Search

• In situ measurements were made close to giant iceberg A-68A during its break-up phase. • Surface meltwater deepened underlying water masses by around 50 m. • Particulates and phytoplankton were rapidly displaced downwards close to A-68 icebergs. • Nutrients concentrations typical of deeper layers were found closer to the surface. • Ice-associated phytoplankton were present both upstream and downstream of the icebergs. • The dynamic oceanographic environment dominated over larger scale iceberg impacts. Large icebergs (>20 km long) are responsible for most of the freshwater discharged into the Southern Ocean. We report on in situ and satellite observations made during the break-up phase around South Georgia of the giant tabular iceberg A-68A. The in situ measurements were obtained during a 4-day visit by a research vessel in February 2021, where physical, chemical and biological measurements were made at a range of distances away from the main and subsidiary icebergs. These results were compared to a far-field station 133 km away. Upstream of the iceberg field, water column structure was similar to ambient water although there was evidence of iceberg-associated phytoplankton as a likely remnant of the passage of the icebergs. Nevertheless, enhancement of primary productivity along the path of the icebergs was not resolved in either in situ or monthly mean satellite observations. There was a considerable brash-ice field moving ahead of the icebergs which limited the number of downstream sampling stations. One downstream station within 2 km of iceberg A-68P showed several ice-melt influenced features that distinguished it from most other stations. Firstly, there was a strong stratified meltwater influenced layer that reached to around 120 m. This had the effect of deepening underlying water masses, with the core of the temperature minimum layer around 50 m deeper than elsewhere. Secondly, there was evidence of rapid downward displacement of both particulate material and certain phytoplankton taxa that may be a further result of this water mass deepening. Thirdly, macronutrient profiles were altered, with concentrations of nitrate, silicic acid and phosphate characteristic of deeper layers being found closer to the surface and a dilution of the ambient nutrient pool just above the iceberg draft that we ascribe to meltwater released from basal melting. Meanwhile, nutrient recycling processes associated with organic matter remineralisation were also modified by the physical restructuring of the water column and biotic components. Finally, the ice-associated phytoplankton taxa Pseudo-nitszchia/Nitszchia , found in both upstream and downstream locations, were abundant at this < 2 km-distant station through melting out from the iceberg and subsequent rapid growth. Overall, we resolved alterations to water column structure, nutrient profiles and phytoplankton community composition at fine to medium scales around the iceberg field. Nevertheless, although there may have been longer term and larger scale impacts, the dynamic oceanographic environment, including the presence of a strong oceanographic front and shelf-edge processes, dominated during the collapse of A-68A. [ABSTRACT FROM AUTHOR]

Antarctic krill is a key prey species for many vertebrate and invertebrate predators in the Southern Ocean; it is also an abundant fishery resource in the Scotia Sea and southern Drake Passage. Here, we identify environmental correlates of krill distribution utilizing acoustic data collected during an extensive international survey in January 2000. Separate models (at scales of 10-80 nautical miles) were derived for the full study area and for each of four subregions: northern and southern shelf waters, the seasonally ice-covered open ocean, and the generally ice-free open ocean. Krill distribution was strongly correlated with bathymetry; densities were higher over island shelves and shelf breaks and decreased with increasing distance offshore. Low krill densities occurred in areas of low chlorophyll concentration and high geostrophic velocity. Krill distribution was also related to sea level anomaly but relationships were not consistent between subregions. The models explained a maximum of 44% of the observed deviance in krill density, but did not reliably identify areas of high krill density in the open ocean, and explained a small proportion of the deviance (16%) in offshore areas covered seasonally by sea ice, probably because of the strong, residual influence of retreated ice. The commercial krill fishery is currently concentrated in shelf areas, where high densities of krill are most predictable. As krill are not predictable in the open ocean, the fishery is likely to remain principally a near-shore operation, and should be managed accordingly. [ABSTRACT FROM AUTHOR]

The South Sandwich Islands form the eastern boundary to the highly biologically productive Scotia Sea in the southwest Atlantic sector of the Southern Ocean and are part of a large Marine Protected Area. The South Sandwich Islands have a complex marine environment that is influenced by both the Antarctic Circumpolar Current and the Weddell Gyre, and seasonal sea ice. Here we investigate the local and regional dynamics and variability of the ocean and sea ice to inform management of the region. Remotely sensed sea surface temperature (SST), sea ice concentration and chlorophyll a data from 2009 to 2021 are used to define the mean seasonal cycle in the environment and the associated temporal and spatial variability. While sea surface temperature and sea ice have a clearly defined seasonality, local chlorophyll blooms are irregular in timing, location and magnitude. Interannual variability in summer SST is strongly positively correlated along the island arc. The islands experience very different winter sea ice conditions from year to year, with marked variability in sea ice distribution and duration. Surface chlorophyll blooms develop in most years close to the island arc, but there is little spatial consistency and there are years where blooms are not observed. The timing and pattern of sea ice retreat appears to be a key driver in the formation of chlorophyll blooms, with their propagation affected by local circulation, but additional local processes are also important. Trajectories of near-surface satellite tracked surface buoys and Argo floats, together with an analysis of sea surface height output from a global reanalysis product, demonstrate the connectivity of the South Sandwich Islands to the wider regional marine system. Enhanced current flows around and between the South Sandwich Islands are likely to affect the transport and exchange of material along the island arc. The South Sandwich Islands are connected with the Scotia and Weddell seas, with contribution from the different regions varying according to latitude along the island arc. There are also connections with islands downstream including Bouvet, Crozet and Kerguelen Islands and seamounts, with possible return flow via the Weddell Gyre. Our analyses indicate that accounting for the complexity and variability in the South Sandwich Islands marine environment will be crucial in the development of conservation and fisheries management procedures. [ABSTRACT FROM AUTHOR]

Abstract: Pathways and rates of ocean flow near the Antarctic Peninsula are strongly affected by frontal features, forcings from the atmosphere and the cryosphere. In the surface mixed layer, the currents advect material from the northwestern Weddell Sea on the eastern side of the Peninsula around the tip of the Peninsula to its western side and into the Scotia Sea, connecting populations of Antarctic krill (Euphausia superba) and supporting the ecosystem of the region. Modelling of subsurface drifters using a particle tracking algorithm forced by the velocity fields of a coupled sea ice-ocean model (ORCA025-LIM2) allows analysis of the seasonal and interannual variability of drifter pathways over 43 years. The results show robust and persistent connections from the Weddell Sea both to the west into the Bellingshausen Sea and across the Scotia Sea towards South Georgia, reproducing well the observations. The fate of the drifters is sensitive to their deployment location, in addition to other factors. From the shelf of the eastern Antarctic Peninsula, the majority enter the Bransfield Strait and subsequently the Bellingshausen Sea. When originating further offshore over the deeper Weddell Sea, drifters are more likely to cross the South Scotia Ridge and reach South Georgia. However, the wind field east and southeast of Elephant Island, close to the tip of the Peninsula, is crucial for the drifter trajectories and is highly influenced by the Southern Annular Mode (SAM). Increased advection and short travel times to South Georgia, and reduced advection to the western Antarctic Peninsula can be linked to strong westerlies, a signature of the positive phase of the SAM. The converse is true for the negative phase. Strong westerlies and shifts of ocean fronts near the tip of the Peninsula that are potentially associated with both the SAM and the El Niño-Southern Oscillation restrict the connection from the Weddell Sea to the west, and drifters then predominantly follow the open paths to South Georgia and the east. Over the 43-year time series, the number of drifters advected into the Bellingshausen Sea decreases significantly by 23% and the travel time to South Georgia shortens significantly by 19% which corresponds to 56 days. We propose that these trends are linked, at least in part, to the increasingly positive trend in the SAM and, as such, this suggests an additional anthropogenic source of change to the regional ecosystem. [Copyright &y& Elsevier]

Abstract: During the austral summer of 2003, a large scale survey of the Scotia Sea was undertaken by the British Antarctic Survey as part of the Southern Ocean Global Ocean Ecosystems Dynamics programme. This cruise provided a unique opportunity to examine the distribution of phytoplankton biomass and primary production in relation to the physico-chemical environment of the Scotia Sea. Phytoplankton were sampled from a range of oceanographic regimes including the open ocean, in the wake of oceanic islands, across major fronts such as the Southern Antarctic Circumpolar Current Front, submarine topographical features such as the South Scotia Ridge and the marginal ice zone. Generally the Scotia Sea was characterised by low biomass (&#60;30 mg chlorophyll-a m−2), low production rates (&#60;0.31 g C m−2 d−1) and an abundance of macronutrients (e.g., surface nitrate >28 mmol m−3) despite favourable environmental conditions for growth caused by shallow mixed layers and deep euphotic depths. Three areas of elevated biomass (77–295 mg chlorophyll-a m−2) and production (0.73–2.04 g C m−2 d−1) as well as substantial macronutrient depletion (e.g., surface nitrate ∼12 mmol m−3) were observed during the cruise: to the northwest of the island of South Georgia, to the southwest of South Georgia and further south over the South Scotia Ridge in a region of rapid ice retreat. These productive regions were also characterised by shallow mixed layer depths, although near to South Georgia euphotic depths were reduced due to the high biomass of phytoplankton in the water column. The biomass was composed of over 80% diatoms. We speculate that the contrasting production regimes observed during the cruise were the result of differences in iron availability. Throughout much of the survey area, the eastward flowing Antarctic Circumpolar Current is unlikely to have encountered any shallow bathymetric features that may introduce sedimentary iron into the euphotic zone. However, shallow topographic features, may cause upwelling of iron into the euphotic zone. This, together with shallow mixed layers and a favourable light environment, may then account for the dense blooms observed near topographic features during our cruise. [Copyright &y& Elsevier]

Abstract: Bioregionalisation, the partitioning of large ecosystems into functionally distinct sub-units, facilitates ecosystem modelling, management and conservation. A variety of schemes have been used to partition the Southern Ocean, based variously on frontal positions, sea ice, productivity, water depth and nutrient concentrations. We have tested the utility and robustness of ecosystem bioregionalisation for the Scotia Sea, by classifying spring, summer and autumn stations on the basis of nutrient concentrations, phytoplankton taxa, meso- and macrozooplankton, fish catches and acoustic data. Despite sampling across different seasons and years, at different spatial scales and taxonomic resolutions, cluster analysis indicated basically consistent spatial divisions across this wide range of trophic levels. Stations could be classified into two main groups, lying broadly to the north and south of the Southern Antarctic Circumpolar Current Front (SACCF). In some aspects the 2 station groups were similar, with both having variable but often high phytoplankton biomass as well as similar biomass of fish. However, the colder water southern group, most of which was covered by seasonal sea ice, had a fundamentally different food web structure to that in the northern Scotia Sea. The cold water community had a depleted, cold-adapted fauna characterised by low zooplankton biomass, Euphausia superba and the fish Electrona antarctica and Gymnoscopelus braueri. In contrast the northern group was richer with higher mesozooplankton biomass and a fauna of warmer or more cosmopolitan species such as Themisto gaudichaudii, Euphausia triacantha and the fish Protomyctophum bolini, Kreffticthys anderssoni and Gymnoscopelus fraseri. The position of the food web transition, broadly consistent with the position of the SACCF, supports a recent circumpolar-scale bioregionalisation. However, there is little evidence that this relatively weak frontal transition represents a significant barrier either here or elsewhere in the Southern Ocean. We suggest that broader-scale factors, namely temperature and possibly the extent of the seasonal ice-zone, within which most of the southern stations lay, were more likely to influence biological zonation. [Copyright &y& Elsevier]

• Beached marine debris has been found in the Southern Ocean for over three decades. • Plastic is the most commonly recovered item. • 10,112 items weighing over 100 kg were recovered from Bird Island, South Georgia. • 1304 items weighing 268 kg were recovered from Signy Island, South Orkneys. • Routine monitoring is essential to catalogue and identify trends in marine debris. We report on three decades of repeat surveys of beached marine debris at two locations in the Scotia Sea, in the Southwest Atlantic sector of the Southern Ocean. Between October 1989 and March 2019 10,112 items of beached debris were recovered from Main Bay, Bird Island, South Georgia in the northern Scotia Sea. The total mass of items (data from 1996 onwards) was 101 kg. Plastic was the most commonly recovered item (97.5% by number; 89% by mass) with the remainder made up of fabric, glass, metal, paper and rubber. Mean mass per item was 0.01 kg and the rate of accumulation was 100 items km−1 month−1. Analyses showed an increase in the number of debris items recovered (5.7 per year) but a decline in mean mass per item, suggesting a trend towards more, smaller items of debris at Bird Island. At Signy Island, South Orkney Islands, located in the southern Scotia Sea and within the Antarctic Treaty area, debris items were collected from three beaches, during the austral summer only, between 1991 and 2019. In total 1304 items with a mass of 268 kg were recovered. Plastic items contributed 84% by number and 80% by mass, with the remainder made up of metal (6% by number; 14% by mass), rubber (4% by number; 3% by mass), fabric, glass and paper (<1% by number; 3% by mass). Mean mass per item was 0.2 kg and rate of accumulation was 3 items km−1 month−1. Accumulation rates were an order of magnitude higher on the western (windward) side of the island (13–17 items km−1 month−1) than the eastern side (1.5 items km−1 month−1). Analyses showed a slight decline in number and slight increase in mean mass of debris items over time at Signy Island. This study highlights the prevalence of anthropogenic marine debris (particularly plastic) in the Southern Ocean. It shows the importance of long-term monitoring efforts in attempting to catalogue marine debris and identify trends, and serves warning of the urgent need for a wider understanding of the extent of marine debris across the whole of the Southern Ocean. [ABSTRACT FROM AUTHOR]