Organic flux control on bathymetric zonation of Mediterranean benthic foraminifera

Abstract

A data set of benthic foraminiferal faunas counted in 138 surface samples from the Mediterranean Sea has been used to investigate whether the bathymetrical distribution of the dominant taxa is controlled by the amount of labile organic matter transported to the sea floor. We find that most of the major taxa show a clear W to E shallowing of their upper or lower depth limit, coinciding with a W to E decrease in the surface water primary production, and in the estimated flux of the labile organic matter to the sea floor. This observation implies that the bathymetrical succession of these taxa is indeed determined by the organic flux. In the western Mediterranean, we find successions from more oligotrophic taxa at greater water depths to more eutrophic taxa in more shallow water. Towards the eastern Mediterranean, most eutrophic taxa tend to become increasingly rare, or even to disappear, whereas the more oligotrophic taxa show a clear shoaling of their depth range. Deep infaunal taxa are mainly limited to the western part of the Mediterranean. This is explained by their dependency on a relatively elevated organic flux, and by the fact that the bacterial stocks on which they feed may become unattainable when the redox front is positioned too deep in the sediment. The close similarity between the flux level controlling our main faunal boundary, and the flux levels coinciding with important faunal changes in other parts of the world ocean, suggests that a flux level of about 2–3 g labile C m −2 y −1 level corresponds to a benthic ecosystem threshold value of global importance.