BENTHIC FORAMINIFERA IN OXYGEN-POOR HABITATS

Numerous eurytolerant species of calcareous and agglutinated benthic foraminifera, belonging to diverse suprageneric taxa, survive under modem dysoxia and suboxia (2-0 ml/l O2), as well as under higher oxygen levels. Under the usual combination of oxygen depletion and organic-matter enrichment of the substrate, the populations of some of these species expand significantly. Thus, generally, a relatively small number of opportunistic species become conspicuously dominant in bathyal oxygen minimum zones; endobenthic adaptations are common within this group. Such high-dominance, low-diversity, foraminiferal biotas are present even in environments where the oxygen minimum stays near 3 ml/l. Various stenobathic but geographically widespread species are known from inshore waters under seasonal or long-term oxygen stress. Some of the species may be facultative anaerobes. Some ecophenotypes with larger pores may be associated with oxygen-poor waters, and small, thin-walled species are common (but not exclusively present) in oxygen minimum zones. Present data from modem environments do not support the hypothesis that most dysoxic-suboxic benthic foraminifers, regardless of their phylogeny, are characterized by a particular test morphology.