Continuous benthic community change along a depth gradient in Antarctic shallows: evidence of patchiness but not zonation

Many shallow water polar communities demonstrate considerable change along a bathymetric gradient. However, it is currently unclear whether community change is generally continuous or discrete. To determine the nature of community change with depth, extensive photographic surveys at three sites at Adelaide Island, West Antarctic Peninsula, were conducted along a bathymetric gradient of 5–35 m depth. Macroalgae were largely absent at the sites, so only distinguishable macrofauna were counted and analysed. Faunal abundance was greatest at the shallowest stations of the depth transects, whilst richness at both species and phylum level increased with depth. Variability in community structure between replicate transects decreased with depth, so that assemblages at >25 m depth were more homogenous. Depth had a highly significant effect on total abundance, species richness and community structure, and it is likely that the frequency of ice disturbance, which also decreases with depth, drives this pattern. Overall, high variability between transects at each site was recorded, which suggested considerable patchiness at the scale of tens of meters. Community change along the bathymetric gradient was continuous and no evidence of discrete zones of assemblages was recorded.