Vertical migratory response of benthic foraminifera to controlled oxygen concentrations in an experimental mesocosm

This experimental study investigated the vertical migratory response of benthic in sediments that were initially oxygenated, to variations in dissolved-oxygen concentrations from well-oxygenated to dysaerobic conditions. Two box cores, with a carpet of polychaete extending above the sediment-seawater interface, were recovered from 71 m water depth Oslofjord, southern Norway. The seawater oxygen concentration of one box core was decreased 4 wk, to a minimum value of <0.2 ml 0; 1'. Aerated seawater circulation was maintained in other (control) box core. In a time course, 2 subcores were taken from each mesocosm every 4 wk sectioned in 0 5 cm mtervals to depths of 2.0 cm, and 1.0 cm intervals from 2.0 to 4.0 cm. The of the polychaete tubes extending into the overlymg water was collected and treated as an sample. Live foramimferal distributions were examined in the > 63 pm fraction of each interval than 2 cm using both an ATP assay and a rose Bengal staining method, while only rose was used to assess distributions from 2 to 4 cm. Results indicate that larger numbers of live and forammifera are found on the polychaete tubes and in the superficial 0.5 cm of sediment after to dysaerobic conditions compared to original and control abundances. After re-oxygenation 4 mo, the foraminifera re-migrated into the sediments, exhibiting distributions similar to those of control mesocosm. These observations suggest that some species actively migrate to a microenvironment a particular oxygen concentration, rather than maintaining a stable position with to the sediment-water interface.