This study shows, for the first time, in sifu images of changes in marine benthic habitats in response to oxygen deficiency. Tight coupling is demonstrated between gradual degradation of benthic habitat and faunal behaviour, species richness, abundance and biomass. The critical oxygen level that forced changes in the benthic faunal successional stages was -10% that of air saturation (-0.7 m1 O2 1-l). Before this critical saturation level was reached, tube-building polychaetes their tubes extended hlgher into the water column, the width of the sub-oxic sediment layer decreased, and vertical animal burrows (formerly oxidised and brown m colour) became sulphidic and black. Over a 10 mo hypoxic period (June 1997 to Apnl 1998) in the Gullmarsfjord (Swe&sh west coast), benthlc community successional stages d e c h e d from equilibrium to virtually azoic conditions. As normoxic conditions returned, pioneering stages gradually recolonised the area.
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Nilsson et al. (2000) studied this question.