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

Sediments collected from the northwestern Adriatic Sea (Mediterranean) w ere 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 densi­ ties being reduced to zero within l i d . Compared to oxic conditions, densities of both nem atodes and soft-shelled foraminifera were significantly lower under prolonged anoxia. In contrast, total hardshelled 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 foraininifera.-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) w ere 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 p.m sieve is examined.