‘Live’ (stained) deep-sea benthic foraminiferans in the western Weddell Sea: trends in abundance, diversity and taxonomic composition along a depth transect

Abstract

‘Entire’ live foraminiferal assemblages (i.e. including soft-walled species) were analysed using replicate subcores (3.45 cm2 surface area, 0–1 cm layer; >63 μm fraction) from multiple corer samples collected along a transect (1100–5000 m water depth) of the continental slope and rise and adjacent abyssal plain to the east of the Antarctic Peninsula. Foraminiferans usually accounted for 61–83% of all meiofaunal organisms. Mean densities were highest (576 individuals (indiv.)10 cm−2) at 2100 m water depth and declined to 240 indiv. 10 cm−2 at 5000 m, although there was considerable variability between replicates at some stations (e.g., 304, 333, 1090 indiv. 10 cm−2 at 2100 m). Foraminiferan and metazoan meiofaunal densities were broadly coherent across the depth range sampled. A total of 158 live foraminiferal species was recognized. Assemblages were most diverse on the lower slope, with species numbers peaking at 3100 m and the Fisher α diversity index at 3100–4100 m. Monothalamous taxa increased in relative abundance from 8% at 1100 m to 33% at 4975 m, and there was a corresponding decrease in the proportion of calcareous taxa from 65% to 16%. On the continental slope (1100–3100 m), the most abundant hard-walled species were Epistominella exigua, Adercotryma glomeratum, Alabaminella weddellensis and Stetsonia hovarthi. In the deeper samples (4100–5000 m), A. glomeratum became the dominant hard-walled species. Important soft-walled species included Tinogullmia riemanni and Bathyallogromia weddellensis. In some samples from 1100 and 2100 m, more than a third of the live assemblage was hidden within phytodetrital aggregates; species such as E. exigua, A. weddellensis and T. riemanni were concentrated within these microhabitats. Many of the Weddell Sea species are typical bathyal and abyssal forms well known from the North Atlantic and elsewhere. The phytodetrital assemblages are strikingly similar to those reported from abyssal sites in the North Atlantic. Our observations suggest that there are close faunal links between the deep-water faunas of the Weddell Sea and those of other oceanic regions, perhaps mediated by thermohaline circulation.