Differential response of benthic meiofauna to anoxia with special reference to Foraminifera (Protista: Sarcodina)

Sediments collected from the northwestern Adriatic Sea (Mediterranean) were exposed to anoxic bottom-water conditions for more than 2 mo in order to examine the resistance of dominant meiobenthic taxa to prolonged anoxia. Copepods appeared to be most sensitive to anoxia, with densities being reduced to zero within 11 d. Compared to oxic conditions, densities of both nematodes and soft-shelled foraminifera were significantly lower under prolonged anoxia. In contrast, total hard-shelled foraminiferal densities did not differ significantly. This differential response resulted in a change in the meiobenthic community structure which was apparent after 1 mo. The change is reflected, and can be followed, in the foraminifera:nematode ratio, which is proposed as a bio-indicator of prolonged anoxia. The results clearly demonstrate that, among the meiobenthos, hard-shelled foraminifera are most resistant to prolonged anoxia. However, conditions resulting from the anoxic treatment (e.g. decreased biological interactions) were beneficial to some foraminiferal genera. This resulted in a shift in faunal patterns eventually leading to a strong reduction in foraminiferal generic diversity. Results also indicated that details of these trends may not be evident if only the part of the foraminiferal assemblage retained on a 63 μm sieve is examined.