VERTICAL MIGRATORY RESPONSE OF BENTHIC FORAMINIFERA TO CONTROLLED OXYGEN CONCENTRATIONS IN AN EXPERIMENTAL MESOCOSM

This experimental study investigated the vertical migratory response of benthic foraminifera in sediments that were initially oxygenated, to variations in dissolved-oxygen concentrations ranging from well-oxygenated to dysaerobic conditions. Two box cores, with a carpet of polychaete tubes extending above the sediment-seawater interface, were recovered from 71 m water depth in Oslofjord, southern Norway. The seawater oxygen concentration of one box core was decreased every 4 wk, to a minimum value of <0.2 ml O-2 l(-1). Aerated seawater circulation was maintained in the other (control) box core. In a time course, 2 subcores were taken from each mesocosm every 4 wk and sectioned in 0.5 cm intervals to depths of 2.0 cm, and 1.0 cm intervals from 2.0 to 4.0 cm. The portion of the polychaete tubes extending into the overlying water was collected and treated as an additional sample. Live foraminiferal distributions were examined in the > 63 Fm fraction of each interval shallower than 2 cm using both an ATP assay and a rose Bengal staining method, while only rose Bengal was used to assess distributions from 2 to 4 cm. Results indicate that larger numbers of live and stained foraminifera are found on the polychaete tubes and in the superficial 0.5 cm of sediment after exposure to dysaerobic conditions compared to original and control abundances. After re-oxygenation for 4 mo, the foraminifera re-migrated into the sediments, exhibiting distributions similar to those df the control mesocosm. These observations suggest that some species actively migrate to a microenvironment with a particular oxygen concentration, rather than maintaining a stable position with respect to the sediment-water interface.