Weight of the Evidence is the Right Approach

Sampling and field experiments were conducted from 1975 to 1990 to test how the structure of marine benthic communities around McMurdo Station, Antarctica varied with levels of anthropogenic contaminants in marine sediments. The structure of communities (e.g., infauna density, species composition, and life history characteristics) in contaminated and uncontaminated areas were compared with the structure of communities influenced by two large‐scale natural disturbances, anchor ice formation and uplift or iceberg scour. Benthic communities changed radically along a steep spatial gradient of anthropogenic hydrocarbon, metal, and PCB contamination around McMurdo Station. The heavily contaminated end of the gradient, Winter Quarters Bay, was low in infaunal and epifaunal abundance and was dominated by a few opportunistic species of polychaete worms, especially Capitella spp., Ophryotrocha claperedii, and Gyptis sp. These are relatively small and motile species. Closely related species live in organically enriched and anthropogenically disturbed sediments throughout temperate latitudes. The edge of the heavily contaminated bay, the transition area, contained several motile polychaete species with less opportunistic life histories, especially Tharyx sp. and Haploscoloplos kerguelensis. Uncontaminated sedimentary habitats harbored dense tube mats of infaunal animals numerically dominated by populations of polychaete worms, crustaceans, and a large suspension feeding bivalve. These species are generally large and relatively sessile, except for several crustacean species living among the tubes. Community patterns along the anthropogenic disturbance gradient were similar to community changes along a natural gradient of anchor ice disturbance. The same motile and opportunistic polychaete worms were most abundant where anchor ice freezing and uplift were most severe. The motile polychaetes, Tharyx sp., Haploscoloplos kerguelensis, and Polygordius sp., were abundant at the edge of the anchor ice disturbance zone. The undisturbed bottom harbored the same dense tube mat community and large bivalve population. Community changes along the anthropogenic disturbance gradient were also similar to colonization patterns into bottom areas gouged by icebergs. The most recent ice gouges along the east side of McMurdo Sound contained large numbers of the motile opportunistic polychaete worms. Ice gouges along the western sound contained more motile species of infauna compared to adjacent undisturbed bottoms with large numbers of tube‐dwelling infauna. Motile species were also the first to colonize experimental sediments placed on the sea floor for 6 yr. Although the community patterns around anthropogenic and natural disturbances were similar, particularly the motile and opportunistic species at heavily disturbed and marginal areas, the natural disturbances cover much greater areas of the sea floor around the entire Antarctic continent. On the other hand, recovery from chemical contamination is likely to take many more decades than recovery from natural disturbances as contaminant degradation is a slow process. This process can be enhanced by ongoing pollution abatement and cleanup operations.