Effect of anoxia on the vertical migration of benthic foraminifera

Abstract

We present the results of a microcosm experiment in which benthic foraminifera had to re-establish microhabitat distributions under simulated anoxic and oxygenated bottom-water conditions after initial homogenization of the sediment (obtained from the northern Adriatic Sea at 32 m water depth). After 3 wk, the vertical distribution of the 12 most abundant species was assessed. These taxa could be divided into 2 distinct groups: (1) typical shallow-living species with maximum abundances in the top 2 cm of the sediment; and (2) deeper-dwelling taxa that were more homogeneously distributed throughout the sediment column. Compared to the oxygenated environment, the average living depth of all species was less under anoxic bottom-water conditions in which the group of shallow dwellers was almost restricted to the top 5 mm of the sediment. This distribution was most probably caused by negative geotaxis, induced by an external stimulus associated with anoxic conditions (e.g. high [H2S]). The deeper-living taxa apparently lack the ability for negative geotaxis or are less sensitive to the stimulus.