Effects of experimentally induced raised levels of organic flux and oxygen depletion on a continental slope benthic foraminiferal community

A laboratory experiment was carried out with 10 mesocosms containing sediment from a 550 m deep station in the Bay of Biscay. Station B is well-oxygenated throughout the year and material for this study was collected just after the spring bloom in May 2000. The aim of the experiment was to assess the separate effect of the principal environmental parameters, oxygen concentration and organic flux, on the benthic foraminiferal assemblage. Oxygen appears to induce the strongest changes, especially on the vertical distribution of the foraminifera. When subjected to anoxic conditions most species, except some intermediate to deep infaunal taxa (Melonis barleeanus, Globobulimina spp.), migrate towards the sediment water interface, apparently trying to escape the hostile conditions. Adding organic matter affected only some shallow living, opportunistic taxa (Epistominella exigua, Adercotryma glomerata). All other species display no significant response to enhanced food conditions, under either oxic or anoxic conditions. The outcome of our experiment suggests that the assemblages of this mesotrophic environment are influenced more strongly by variation in oxygen levels than by changes in organic flux on a short time scale (days to weeks). On a longer time scale organic flux is important in regulating abundances.