suggest a strong impact of seasonal outer-shelf phytoplankton bloom events on the composition and density of the benthic foraminiferal faunas.

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