Tolerance of benthic macrofauna to hypoxia and anoxia in shallow coastal seas: a realistic scenario

Oxygen depletion can lead to the collapse of benthic ecosystems, i.e. to dead zones, and large-scale biodiversity loss. Based on mortality and survival, we evaluated ranges of sensitivity and tolerance to hypoxia levels, duration of anoxia and H2S exposure across taxa and key life habits. Experiments were conducted on a sublittoral soft-bottom under realistic in situ conditions in a community setting featuring both a well-developed epi- and infauna. Overall, 495 individuals representing 40 species were examined over almost 1000 h (using time-lapse camera and sensor arrays). Mortality started at dissolved oxygen (DO) concentrations <0.5 ml l(-1) and centred at the transition from severe hypoxia to early anoxia. A total of 58% of the individuals belonging to 27 species died. Thirteen species (39 individuals) died exclusively during anoxia. All of the individuals that died during hypoxia, and most of those that died during anoxia, did so before the onset of hydrogen sulphide (H2S). In 11 species, all individuals survived: survival rates were highest among molluscs, anthozoans and ascidians. In contrast, most polychaetes, decapods and echino-derms died. Epifauna was more vulnerable than infauna, mobile forms were more vulnerable than sessile forms, and predators more vulnerable than deposit-feeders and suspension-feeders. While hypoxia primarily affected total abundance, anoxia significantly reduced the number of species. The former represents a quantitative, the latter a qualitative decline in ecosystem function. Most of the macrofauna may initially survive shorter-term (day-long) or intermittent hypoxia, but the onset of anoxia marks community collapse and biodiversity loss.