Worm burrows and oxic microniches in marine sediments .1. Spatial and temporal scales

The upper 2 to 10 cm of shallow-water sediments may represent a mosaic of oxic and anoxic microhabitats. In the studied sediments the volume fraction of patches of oxic sediments decreased with depth from around 0.2 in the upper centimetres to zero at 5 to 10 cm depth. The mean volume fraction of oxic microhabitats beneath a unit surface and down to 5-8 cm depth ranged between 0.06 and 0.16. The mean distance between such neighbouring oxic microniches was several millimetres in the upper centimetres of the sediment. The oxic microniches are caused by small (diameter: 0.5 to 2 mm), inhabited worm burrows which have not previously drawn attention in bioturbation studies. An implication is that anaerobic and aerobic microbial processes take place at identical depths. In such sediments the apparent vertical zonation of microbial processes and of microbial biota over a scale of several centimetres reflects the diminishing fraction of oxic habitats with depth rather than an idealised vertical redox sequence (such as found over a few millimetres in microbial mats which develop in the absence of bioturbation).