DIVERSITY AND RELATIVE ABUNDANCE OF ENCRUSTING FORAMINIFERA ON THE ANTARCTIC SCALLOP ADAMUSSIUM COLBECKI IN RELATION TO SEA-ICE CONDITIONS AND SEA-WATER CHEMISTRY

With global climate change affecting seawater chemistry and temperature, organisms that depend on calcite biomineralization are at risk. This is especially true for Antarctic foraminifera with calcite skeletons that settle on carbonate substrates, such as epibenthic mollusk shells. We examined foraminifera that encrust the valves of Adamussium colbecki to determine if species diversity, relative abundance, calcareous vs. agglutinated species, and distribution on bottom vs. top valves differed between two sites from western McMurdo Sound: Bay of Sails (BOS), a locality with annual sea-ice cover and relatively open-ocean conditions, and Explorers Cove (EC), a protected embayment with multi-annual sea-ice cover. BOS had fine-grained sediment, normal salinity water (55.96 mS/cm), and a seawater pH of 7.84; EC had fine- to coarse-grained sediment, lower salinity water (53.36 mS/cm), and lower pH (7.65). Bottom water temperatures were the same for both sites (-1.96^oC).

Ten encrusting foraminiferal species and 1,405 individuals (506/m²) were present at the BOS site; six species were calcitic representing 771 individuals (256/m²). The EC site had ten encrusting species and higher foraminiferal density (1,336 individuals, 892/m²); five were calcitic, for 859 individuals (693/m²). The BOS site had a Shannon-Wiener Index of H’ = 1.46 (E = 0.7), whereas the EC site had a lower diversity score of H’ = 0.89 (E = 0.39), resulting from the dominance of calcitic Cibicides refulgens (recent genetic analysis suggests C. refulgens represents a species complex; Antarctic forms are referred to as Cibicides sp.). At the BOS site, bottom valves resting on the sediment were dominated by agglutinated species; top valves (exposed surface) were dominated by calcitic species. In contrast, the EC site had a dominance of calcitic species on both bottom and top valves. This bottom vs. top variation in encrusting foraminifera may be a consequence of difference in sediment size and seawater chemistry between the sites, but further analysis is needed. While evenness and diversity of encrusting foraminifera is higher at the BOS open-ocean site, carbonate-secreting foraminiferans are more abundant at EC where salinity and pH is lower, perhaps in relation to sea-ice dynamics and localized melt-out during Austral summer.