ALBERT

Description: Antarctic krill ( Euphausia superba ) are a key species in Southern Ocean ecosystems, maintaining very large numbers of predators, and fluctuations in their abundance can affect the overall structure and functioning of the ecosystems. The interannual variability in the abundance and biomass of krill was examined using a 17-year time-series of acoustic observations undertaken in the Western Core Box (WCB) survey area to the northwest of South Georgia, Southern Ocean. Krill targets were identified in acoustic data using a multifrequency identification window and converted to krill density using the Stochastic Distorted-Wave Born Approximation target strength model. Krill density ranged over several orders of magnitude (0–10 000 g m –2 ) and its distribution was highly skewed with many zero observations. Within each survey, the mean krill density was significantly correlated with the top 7% of the maximum krill densities observed. Hence, only the densest krill swarms detected in any one year drove the mean krill density estimates for the WCB in that year. WCB krill density ( µ , mean density for the area) showed several years (1997/1998, 2001–2003, 2005–2007) of high values ( µ 〉 30 g m –2 ) interspersed with years (1999/2000, 2004, 2009/2010) of low density ( µ 〈 30 g m –2 ). This pattern showed three different periods, with fluctuations every 4–5 years. Cross correlation analyses of variability in krill density with current and lagged indices of ocean (sea surface temperature, SST and El Niño /Southern Oscillation) and atmospheric variability (Southern Annular Mode) found the highest correlation between krill density and winter SST (August SST) from the preceding year. A quadratic regression ( r 2 = 0.42, p 〈 0.05) provides a potentially valuable index for forecasting change in this ecosystem.

Description: Chereskin, T. K., and Tarling, G. A. 2007. Interannual to diurnal variability in the near-surface scattering layer in Drake Passage. – ICES Journal of Marine Science, 64: 1617–1626. Backscattering strength was estimated from 127 shipboard surveys with an acoustic Doppler current profiler (ADCP) made during Drake Passage transits from 1999 to 2004. The backscattering strength is used to determine the characteristics of the near-surface scattering layer, which south of the Southern Antarctic Circumpolar Current Front (SACCF) is dominated by Antarctic krill (Euphausia superba). Diel vertical migration in the upper 150 m was the dominant variability observed in any single transect. When averaged over depth, there was a well-defined annual cycle in backscattering strength, with a factor of four increase from a late-winter minimum to a spring-summer maximum over a period of four months, followed by a more gentle decline during late summer and autumn. In addition, there were significant differences in scattering strength north and south of the Polar Front (PF) on both seasonal and interannnual time-scales. The average summer maximum to the north of the PF was more than twice the maximum to the south, but the winter minima were about the same. On interannual time-scales, scattering strength south of the PF displayed a negative linear trend primarily attributable to a fourfold decrease in backscattering strength south of the SACCF. No significant long-term trend in the scattering strength north of the SACCF was observed.

Description: The polar oceans are experiencing some of the largest levels of ocean acidification (OA) resulting from the uptake of anthropogenic carbon dioxide (CO2). Our understanding of the impacts this is having on polar marine communities is mainly derived from studies of single species in laboratory conditions, while the consequences for food web interactions remain largely unknown. This study carried out experimental manipulations of natural pelagic communities at different high latitude sites in both the northern (Nordic Seas) and southern hemispheres (Scotia and Weddell Seas). The aim of this study was to identify more generic responses and achieve greater experimental reproducibility through implementing a series of short term (4 d), multilevel (3 treatment) carbonate chemistry manipulation experiments on unfiltered natural surface-ocean communities, including grazing copepods. The experiments were successfully executed at six different sites, covering a diverse range of environmental conditions and differing plankton community compositions. The study identified the interaction between copepods and dinoflagellate cell abundance to be significantly altered by elevated levels of dissolved CO2 (pCO(2)), with dinoflagellates decreasing relative to ambient conditions across all six experiments. A similar pattern was not observed in any other major phytoplankton group. The patterns indicate that copepods show a stronger preference for dinoflagellates when in elevated pCO(2) conditions, demonstrating that changes in food quality and altered grazing selectivity may be a major consequence of ocean acidification. The study also found that transparent exopolymeric particles (TEP) generally increased when pCO(2) levels were elevated, but the response was dependent on the exact set of environmental conditions. Bacteria and nannoplankton showed a neutral response to elevated pCO(2) and there was no significant relationship between changes in bacterial or nannoplanlcton abundance and that of TEP concentrations. Overall, the study illustrated that, although some similar responses exist, these contrasting high latitude surface ocean communities are likely to show different responses to the onset of elevated pCO(2).

Description: Biogeochemical and diatom export fluxes are presented from two bathypelagic sediment trap deployments in the Antarctic Zone of the Southern Ocean. One of the sediment traps was deployed in very productive, naturally iron-fertilized waters downstream of South Georgia (P3, 2000 m) and compared to a deployment in moderately productive waters upstream of the island system (P2, 1500 m). At both sites significant diatom export events occurred in spring (November) and contained mostly empty cells that were associated with low particulate organic carbon (POC) fluxes. A summer export pulse occurred one month later at P2 (end February/March) compared to P3 (end January). Diatom fluxes at P3 were one order of magnitude higher than at P2, a difference mainly attributed to the short and intense export of resting spores from Chaetoceros Hyalochaete and Thalassiosira antarctica species. Aside from these resting spores, diatom export assemblages at both sites were dominated by empty Fragilariopsis kerguelensis frustules. The fraction of diatoms exported as empty frustules was considerably lower at P3 (52%) than P2 (91%). This difference was related to the flux of intact diatom resting spores at P3 and may partially explain the lower Si:C export stoichiometry observed at P3 (1.1) compared to P2 (1.5). Through the enumeration of full diatom frustules and subsequent biomass calculations we estimate that diatom resting spores account for 42% of annual POC flux in the productive waters downstream of South Georgia. At both sites the contribution of diatom vegetative stages to POC fluxes was considerably lower (〈5%). From these analyses we conclude that resting spore export contributes towards the slightly higher bathypelagic (POC) flux at P3 (40.6 mmol m−2 y−1) compared to P2 (26.4 mmol m−2 y−1). We compared our sediment trap records with previously published diatom assemblage data from the mixed layer and surface sediments (3760 m) around South Georgia. The relative proportion of diatom resting spores within diatom assemblages increases as a function of depth and is explained by selective preservation of their robust frustules. Our study highlights the significance of diatom resting spore export as a carbon vector out of the mixed layer. Furthermore, the contribution or resting spores to POC flux in the bathypelagic ocean and sediments suggests they play a particularly important role in sequestering biologically fixed CO2 over climatically relevant timescales.