Benthic ecology of pockmarks in the Inner Oslofjord, Norway

Abstract

Despite pockmarks being one of the most widespread small-scale topographic features of the seabed, almost nothing is known of their influence on fauna. Here we present the first ecolog- ical study of fjordic pockmarks, focusing on the contrast between macrofauna inside and outside of these craters. We report an analysis of macrofauna from 27 pockmarks of the Inner Oslofjord, Nor- way. Five replicate grab samples were collected from each of 3 pockmarks at 3 sites plus a further control (non-pockmark) sample at each site. A single grab sample was collected from an additional 3 pockmarks at 6 sites and 6 control locations. We compared macrofaunal assemblages inside and outside of pockmarks and found important but subtle differences to those on non-pockmarked sub- strata. The fauna of pockmarks were typical of a disturbed fjord environment with a dominance of small opportunistic taxa, such as pioneer bivalves and polychaetes. The position of sites in the fjord gradient drove the most obvious faunal differences, but contrary to expectations, we could find no influence of pockmarks on the composition of the fauna at any taxonomic level. This makes them very unusual amongst marine topographic features, which usually have considerable influence on the nature of benthic communities. However, we found that pockmarks do significantly alter the abun- dances of key species and, as such, we suggest that the presence of pockmarks in the Oslofjord has a considerable cumulative influence on densities and populations of benthic organisms.