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

This experimental study investigated the vertical migratory response o f benthic foraminifera in sediments that were initially oxygenated, to variations in dissolved-oxygen concentrations ranging from well-oxyge nated to dysaerobic conditions. Two box cores, with a carpet of polych aete tubes extending above the sediment-seawater interface, were recov ered from 71 m water depth in Oslofjord, southern Norway. The seawater oxygen concentration of one box core was decreased every 4 wk, to a m inimum value of <0.2 ml O-2 l(-1). Aerated seawater circulation was ma intained 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 inte rvals to depths of 2.0 cm, and 1.0 cm intervals from 2.0 to 4.0 cm. Th e portion of the polychaete tubes extending into the overlying water w as collected and treated as an additional sample. Live foraminiferal d istributions were examined in the > 63 Fm fraction of each interval sh allower than 2 cm using both an ATP assay and a rose Bengal staining m ethod, while only rose Bengal was used to assess distributions from 2 to 4 cm. Results indicate that larger numbers of live and stained fora minifera are found on the polychaete tubes and in the superficial 0.5 cm of sediment after exposure to dysaerobic conditions compared to ori ginal and control abundances. After re-oxygenation for 4 mo, the foram inifera re-migrated into the sediments, exhibiting distributions simil ar to those df the control mesocosm. These observations suggest that s ome species actively migrate to a microenvironment with a particular o xygen concentration, rather than maintaining a stable position with re spect to the sediment-water interface.