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Abstract: The geographic distribution, stocks and vertical profiles of phytoplankton of the seasonal ice zone off east Antarctica were determined during the 2005–2006 austral summer during the Baseline Research on Oceanography, Krill and the Environment-West (BROKE-West) survey. CHEMTAX analysis of HPLC pigment samples, coupled with microscopy, permitted a detailed survey along seven transects covering an extensive area between 30°E and 80°E, from 62°S to the coast. Significant differences were found in the composition and stocks of populations separated by the Southern Boundary of the Antarctic Circumpolar Current (SB), as well as a small influence of the Weddell Gyre in the western sector of the ‘zone south of the Antarctic Circumpolar Current’ (SACCZ). Within the SACCZ, we identified a primary bloom under the ice, a secondary bloom near the ice edge, and an open-ocean deep population. The similarity of distribution patterns across all transects allowed us to generalise a hypothesized sequence for the season. The primary phytoplankton bloom, with stocks of Chl a up to 239mgm−2, occurred about 35 days before complete disappearance of the sea ice, and contained both cells from the water column and those released from melting ice. These blooms were composed of haptophytes (in particular, colonies and gametes of Phaeocystis antarctica), diatoms and cryptophytes (or the cryptophyte symbiont-containing ciliate Myrionecta rubrum). Aggregates released by melting ice quickly sank from the upper water column and Chl a stocks declined to 56–92mgm−2, but the bloom of diatoms and, to a lesser extent, cryptophytes continued until about 20 days after ice melt. The disappearance of sea ice coincided with a sharp increase in P. antarctica and grazing, as indicated by increasing phaeophytin a and phaeophorbide a. Chlorophyllide content suggests that the diatom bloom then senesced, probably due to iron exhaustion. Stocks rapidly declined, apparently due to grazing krill that moved southward following the retreating sea ice. We suggest that grazing of the bloom and export of faecal pellets stripped the upper water column of iron (as suggested by low Fv/Fm ratios and CHEMTAX pigment ratios in Haptophytes – iron was not measured). Thus, export of iron by grazing, and possibly sedimentation, created a southward migrating iron gradient, limiting growth in the upper water column. North of the postulated iron gradient, a nanoflagellate community developed at depth, with Chl a stocks from 36–49mgm−2. This community was probably based on regenerated production, sustained by residual and/or upwelling iron, as indicated by a close correspondence between distributions of Chl a and profiles of Fv/Fm. The community consisted of haptophytes (chiefly Phaeocystis gametes), dinoflagellates, prasinophytes, cryptophytes, and some small diatoms. Selective grazing by krill may have fashioned and maintained the community. North of the SB, Chl a ranged from 40–67mgm−2 and was found predominantly in the mixed layer, but Fv/Fm ratios remained low, suggesting the community of P. antarctica and diatoms was iron-limited. These interpretations provide a cogent explanation for the composition and structure of late summer microbial populations in the marginal ice zone. [Copyright &y& Elsevier]

Presents information on a study which investigated the distribution and abundance of phytoplankton communities off east Antarctica, as part of the Baseline Research on Oceanography, Krill and Environment survey. Brief background on the survey; Use of the chemical taxonomy software CHEMTAX and high performance liquid chromatography in the study; Findings and discussion.

Abstract: In situ primary productivity (PP) in the Sub-Antarctic Zone (SAZ) and the Polar Frontal Zone (PFZ) south of Australia was estimated using fast repetition rate fluorometry (FRRF). FRRF-derived PP at Process station 3 (P3) southeast of Tasmania (46°S, 153°E) were higher than P1 in the southwest of Tasmania (46°S, 140°E) and P2 in the Polar Frontal Zone (54°S, 146°E). The FRRF-derived PP rates were well correlated with 14C-uptake rates from one-hour incubations (r 2=0.85, slope=1.23±0.05, p<0.01, n=85) but the relationship between both methods differed vertically and spatially. There was a linear relationship between FRRF-based PP and 14C-based PP under light-limited conditions in deeper waters. Under light-saturated conditions near the surface (0–45m), the relationship was less clear. This was likely associated with the effects of physiological processes such as cyclic electron flow and the Mehler reaction, which are stimulated at high irradiance. Our results indicate that FRRF can be used to estimate photosynthesis rates in the SAZ and PFZ but to derive an accurate estimation of C-fixation requires a detailed understanding of the physiological properties of the cells and their response to oceanographic parameters under different environmental conditions. [Copyright &y& Elsevier]

Abstract: The Sub-Antarctic Zone (SAZ) in the Southern Ocean provides a significant sink for atmospheric CO2 and quantification of this sink is therefore important in models of climate change. During the SAZ-Sense (Sub-Antarctic Sensitivity to Environmental Change) survey conducted during austral summer 2007, we examined CO2 sequestration through measurement of gross primary production rates using 14C. Sampling was conducted in the SAZ to the south-west and south-east of Tasmania, and in the Polar Frontal Zone (PFZ) directly south of Tasmania. Despite higher chlorophyll biomass off the south-east of Tasmania, production measurements were similar to the south-west with rates of 986.2±500.4 and 1304.3±300.1mgCm−2 d−1, respectively. Assimilation numbers suggested the onset of cell senescence by the time of sampling in the south-east, with healthy phytoplankton populations to the south-west sampled three weeks earlier. Production in the PFZ (475.4±168.7mgCm−2 d−1) was lower than the SAZ, though not significantly. The PFZ was characterised by a defined deep chlorophyll maximum near the euphotic depth (75m) with low production due to significant light limitation. A healthy and less light-limited phytoplankton population occupied the mixed layer of the PFZ, allowing more notable production there despite lower chlorophyll. A hypothesis that iron availability would enhance gross primary production in the SAZ was not supported due to the seasonal effect that masked possible responses. However, highest production (2572.5mgCm−2 d−1) was measured nearby in the Sub-Tropical Zone off south-east Tasmania in a region where iron was likely to be non-limiting (. Biogeochemical iron budgets of the Southern Ocean south of Australia: decoupling of iron and nutrient cycles in the subantarctic zone by the summertime supply. Global Biogeochemical Cycles 23(4), doi:10.1029/2009GB003500). [Copyright &y& Elsevier]

Abstract: Protistan species composition and abundance in the Sub-Antarctic Zone (SAZ) and Polar Front Zone (PFZ) south of Tasmania were determined by microscopy and pigment analysis from samples collected during the Sub-Antarctic Zone—Sensitivity of the sub-Antarctic Zone to Environmental Change (SAZ-Sense) voyage, in January and February of 2007. A primary goal of this voyage was to determine the potential effects of climate change-induced natural iron fertilisation of the SAZ on the protistan community by exploring differences between communities in waters west of Tasmania, which are low in iron, and eastern waters, which are fertilised by continental iron input and mixing across the subtropical front. The SAZ is a sink for anthropogenic CO2 in spring, but the magnitude of this may vary depending on seasonal changes in protistan abundance, composition and trophodynamics. Protistan species composition and abundance in the western Sub-Antarctic Zone at process station 1 (P1) showed a community in which low carbon biomass was dominated by a Thalassiosira sp., which was very weakly silicified under strong silica limitation. Protistan cell carbon was dominated by diatoms and nano-picoflagellates at process station 2 (P2) in the Polar Front Zone (PFZ), while dinoflagellates dominated in the iron-enriched waters of eastern SAZ at station 3 (P3). Iron enrichment enhanced production and favoured proliferation of small flagellates during summer in the silica-depleted eastern SAZ rather than large diatoms, though the effect this may have on the vertical export of particulate organic carbon (POC) is still unclear. [Copyright &y& Elsevier]

Abstract: Despite the long-standing belief that seawater viscosity is driven by temperature and salinity, biologically increased seawater viscosity has repeatedly been reported in relation to phytoplankton exudates in shallow, productive coastal waters. Here, seawater viscosity was investigated in relation to microbial and phytoplanktonic communities off the coast of East Antarctica along latitudinal transects located between 30°E and 80°E in sub-surface waters and at the deep chlorophyll maximum (DCM). The physical component of seawater viscosity observed along each transects ranged from 1.80 to 1.95cP, while the actual seawater viscosity ranged from 1.85 to 3.69cP. This resulted in biologically increased seawater viscosity reaching up to 84.9% in sub-surface waters and 77.6% at the DCM. Significant positive correlations were found between elevated seawater viscosity and (i) bacterial abundance in sub-surface waters and (ii) chlorophyll a concentration and the abundance of flow cytometrically-defined auto- and heterotrophic protists at the DCM. Among the 12 groups and 108 species of protists identified under light microscopy, dinoflagellates and more specifically Alexandrium tamarense and Prorocentrum sp. were the main contributors to the patterns observed for elevated seawater viscosity. Our observations, which generalised the link previously identified between seawater viscosity and phytoplankton composition and standing stock to the Southern Ocean, are the first demonstration of increases in seawater viscosity linked to marine bacterial communities, and suggest that the microbially-increased viscosity might quantitatively be at least as important as the one related to phytoplankton secretion. [Copyright &y& Elsevier]

Abstract: Protists are critical components of the Antarctic marine ecosystem as they comprise most of the living carbon and are the base of the Antarctic food web. They are also key determinants of vertical carbon flux and mediate draw-down of atmospheric CO2 by the ocean. The community composition, abundance and distribution of marine protists (phytoplankton and protozoa) was studied during the Baseline Research on Oceanography, Krill and the Environment-West (BROKE-West) survey, in the seasonal ice zone during the 2005-2006 austral summer between 30°E and 80°E. Light and electron microscopy were used to determine the protistan composition and abundance in samples obtained at 30 sites from surface waters and at 26 sites from the depth of the maximum in situ chlorophyll fluorescence (Chl max). Cluster analysis was used to identify 5 groups of sample sites at the surface and 5 at the Chl max that were of similar protist composition and abundance. The physical characteristics, taxonomic composition, indicator taxa, and taxonomic diversity were determined for each group. In the southwest, a bloom of colonial Phaeocystis antarctica dominated the protistan community composition and biomass amongst the receding ice, but this was replaced by the flagellate life stage/s of this haptophyte in waters to the north. In the southeast, a diatom bloom had the highest diversity of protist taxa observed during the survey and centric diatoms dominated the biomass. Outside these blooms, grazing by krill probably reduced the composition and abundance of large diatoms and autotrophic dinoflagellates in coastal to mid-inshore waters. Only in offshore waters did large diatoms and dinoflagellates increase in abundance and diversity, despite low concentrations of iron and silicate at many of these sites. This increase was probably due to reduced top-down control by krill and other large zooplankton. Large diatoms dominated in offshore waters, despite other coincident studies showing that the trophic structure and function of the microbial community was frequently typical of nanoflagellate-dominated systems in high-nutrient low-chlorophyll waters. Nanoflagellate abundances were low during the survey and were either poorly resolved by our study or limited by microheterotrophic grazing. We propose that protistan abundance and composition in the sea-ice zone of the Indian Sector were determined by synoptic-scale oceanographic features, meso-scale changes caused by sea-ice retreat and meso- to nano- scale interactions between grazers and the composition and abundance of their protistan prey. [Copyright &y& Elsevier]