Metal lability, sea ice cover, and physicochemical properties of sediments as predictors of benthic communities in Antarctic coastal environments

Metals are important nutrients to marine life but may cause toxicity at elevated concentrations. There is uncertainty about how sediment metal concentrations influence benthic biodiversity, in part because metal bioavailability depends on sediment properties. We show that labile metal fluxes to sediment porewaters, as well as environmental and sediment physicochemical properties, are key predictors of benthic biodiversity in the fragmented nearshore marine ecosystems of the Windmill Islands, East Antarctica. Sediment metal lability was assessed using dilute- and concentrated-acid extractions and the Diffusive Gradients in Thin-Films (DGT) technique which measures labile metal fluxes to sediment porewaters. These were related to benthic community composition at 8 sites, assessed with eDNA metabarcoding using 16S and 18S rRNA gene primers for prokaryotic and eukaryotic communities, respectively. Benthic communities were highly varied and significantly different between sites (p ≤ 0.037) with the exception of Wilkes Tip North and South eukaryotic communities (p = 0.063). The eukaryotic community had minimal similarities with macrofaunal samples sampled for previous studies at the same sites in previous years, but this was not unexpected and patterns of differences among sites were broadly consistent between the two methods. DGT-labile metal fluxes, median particle size, sea ice cover, and phosphorous content were the most important predictors of prokaryotic and eukaryotic communities, explaining a cumulative 52.3 % and 56.1 % of variance in distance-based linear models, respectively. Contaminants in sediments were low at all sites, despite some sites being selected for their proximity to abandoned waste disposal sites on land. This brings into question whether terrestrial sources of pollution near these sites are mobile to the nearshore marine environment. We anticipate our results will inform the environmental management of historical waste in Antarctica and reaffirms the importance of metal lability, rather than total concentrations, when considering relationships with benthic communities.