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

Abstract

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