Migratory responses of deep-sea benthic foraminifera to variable oxygen conditions: laboratory investigations

Field studies of recent benthic foraminifera have shown that the vertical distribution in the sediment is controlled mainly by two parameters: oxygen concentration and food availability. The aim of the present paper is to determine the response of deep-sea foraminifera to changing bottom and pore water oxygen concentrations in the laboratory, under conditions of controlled temperature and salinity. The experiment, using spectrofluorimetry to distinguish living from dead specimens, has shown that all selected species (Globobulimina affinis, Hoeglundina elegans, Pyrgo murrhina, Uvigerina peregrina, Uvigerina mediterranea) can live in the oxic sediment layer. However, G. affinis was also found living in the anoxic layers and even has its maximum density there. An accidentally obtained inverse profile of oxygen, with anoxic conditions on the surface and oxic conditions at the bottom, resulted in an inverse distribution of foraminifera, with “shallow infaunal” species (H. elegans, U. peregrina and U. mediterranea) at the bottom and the “deep infaunal” species G. affinis at the sediment surface. Our results show that some deep-sea species can actively migrate along the sediment column to reach their favourite microhabitat. Furthermore, they suggest that changes in oxygen content are mainly responsible for those migrations. Based on a comparison between our experimental and field results, the ecological requirements of some selected species have been refined.