Ice is a major structuring force in marine and freshwater environments at high latitudes. Although recovery from scouring has been quantified in time, the frequency of scouring in the Antarctic has not. We placed grids of markers at 9-17 m depth at two sites, to study ice-scouring over 2 years at Adelaide island (Antarctic Peninsula). We quantified the time scale of scour frequencies, and linked this to community mortality, age and diversity. Markers were hit from zero to at least three times in 2 years. At the least disturbed site (South Cove) 24% of markers were destroyed per year, whereas in North Cove 60% of markers were destroyed. There were significant differences in scouring frequency between our two sites: a given area in North Cove was on average hit twice as often as one in South Cove. Compared with near shore environments elsewhere, faunas of both sites were characteristic of high disturbance regimes, exhibiting low percent cover, diversity, ages and a high proportion of pioneers. Aspects of the encrusting communities studied reflected the differences between site disturbance regimes. North Cove was scoured twice as often, and bryozoan communities there had half the number of species, two-thirds the space occupation and twice the mortality level of those in South Cove. Maximum age in North Cove bryozoans was also half that in South Cove. Although there are natural disturbance events that rival ice-scouring in either frequency or catastrophic power at lower latitudes, none do both nor across such a wide depth range. We suggest that ice scour effects on polar benthos are even more significant than the same magnitude of disturbance at lower latitudes as recovery rates of high latitude communities are very slow. Climate warming seems likely to increase iceloading of near shore polar waters, so that some of the world's most intensely disturbed faunas may soon suffer even more disturbance.
The West Antarctic Peninsula is one of the fastest warming regions on Earth, and, as a consequence, most maritime glaciers and ice shelves in the region have significantly retreated over the past few decades. We collected a multiyear data set on ice scouring frequency from Antarctica by using unique experimental markers and scuba diving surveys. We show that the annual intensity of ice scouring is negatively correlated with the duration of the winter fast ice season. Because fast ice extent and duration is currently in decline in the region after recent rapid warming, it is likely that marine benthic communities are set for even more scouring in the near future.
The southern continental margin of Australia, the largest area of cool-water carbonate sedimentation on the globe, is characterized by extensive marine grassbeds in many inshore environments. The most important seagrasses in terms of calcareous epiphyte production are Posidonia sinuosa, P. angustifolia, P. australis, Amphibolis antarctica and A. griffi thii. The predominant control on relative abundance of calcareous epiphytes is seagrass biomass. These grasses have a biomass of 50-500 g m 2 , which peaks at 2-4 m water depth. The most abundant calcareous epiphytes are geniculate (articulated) and non-geniculate (encrusting) coralline algae that together comprise ~38-80% of the epiphyte carbonate. The only other signifi cant epiphytes are bryozoans and benthic foraminifera, which contribute roughly equal amounts (~8-33% each) of carbonate. Unlike the seagrass biomass, calcareous epiphyte abundance peaks at water depths of ~10 m. The rates of epiphyte production are roughly similar to those from epiphytes in tropical environments, averaging 210 26 g m 2 yr 1 . Posidonia is morphologically similar to tropical seagrasses (e.g. Thalassia) and produces largely carbonate mud from the disintegration of blade-encrusting corallines. Amphibolis, on the other hand, has an extensive upright, exposed shoot system that is much longer lived (biennial) and so is encrusted with prolifi c articulated corallines thus producing ~3 more carbonate in terms of g kg 1 from the stems than the blades. Average accumulation rates of epiphytic carbonate are calculated to be ~7.4 cm kyr 1 . This accounts for a major proportion of the carbonate sequestered in grass beds in this cool-water realm, and probably accounts for much of the nearshore and supratidal carbonate mud. Thus, the nearshore, grass-covered habitat is a cool-water carbonate factory surprisingly similar to the shallow-water tropical system, except that the sediment produced is poorly sorted Mg-calcite carbonate with little or no aragonite.
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