Worm burrows and oxic microniches in marine sediments .2. Distribution patterns of ciliated protozoa

It is shown that in sandy sediments the occurrence of aerobic and microaerobic ciliated protozoa at several centimetres depth is due to the presence of oxic microhabitats brought about by small burrowing invertebrates. The characteristic vertical zonation patterns, which have previously been described in the literature, are due to the diminishing volume fraction of such aerobic microhabitats rather than to redox conditions which become monotonously more reducing with depth. At the surface of defaunated sediments as well as in sediments surrounding worm burrows the zones harbouring aerobes and microaerobes are compressed to a thickness of a few millimetres. Chemosensory behaviour explains how the organisms find and maintain themselves in oxic microniches.