Southern Antarctic Circumpolar Front Research Articles

Multivariate analyses of Antarctic and sub-Antarctic seaweed distribution patterns: An evaluation of the role of the Antarctic Circumpolar Current

Biogeographic barriers and ecological corridors are fundamental in defining macroecological and evolutionary processes. Ocean circulation, considering present and past patterns of continental drift, can isolate or connect many groups of marine organisms, including seaweeds. These benthic organisms present spores and propagules as planktonic stages that drift with currents and or tides, and have been a sensible indicator of changes to biogeographical distribution patterns. Phycological studies have been exhaustive in the sub-Antarctic and Antarctic seaweed communities. However, the role of the Antarctic Circumpolar Current (ACC) in shaping marine phytogeographic diversity has been poorly investigated. The ACC connects the major world oceans and redistributes oceanic properties, such as heat, salt, and nutrients, consisting of three major circumpolar fronts (in order of north to southward): the Sub-Antarctic Front (SAF), the Antarctic Polar Front (PF), and the Southern Antarctic Circumpolar Front (SACF). This paper's aim is to understand the role of the ACC fronts as a constraint on seaweed distribution patterns in different taxonomic levels, in relation to the southern Sub-Tropical areas, as well as to compare sections connected with South America (1: north), influenced by the Ross Sea Gyre (2: Western Antarctica Peninsula) and by the Weddell Sea Gyre (3: Eastern Antarctica Peninsula). nMDS showed differences in the distributional patterns of species and genera in relation to the zones and sections. The predicted latitudinal gradient of species richness was observed, and by comparing biogeographic zones, two main clusters were observed: Sub-tropical and Sub-Antarctica; and Polar Front (Antarctic Peninsula) and SACF (areas surrounding continental Antarctica), suggesting, for this sort of macroscale analysis, that ACC still has a role of “watershed barrier”. However, a lower dissimilarity (higher similarity) was observed between the 2nd (Eastern Antarctica Peninsula – EAP – or under influence of the Weddell Sea Gyre, and some sub-Antarctic islands as South Orcadas, South Georgia and Crozet Archipelago) and the 1st sectors (Chilean and Argentinean Patagonia and surrounding islands, Falklands, South Shetlands and including Antarctic Peninsula). When considering recently reported changes in diversity patterns of these locations, this result could demonstrate the existence of a clear species distributional flux, despite evidence of the ACC being a limit to dispersal. Therefore, this contribution has used previously published data to provide a tool for monitoring future biogeographical changes to the flora and fauna of this region of the Antarctic and sub-Antarctic areas due to changes in biogeographical distribution, resulting either from natural dispersion due to global meteoceanographic changes, and or biological invasions related to anthropogenic activities. We could be witness to a period of changes in Antarctic diversity, suggesting that Antarctica may not be as isolated as was once thought.

Latitudinal distribution of extant fossilizable phytoplankton in the Southern Ocean: Planktonic provinces, hydrographic fronts and palaeoecological perspectives

Abstract We present the combined abundance of all extant fossilizable planktonic groups (Dinoflagellates, Coccolithophores, Silicoflagellates, Diatoms, Parmales, Archaeomonads and micro-zooplankton) from surface waters collected along a latitudinal transect in the western Pacific sector of the Southern Ocean, ranging from ~ 48°S, offshore New Zealand, to ~ 70°S in the Ross Sea, Antarctica. Latitudinal shifts in species' distribution correspond with the Antarctic Circumpolar Current fronts and with the seasonal position of the ice-edge. Distinct bioprovinces are defined by clustering samples with a high degree of similarity. Our data confirm the importance of previously-defined taxa as palaeoceanographic proxies but also reveal some differences: the shift in dominance between the silicoflagellates genera Dictyocha and Stephanocha, used as a proxy of palaeo sea-surface temperatures, occurs slightly north of the Southern Sub-Antarctic Front rather than at the Polar Front; the shifts in abundance between the open-ocean diatom species Fragilariopsis kerguelensis and sea-ice related F. curta and F. cylindrus, as well as the drop in the coccolithophore Emiliania huxleyi, occur at the southern Antarctic Circumpolar Front rather than at the Polar Front. Finally, we introduce the Corona-index, based on the ratio of the coronatid to non-coronatid silicoflagellates species Stephanocha speculum, as a new proxy for sea-ice and we confirm the occurrence of abundant Archaeomonads and Parmales (Triparma laevis subsp. ramispina) in the marginal ice-edge zone.

Extremophiles in an Antarctic Marine Ecosystem.

Recent attempts to explore marine microbial diversity and the global marine microbiome have indicated a large proportion of previously unknown diversity. However, sequencing alone does not tell the whole story, as it relies heavily upon information that is already contained within sequence databases. In addition, microorganisms have been shown to present small-to-large scale biogeographical patterns worldwide, potentially making regional combinations of selection pressures unique. Here, we focus on the extremophile community in the boundary region located between the Polar Front and the Southern Antarctic Circumpolar Current in the Southern Ocean, to explore the potential of metagenomic approaches as a tool for bioprospecting in the search for novel functional activity based on targeted sampling efforts. We assessed the microbial composition and diversity from a region north of the current limit for winter sea ice, north of the Southern Antarctic Circumpolar Front (SACCF) but south of the Polar Front. Although, most of the more frequently encountered sequences were derived from common marine microorganisms, within these dominant groups, we found a proportion of genes related to secondary metabolism of potential interest in bioprospecting. Extremophiles were rare by comparison but belonged to a range of genera. Hence, they represented interesting targets from which to identify rare or novel functions. Ultimately, future shifts in environmental conditions favoring more cosmopolitan groups could have an unpredictable effect on microbial diversity and function in the Southern Ocean, perhaps excluding the rarer extremophiles.

Distribution of particulate organic matter (POM) in the Southern Ocean during BROKE-West (30°E - 80°E)

Abstract The distribution of particulate organic carbon (POC), nitrogen (PON) and biogenic silica (BSi) in the south Indian sector of the Southern Ocean was investigated during summer 2006 as part of the BROKE-West survey. Differences in the distribution of particulate organic matter (POM) were attributed to spatial pattern related to the influence of regional hydrography and sea-ice retreat pattern. Offshore to the west, low NCP and POM accumulation were observed in a region influenced by the Weddell gyre, whereas to the east higher NCP and POM were found in a region influenced by the southern Boundary of the ACC and the southern Antarctic Circumpolar Front circulation (SB-sACCf). The highest NCP (5.4 molC m −2 ), POC (1.6 molC m −2 ) and BSi stocks (0.5 molSi m −2 ) were observed in the coastal/Marginal Ice Zone (MIZ). Spatial differences were also found in particulate elemental composition, with lower POC:PON found in the coastal zone, illustrating the presence of an active autotrophic community. Analysis of elemental ratios also showed a strong decoupling between silica and nitrogen cycling over the whole survey region, particularly in association with the circulation of SB-sACCf. Estimates of silica and nitrogen export production illustrate this decoupling with BSi exported production. BSi export ranged between 54 and 94% of BSi production whereas PON ranged between 4 and 70%, suggesting that organic matter was actively recycled in the surface water, hence illustrating the efficiency of the silicate pump in this region.

Upper ocean nitrogen fluxes in the Polar Antarctic Zone: Constraints from the nitrogen and oxygen isotopes of nitrate

We report nitrate nitrogen (N) and oxygen (O) isotope measurements from the seasonally sea ice covered Polar Antarctic Zone (PAZ) south of the Southern Antarctic Circumpolar Front. The 15N/14N and 18O/16O ratios of nitrate both increase into the summertime surface mixed layer, in strong correlation with the upward decrease in nitrate concentration, the expected result of nitrate assimilation by phytoplankton. Culture studies indicate that algal assimilation of nitrate fractionates the nitrate N and O isotopes equally, while previous field studies suggest that nitrate N and O isotope behavior can be decoupled by euphotic zone nitrification. Our data for the PAZ show strong coupling of the dual isotopes of nitrate, and a numerical model of Antarctic summertime surface layer N cycling fits our observations (including isotopic compositions of both nitrate and suspended particulate N) if the nitrification rate is no more than 6% of the nitrate assimilation rate by phytoplankton. The model estimates that the N isotope effect of nitrate assimilation is 5.0 ± 0.7‰. This estimate lacks some of the uncertainties associated with previous studies within the Antarctic Circumpolar Current, and it is at the low end of most recent estimates from the Southern Ocean, the range of which we speculatively attribute to an effect of mixed layer depth on the amplitude of isotope discrimination.

Bellingshausen and western Antarctic Peninsula region: Pigment biomass and sea-ice spatial/temporal distributions and interannual variabilty

The Palmer Long-Term Ecological Research (LTER) program seeks to obtain a comprehensive understanding of various components of the Antarctic marine ecosystem—the assemblage of plants, microbes, animals, ocean, and sea-ice south of the Antarctic Polar Front. A central hypothesis of the Palmer LTER is that the seasonal and interannual variability of sea ice affects all levels of the Antarctic marine ecosystem, from the timing and magnitude of seasonal primary production to the breeding success and survival of apex predators. In the context of this high-latitude ecosystem we use satellite imagery to examine physical forcing and possible mechanisms influencing the distribution of phytoplankton biomass in the region to the west of the Antarctic Peninsula. We evaluate the spatial and temporal variability of pigment biomass (estimated as chlorophyll-a concentrations using SeaWiFS data) in response to the spatial and temporal variability of sea-ice extent (estimated from passive microwave satellite data). While the ocean-color data record is relatively short (7 years) and contains high interannual variability, there are persistent spatial patterns of phytoplankton biomass that indicate important regional-scale physical mechanisms including: the marginal ice zone and its impact on the mixed-layer depth, the timing of spring sea-ice retreat, the importance of the southern Antarctic Circumpolar front, and teleconnections with sub-polar regions. The SeaWIFS imagery presented here provides the most complete synoptic space/time views of phytoplankton biomass within this region to date. These observations suggest that the southern Antarctic Circumpolar front may have a more profound influence on the western Antarctic Peninsula ecosystem than previously thought.