Austral autumn and winter seasonal affects on benthic foraminiferal communities: Bransfield and northern Gerlache Straits.

摘要

The Southern Ocean has unique seasonal qualities due to the Antarctic Circumpolar Current (ACC) thermo-isolating the Antarctic continent. During summer months, surface primary productivity from algal blooms is very high. In the winter months, limited daylight hours (4 in winter) and formation of sea ice prevents sun light from reaching surface waters, therefore limiting productivity. The short seasons of productivity and long winters in Antarctica combined with seasonal changes in deep ocean temperatures, salinity, and fluxes of organic matter impact foraminiferal population dynamics. Fluctuations in surface primary productivity, as well as living foraminiferal assemblages have been documented around the Antarctica Peninsula, but the impact on benthic foraminiferal assemblages is poorly understood. This is a study of seasonal affects on benthic foraminiferal assemblages in the southern Bransfield-northern Gerlache Straits of the Southern Ocean. Surface sediment samples from 600 meters and 1200 meters water depth were collected during two seasonal cruises: early April to record the productivity of the end-of-summer bloom and late June to sample the less-productive winter period. Three hundred and sixty samples were collected from 7 sites and processed using standard techniques. To identify living foraminifera, samples were treated with Rose Bengal, and CellTracker Green on a select set of samples for comparison. Ninety total species were identified; seventy species from June and seventy-one from April, fifteen species of foraminifera unique to April, thirteen to June and two unique species in the CellTracker Green samples. The abundance of total living (stained tests) opportunistic benthic foraminiferal species from the 7 sampled sites show distinct temporal differences related to seasonality. An assemblage of deep water species was also found, as well as an assemblage of shallow water species. ANOVA and post hoc Tukey tests showed that the full cores must be analyzed to determine seasonal species assemblage changes. Cluster analysis and species abundances in CellTracker Green samples showed a marked difference from the Rose Bengal samples, consistent with literature that suggests the two methods differ. Fluctuating populations of foraminifera in fossil samples can be interpreted as changes in local or global climate. This study stands as a modern analog for fossil foraminiferal assemblages, and provides important information to help interpret paleoenvironmental conditions related to seasonality by defining seasonally and geographically distinct species assemblages.