Search

Abstract: A phytoplankton bloom occurs north of the Crozet Plateau annually from September to January. The area, bounded to the north by the Sub-Antarctic Front, is the most northerly of the areas of regular high productivity in the otherwise high-nutrient low-chlorophyll Southern Ocean. Chlorophyll concentrations are at background values to the south and only slightly enhanced over the shallow plateau, producing three contrasting productivity regimes. The CROZet natural iron bloom and EXport experiment (CROZEX) project was aimed at testing the hypothesis that the bloom is caused by natural iron fertilisation from the sediments and islands of the Crozet Plateau. In this paper, the temporal and spatial progression of the bloom and the contrasting productivity regimes are investigated using SeaWiFS and MODIS chlorophyll-a (chl-a), photosynthetically available radiation (PAR) from SeaWiFS, QuikSCAT wind-stress data, Argo float profiles and data from three research cruises to the region. Comparison of satellite chl-a data against in situ measurements showed that the satellite data were lower by a factor of approximately 2. Iron, light and grazing are all important in explaining the different productivity regimes. To the north, light is dominant in controlling the timing and location of the bloom initiation, but the spatial distribution of the peak chl-a values obtained through the season is associated with the flow pattern and their proximity to the plateau and so are likely driven by nutrient availability. Mixed-layer depths in Argo float density profiles show a shallowing of the mixed-layer depth to the north of 10.8±1.0mdegree−1 across the study area. This gradient, together with a latitudinal gradient in PAR, is sufficient to explain the observed spatial progression of the bloom. [Copyright &y& Elsevier]

The Southern Ocean is a key region for silica (Si) cycling, and the isotopic signatures established here influence the rest of the world's oceans. The climate and ecosystem of the Southern Ocean are changing rapidly, with the potential to impact Si cycling and isotope dynamics. This study examines high-resolution time-series dataset of dissolved Si concentrations and isotopic signatures, particulate Si concentrations and diatom speciation at a coastal site on the western Antarctic Peninsula (WAP), in order to characterise changes in Si cycling with respect to changes occurring in productivity and diatom assemblages. Dissolved and particulate Si phases reflect the dominant control of biological uptake, and combined with isotopic fractionation were consistent with a season of low/intermediate productivity. Biogenic Si is tightly coupled to both chlorophyll and particulate organic carbon at the sampling site, consistent with diatom-dominated phytoplankton assemblages along the WAP. Variability in diatom speciation has a negligible impact on the isotopic signature of dissolved Si in surface waters, although this is unlikely to hold for sediments due to differential dissolution of diatom species. A continued decline in diatom productivity along the WAP would likely result in an increasing unused Si inventory, which can potentially feed back into Si-limited areas, promoting diatom growth and carbon drawdown further afield. [ABSTRACT FROM AUTHOR]

The Western Antarctic Peninsula warmed significantly during the second half of the twentieth century, with a concurrent retreat of the majority of its glaciers, and marked changes in the sea-ice field. These changes may affect summertime upper-ocean stratification, and thereby the seasonal dynamics of phytoplankton and bacteria. In the present study, we examined coastal Antarctic microbial community dynamics by pigment analysis and applying molecular tools, and analysed various environmental parameters to identify the most important environmental drivers. Sampling focussed on the austral summer of 2009–2010 at the Rothera oceanographic and biological Time Series (RaTS) site in northern Marguerite bay, Antarctica. The Antarctic summer was characterized by a salinity decrease (measured at 15 m depth) coinciding with increased meteoric water fraction. Maximum Chl-a values of 35 µg l -1 were observed during midsummer and mainly comprised of diatoms. Microbial community fingerprinting revealed four distinct periods in phytoplankton succession during the summer while bacteria showed a delayed response to the phytoplankton community. Non-metric multidimensional scaling analyses showed that phytoplankton community dynamics were mainly directed by temperature, mixed layer depth and wind speed. Both high and low N/P ratios might have influenced phytoplankton biomass accumulation. The bacterioplankton community composition was mainly governed by Chl-a, suggesting a link to phytoplankton community changes. High-throughput 16S and 18S rRNA amplicon sequencing revealed stable eukaryotic and bacterial communities with regards to observed species, yet varying temporal relative contributions. Eukaryotic sequences were dominated by pennate diatoms in December followed by polar centric diatoms in January and February. Our results imply that the reduction of mixed layer depth during summer, caused by meltwater-related surface stratification, promotes a succession in diatoms rather than in nanophytoflagellates in northern Marguerite Bay, which may favour higher trophic levels. [ABSTRACT FROM AUTHOR]

Abstract: Mesopelagic fish are a key component of the pelagic ecosystem throughout the world’s oceans. Opening and closing nets were used to investigate patterns in the distribution and abundance of mesopelagic fish from the surface to 1000m on a series of transects across the Scotia Sea from the ice-edge to the Antarctic Polar Front. A total of 141 non-target net hauls were undertaken during three cruises (Nov 2006, Jan 2008 and Mar 2009), with 7852 teleost fish captured, representing 43 species in 17 families. A further 1517 fish were caught in targeted net hauls. The dominant families were the Myctophidae (6961 specimens; 21 species) and Bathylagidae (1467 specimens; 4 species). Few fish were caught in the upper 400m during daylight, which was attributed to a combination of net avoidance and diurnal vertical migration. Species composition was linked to depth and location and was closely associated with oceanographic features. Diversity was lowest in cold water at the most southerly stations, which were dominated by Electrona antarctica, Gymnoscopelus braueri and Bathylagus antarcticus. Further north, diversity increased with the addition of species such as Krefftichthys anderssoni, Protomyctophum bolini and Electrona carlsbergi. The depth integrated biomass of myctophids was similar across the latitudinal transect and produced an estimate of 4.5 million tonnes in the Scotia Sea. Bathylagids were patchily distributed, but were abundant in the lower mesopelagic zone (>400m) and are potentially significant zooplankton consumers. Given the biomass of the myctophids and bathylagids coupled with the vertical migrations of many species, these fish are likely to play a significant role in carbon export from the surface waters to the deep ocean. [Copyright &y& Elsevier]

Abstract: Marine productivity is often higher downstream than upstream of islands. This so-called island mass effect was tested and quantified with respect to biological carbon uptake and air–sea exchange of carbon dioxide (CO2) at the Crozet Plateau between November 2004 and January 2005 during two CROZEX cruises. The remote plateau is situated at 45.5–47.0°S 49.0–53.0°E, south of the Subantarctic Front (SAF) in the Polar Frontal Zone (PFZ). Surface waters upstream (south) of the plateau had high nutrient and low chlorophyll (HNLC) concentrations. The fugacity of carbon dioxide (fCO2) in surface water was just below the atmospheric value and oceanic CO2 uptake was small (0.2±0.1molm−2) throughout CROZEX. The mixed-layer concentration of dissolved inorganic carbon (DIC) decreased by 15μmolkg−1 from November to January in these HNLC waters, indicating significant biological carbon uptake. Extensive phytoplankton blooms occurred downstream (north) of the plateau in austral spring. These reduced surface water fCO2 by 30–70μatm and DIC by 30–60μmolkg−1 and created an important oceanic sink for atmospheric CO2 of 0.6–0.8±0.4molm−2, corresponding to a total uptake of 1.3±0.8TgC (1Tg=1012 g). The reduction of DIC in the upper 100m was much larger downstream (2–3molm−2) than upstream (1molm−2) of the plateau in January, further confirming the existence of the island mass effect for the Crozet Archipelago. An additional finding is the sizeable DIC deficit in the HNLC waters upstream (south) of the plateau, suggesting that some HNLC waters of the PFZ are more productive than commonly thought. Deep mixed layers of 60–90m may hide such sustained, modest marine productivity from detection by satellite. [Copyright &y& Elsevier]