The influence of ice disturbance on nearshore benthic communities at Adelaide Island, Antarctica

Abstract

Ice disturbance is frequently cited as having a major role in structuring benthic communities in shallow polar waters, and yet comprehensive field studies on the direct effects of ice disturbance are rare. This thesis aimed to describe the role of ice disturbance in determining benthic community structure at sites around Rothera Point, Adelaide Island (67° 34.5' S, 68° 07.0' W). The work comprised of two main components; a range of observational studies to describe the general effects of ice disturbance, and an experimental quantification of the disturbance pressure, which was then used to make novel links between disturbance and community structure. A photographic survey using depth transects (0-35 in depth) at three sites showed that benthic assemblages changed continuously along a bathymetric gradient. Assemblages were most patchy at shallow depths and the relative abundance of sessile forms increased with depth, which suggested that disturbance intensity was greatest in the shallows. The immediate effects of iceberg impacts were also investigated. Communities within newly formed iceberg scours were sampled and compared with those in undisturbed areas; scoured assemblages were 95% lower in mean macrofaunal abundance and 75.9% lower in species richness. The recovery of three scour assemblages was monitored for ~30 months following the disturbance event. Scoured assemblages became increasingly similar to undisturbed assemblages over time and marked spatial variability was observed in both scoured and unscoured zones. Experimental markers were designed to detect iceberg impacts and were deployed as 24 grids at four depth increments and two study sites. Markers were surveyed regularly for two years to quantify the frequency of iceberg impacts, which varied significantly with depth, site, season and year. The intensity of disturbance was greatest in the shallows (0-5 m depth) and significantly reduced at 25 m depth. Variation between site and season could be largely explained by the duration of winter fast ice. Extensive sampling at each of the disturbance grids showed that disturbance intensity has a significant and wide-ranging influence on macrobenthic community structure.