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Little is known about the diet of planktonic tunicates such as salps, which can comprise around 80% of zooplankton abundance under bloom conditions. The gut contents of solitary and aggregate phases of the salps Thalia democratica and Salpa fusiformis were analysed with scanning electron microscopy (SEM), high-performance liquid chromatography (HPLC) and stable isotope analyses (SIA) to describe their diets under field conditions. The gut contents contained representatives of diatoms, dinoflagellates, haptophyte flagellates (Order Prymnesiales and Coccolithophorales), prasinophytes (Order Chlorodendrales) and, in a few instances, copepods (Crustacea). SEM confirmed the presence of many species of phytoplankton in the salp guts and was broadly supported by HPLC and SIA. The dominant peaks in the HPLC chromatograms corresponded to fucoxanthin, alloxanthin, chlorophyll b and β-carotene, indicating the ingestion of diatoms, cryptophytes and green algae. Solitary stages of both S. fusiformis and T. democratica fed on items that were not common in the phytoplankton samples, namely coccolithophores and copepods, respectively. [ABSTRACT FROM AUTHOR]

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: Our study, as part of the Baseline Research on Oceanography, Krill and the Environment, West (BROKE-West) survey, emphasised the vital role of sea-ice retreat and upwelling in controlling the distribution, abundance and composition of marine microbial communities in the seasonal ice zone (SIZ). Autofluorescence or stains were used to detect the abundance of nanophytoplankton, heterotrophic nanoflagellates (HNF), virus like particles (VLP) and bacteria by flow cytometry. Correlations among microbial concentrations were determined and cluster analysis was performed to group sites of similar microbial composition and abundance. Distance to sea ice was the primary determinant of nanophytoplankton abundance and nanophytoplankton contributed up to 84% of the phytoplankton carbon biomass where melting sea ice caused shallow summer mixed layer depths. To the north, nanophytoplankton abundance was generally low except adjacent to the Southern Boundary (SB). HNF and bacterial abundance was positively correlated with the abundance of nanophytoplankton. Cluster analysis identified 5 groups of sites over the BROKE-West survey area. Clusters 1-4 grouped sites of different successional maturity of the microbial community along the continuum between bloom formation and senescence. Maturity increased with distance from the sea ice and, in areas of upwelling, with time since the development of phytoplankton blooms. Sites in cluster 5 occurred at the northernmost extreme of the survey area and were typical of communities in high nutrient low chlorophyll (HNLC) waters of the permanent open-ocean zone (POOZ) where phytoplankton growth was matched by mortality and decomposition. Synoptic-scale studies in Antarctic waters are rare but provide vital information about the control of microbial productivity, abundance and distribution in the Southern Ocean. Our study, covering over 40% of the SIZ off East Antarctica, enhances our understanding of the synoptic-scale factors that determine the structure and function of the microbial loop. [Copyright &y& Elsevier]

Abstract: The high nutrient low chlorophyll (HNLC) surface waters of the Southern Ocean are characterised by high concentrations of nitrate and phosphate, low concentrations of dissolved iron and deep vertical mixing. Future climate scenarios predict increased surface temperatures and ocean stratification in the region. These changes to vertical mixing will result in a slowdown of nutrient supply to surface waters and an increase in the integrated irradiance in the upper mixed layer. To investigate the influence of iron-limitation and high irradiance on phytoplankton growth and physiology, a 6-day shipboard incubation experiment was conducted during the Sub-Antarctic Zone Sensitivity to Environmental Change (SAZ Sense) voyage using phytoplankton populations from the upper mixed layer in the north-eastern SAZ region. Iron-limitation was induced with an organic siderophore and was compared with a 1nM iron-enriched incubation and an unamended treatment (under silicate replete conditions). As expected, iron enrichment led to dominance by large diatoms and enhanced photosynthetic performance, while the iron-limited community showed a decline in total chl a and photochemical efficiency. Under the added stress of high light, the iron-limited community was able to cope with the shift from in situ (<150μmolphotonsm−2 s−1) to incubation (mean=765μmolphotonsm−2 s−1) irradiance by increasing the proportion of photoprotective pigments and diverting excess light energy via energy-dependent quenching (q E). The responses to iron-limitation under high light showed that the phytoplankton community was able to acclimate to these conditions, but exhibited an overall decline in photosynthetic activity. Data presented here suggest the community shifts, in particular the decrease in diatoms, and the decline in photosynthetic performance of phytoplankton under low iron-high irradiance conditions has the potential to impact future ocean productivity and biogeochemical cycling. [Copyright &y& Elsevier]

Abstract: The sub-Antarctic zone (SAZ) of the Southern Ocean is considered one of the largest sinks for atmospheric CO2 and as such is an important region for climate change research. To determine the importance of micro- and nano-heterotrophs in controlling microbial abundance within this region, we determined microbial standing stocks and rates of herbivory and bacterivory in relation to changes in the water masses south of Tasmania. The SAZ-Sense (‘Sensitivity of the sub-Antarctic zone to environmental change’) cruise traversed the SAZ during mid-late austral summer and focussed on process stations to the southeast (45°S, 153°E) and southwest (46°S, 140°E) of Tasmania and at the Polar Front (54°S, 147°E). Growth and grazing mortality of phytoplankton and bacteria were estimated by the grazing dilution technique using seawater from 10m depth at 15 sites along the survey, along with concentrations of heterotrophic nanoflagellates (HNF), microzooplankton, bacteria, cyanobacteria and size fractionated (pico-, nano- and micro-sized) chlorophyll a (Chl a). Rates of herbivory ranged from 0.12 to 1.39d−1 and were highest in the north-eastern SAZ (NE-SAZ) where concentrations of prey (as indicated by Chl a) and microzooplankton were also highest. Rates of herbivory were correlated with total rates of phytoplankton growth, bacterial growth and concentrations of microzooplankton. On average 82%, 67% and 42% primary production d−1 was consumed by microzooplankton and HNF at process stations in the north-western SAZ (NW-SAZ), NE-SAZ and polar frontal zone (PFZ), respectively. In the NW-SAZ, grazing pressure was highest on the pico-sized Chl a fraction, whereas in the NE-SAZ, grazing pressure was more evenly distributed across all three size fractions of Chl a. Bacterivory removed 77%, 93% and 39% of bacterial production d−1 in the NW-SAZ, NE-SAZ and PFZ, respectively, and rates of bacterivory ranged from 0.12 to 1.03d−1. Rates of bacterivory were highest in the NE-SAZ where concentrations of bacteria were significantly higher than elsewhere in the region and bacterivory was correlated with bacterial growth rates and rates of cyanobacterivory. Cluster analysis of the concentrations of marine microbes and their rates of growth and grazing mortality identified 5 groups of sampling sites that differed in community structure. Analysis distinguished between high nutrient, low Chl a (HNLC) communities in the NW-SAZ that were iron-limited; iron-limited low Chl a PFZ communities; and iron-replete NE-SAZ communities where high rates of remineralisation correlated with higher concentrations of Chl a. Our findings show that much of the carbon sequestered by photosynthesis in the SAZ during summer is reprocessed via the microbial loop rather than contributing to vertical flux, particularly to the southeast of Tasmania. This suggests strong seasonality in carbon export in the region and that future climate-driven changes in oceanography may reduce carbon export from the region in summer. [Copyright &y& Elsevier]

Abstract: Changes in oceanic circulation and physiochemical parameters due to climate change may alter the distribution, structure and function of marine microbial communities, thereby altering the action of the biological carbon pump. One area of current and predicted future change is the sub-Antarctic zone (SAZ) to the southeast of Tasmania, Australia, where a southward shift in westerly winds appears to be forcing warmer and macronutrient-poor subtropical waters into the sub-Antarctic zone (SAZ). We investigated the impact of these subtropical waters on the microbial community of the SAZ on the SAZ-Sense cruise during the austral summer of 2007. The abundance of pico- and nanoeukaryotic algae, cyanobacteria, heterotrophic nanoflagellates, bacteria and viruses was determined by flow cytometry at stations in the Polar Frontal Zone (PFZ), the SAZ and in Subtropical Zone (STZ). Using cluster and similarity profile analyses on integrated microbial abundances over the top 200m, we found that microbial communities located in the potential future SAZ to the southeast of Tasmania formed two distinct groups from those of the remainder of the SAZ and the PFZ. In the waters of the potential future SAZ, shallow mixed layers and increased iron concentrations elevated cyanobacterial, bacterial and viral abundances and increased percentage high DNA bacteria, resulting in communities similar to those of subtropical waters. Conversely, waters of the PFZ exhibited relatively low concentrations of autotrophic and heterotrophic microbes and viruses, indicative of the iron limitation in this region. A Distance Based Linear Model determined that salinity and nitrogen availability (nitrate, nitrite and ammonia concentrations) were the most influential environmental parameters over the survey, explaining 72% of the variation in microbial community structure. The microbial community of the potential future SAZ showed a shift away from particulate carbon export from the photic zone towards increased production by smaller cells, increased significance of the microbial loop and viral lysis. These changes would promote carbon recycling within the photic zone, thereby potentially decreasing the capacity of the future SAZ to absorb CO2. [Copyright &y& Elsevier]

Abstract: Marine primary production is a fundamental measure of the ocean''s capacity to convert carbon dioxide to particulate organic carbon for the marine foodweb, and as such is an essential variable used in ecosystem and biogeochemical models to assess trophic dynamics and carbon cycling. The Sub-Antarctic Zone (SAZ) is a major sink for atmospheric carbon and exhibits large gradients in ocean conditions on both temporal and spatial scales. In this dynamic system, an understanding of small-scale temporal changes is critical for modelling primary production at larger scales. Thus, we investigated diel effects on maximum quantum yield of PSII (F V/F M), photosynthetic pigment pools and primary productivity in the western (Diel 1) and eastern SAZ region (Diel 3) south of Tasmania, Australia, and compared this to a station at the polar front (Diel 2). Phytoplankton in the eastern SAZ had the greatest diel response, with cells showing decreased F V/F M and increased biosynthesis and transformation of xanthophyll and other photoprotective pigments during the day, but only in the surface waters (0 and 10m). Diel responses diminished by 30m. Cells in the western SAZ had similar responses across the depths sampled, increasing their F V/F M during the night and increasing their xanthophyll pigment content during the day. Phytoplankton at the polar front (Diel 2) showed intermediate diel-related variations in photophysiology, with xanthophyll conversion and increases in photoprotective pigments during the day but constant F V/F M. These diel changes at all sampling stations had little impact on carbon fixation rates, although cells sampled from the deep chlorophyll maximum at the polar front had significantly lower maximum carbon fixation and minimum saturating irradiance (E k ) compared to the other depths and stations. Considering the oceanographic context, cells at Diel 1 and 2 received less light and were more deeply mixed than cells at Diel 3, causing a dampening of the diel response. These results highlight that phytoplankton in the SAZ is regulated by the physical processes of mixing and light provision, but short-term diel effects on maximum quantum yield of PSII and photoprotective pigments may not propagate to changes in carbon fixation, particularly when cells are nutrient replete. If however, the more stratified eastern SAZ (which had the greatest diel responses) is indicative of how the SAZ region might respond to climate change, then diel effects may become more prominent in the future. [Copyright &y& Elsevier]

Abstract: Areas of high nutrients and low chlorophyll a comprise nearly a third of the world’s oceans, including the equatorial Pacific, the Southern Ocean and the Sub-Arctic Pacific. The SOLAS Sea-Air Gas Exchange (SAGE) experiment was conducted in late summer, 2004, off the east coast of the South Island of New Zealand. The objective was to assess the response of phytoplankton in waters with low iron and silicic acid concentrations to iron enrichment. We monitored the quantum yield of photochemistry (F v /F m ) with pulse amplitude modulated fluorometry, chlorophyll a, primary productivity, and taxonomic composition. Measurements of F v /F m indicated that the phytoplankton within the amended patch were relieved from iron stress (F v /F m approached 0.65). Although there was no significant difference between IN and OUT stations at points during the experiment, the eventual enhancement in chlorophyll a and primary productivity was twofold by the end of the 15-day patch occupation. However, no change in particulate carbon or nitrogen pools was detected. Enhancement in primary productivity and chlorophyll a were approximately equal for all phytoplankton size classes, resulting in a stable phytoplankton size distribution. Initial seed stocks of diatoms were extremely low, <1% of the assemblage based on HPLC pigment analysis, and did not respond to iron enrichment. The most dominant groups before and after iron enrichment were type 8 haptophytes and prasinophytes that were associated with ∼75% of chlorophyll a. Twofold enhancement of biomass estimated by flow cytometry was detected only in eukaryotic picoplankton, likely prasinophytes, type 8 haptophytes and/or pelagophytes. These results suggest that factors other than iron, such as silicic acid, light or physical disturbance limited the phytoplankton assemblage during the SAGE experiment. Furthermore, these results suggest that additional iron supply to the Sub-Antarctic under similar seasonal conditions and seed stock will most likely favor phytoplankton <2μm. This implies that any iron-mediated gain of fixed carbon will most likely be remineralized in shallow water rather than sink and be sequestered in the deep ocean. [Copyright &y& Elsevier]

Abstract: Marine microbes (<200μm) contribute most of the living matter and carbon flow in the Southern Ocean, yet the factors that control the composition and function of these microbial communities are not well understood. To determine the importance of microbial grazers in controlling microbial abundance, we determined microbial standing stocks and rates of herbivory and bacterivory in relation to the physical environment off East Antarctica during the Baseline Research on Oceanography, Krill and the Environment: West (BROKE-West) survey, which covered waters from the Polar Front to the coast between 30 and 80°E. Concentrations of heterotrophic nanoflagellates (HNF) (∼2 to 20μm), microzooplankton (∼20 to 200μm), bacteria, and chlorophyll a (Chl a) were determined and the growth and grazing mortality of phytoplankton and bacteria were estimated using the grazing dilution technique at 22 sites along the survey. Results showed that microzooplankton and HNF consumed on average 52% of bacterial production d−1 and 62% primary production d−1 but consumed >100%d−1 at the western ice-edge sites. Rates of bacterivory ranged from 0.4 - 2.6d−1 and were correlated with bacterial concentrations, bacterial growth rates and longitude. Rates were highest in the eastern-most part of the survey, which was sampled last, reflecting the transition along the successional continuum toward a respiration-based, senescent, microbial community. Rates of herbivory ranged from 0.3 to 2.4d−1 and were correlated with concentrations of microzooplankton and HNF combined, rates of phytoplankton growth, and latitude. Rates were highest at southern ice edge sites where concentrations of prey (as represented by Chl a) and microzooplankton were also highest. Cluster analysis of the concentrations of marine microbes and their rates of growth and grazing mortality identified 5 groups of sample sites that conveniently summarised the variability in the composition and function of the microbial community. Cluster groups differentiated between low Chl a (∼0.3μgl−1) open ocean Antarctic Circumpolar Current (ACC) communities; and high Chl a (∼2.4μgl−1) ice-associated coastal blooms at various stages between bloom formation and senescence. This partitioning of cluster groups can be used to determine spatial and temporal patterns of carbon transfer by the microbial loop within the BROKE-West survey area. [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]