Species composition and ecology of dinoflagellates from the Southern Ocean near South Georgia

Dinoflagellates were identified and counted from preserved verticalnet haul samples collected from a station grid centred on SouthGeorgia in austral summer 1981 –1982. All species werescarce in comparison with diatoms, the dominant large-celledmicroplankton. Dinophysis antarctica Balech was the most abundantand widespread species. Maximum dinoflagellate numbers werefound at stations forming a belt to the south and west of theisland. Whilst previous analysis of diatom community compositionhad revealed distinct species associations, no differences inspecies composition between this region and areas of low dinoflagellateabundance could be resolved. Since both heterotrophic and photoautotrophictaxa were found in the samples, it is difficult to identifyenvironmental features which could act on most of the dinoflagellatetaxa. The large-scale pattern of dinoflagellate distributionmay reflect the pattern of water circulation along the edgeof the South Georgia continental shelf.     

Trophic cascading in the Polar Frontal Zone of the Southern Ocean during austral autumn 2002

Trophic cascading resulting from coupling among phytoplankton, protozooplankton (2–200 µm) and the mesozooplankton fractions <1,000 µm and <2,000 µm was investigated at three stations in the Polar Frontal Zone of the Southern Ocean during austral autumn 2002. In the absence of any grazers, net growth rate of the phytoplankton was positive and ranged between 0.209 and 0.402 day^–1. Among the heterotrophic components of the zooplankton, protozooplankton were identified as the most important consumers of the phytoplankton biomass. The low impact of the mesozooplankton on the phytoplankton may have been related to the inability of the larger grazers to feed efficiently on the small phytoplankton cells, which dominated the total chlorophyll concentration at two of the three sampling stations. It appears that the mesozooplankton <1,000 µm consumed protozooplankton, which resulted in a decrease in the impact of the latter organisms on the phytoplankton. The presence of predatory zooplankton (mainly chaetognaths and amphipods) >1,000 µm led to a decrease in the carnivory of the <1,000-µm mesozooplankton fraction on the protozooplankton. In this situation, protozooplankton were again able to exert a considerable impact on the phytoplankton.     

Trophodynamics of selected mesozooplankton in the west-Indian sector of the Polar Frontal Zone, Southern Ocean

Mesozooplankton community structure and grazing impact were investigated at 15 stations in the west-Indian sector of the Polar Frontal Zone during the third dynamics of Eddie impacts on Marions ecosystem cruise, conducted during April 2004. An intense frontal feature, likely the convergence of the Sub-Antarctic and Antarctic Polar Fronts, was identified running in a north-eastward direction across the survey area. Total integrated chlorophyll-a (chl-a) biomass ranged from 4.15 mg m^–2 to 22.81 mg m^–2 and was dominated by picophytoplankton at all stations. Mesozooplankton abundances ranged from 163.84 ind m^–2 to 2,478.08 ind m^–2 and biomass between 6.70 mg Dwt. m^–2 and 23.40 mg Dwt. m^–2. The mesozooplankton community was dominated almost entirely by copepods, which contributed between 35% and 79% (mean=63%; SD=±12%) of the total numbers. The pteropoda, Limacina retroversa, contributed up to 30% (mean=10%; SD=± 8%) of the total numbers. Numerical analysis identified two distinct mesozooplankton communities separated by the intense frontal feature, namely the Antarctic and the Sub-Antarctic Zone Groups. Ingestion rates of the four numerically dominant copepod species (Calanus simillimus, Clausocalanus spp., Ctenocalanus spp. and Oithona similis) and the pteropod, L. retroversa, were estimated using the gut fluorescence technique. Total grazing impact ranged from 0.156 mg (pigm) m^–2 to 2.958 mg (pigm) m^–2 or between 1% and 29% of the available chl-a per day. The four copepods contributed approximately 36% of the total daily grazing impact, while the pteropod contributed to a mean of 64%, indicating that this zooplankton group may play an important role in the Southern Ocean carbon cycle. In general, the highest daily grazing impact was exhibited in the Antarctic Zone Group (mean=12% phytoplankton standing stock per day).     

Size-fractionated Primary Production in the South Atlantic and Atlantic Sectors of the Southern Ocean

Results are presented from size fractionated chlorophyll a (Chla) and primary production studies along a transect between Antarcticaand southern Africa during the second South African AntarcticMarine Ecosystem Study (SAAMES II), conducted in late australsummer (January to February) 1993. Total integrated Chl a alongthe transect was highest in the vicinity of the Marginal IceZone (MIZ) and Antarctic Polar Front (APF). At these stations,integrated Chl a biomass was always >25 mg Chl a m^–2and was dominated by microphytoplankton. Although nominal increasesinChl a biomass were also associated with the Subantarctic Front(SAF) and Subtropical Convergence (STC), total Chl a biomassin these regions was dominated by nanophytoplankton. Withinthe inter-frontal regions, total integrated Chl a biomass waslower, generally <25 mg Chl a m^–2, and was always dominatedby nanophytoplankton. An exception was found in the AgulhasReturn Current (ARC) where picophytoplankton dominated. Totaldaily integrated production along the transect ranged between60 and 436 mg C m^–2 day^–1. Elevated production rateswere recorded at stations occupied in the vicinity of the MIZand at all the major oceanic frontal systems. The contributionsof the various size fractions to total daily production displayedthe same spatial pattern as integrated biomass, with microphytoplanktonbeing the most important contributor in areas characterizedby elevated phytoplankton biomass. Outside these regions, nanophytoplanktondominated the total phytoplankton production. Again, an exceptionwas found in the ARC north of the STC where picophytoplanktondominated total production. There, the lowest production alongthe entire transect was recorded, with total daily integratedproduction always <90 mg C m^–2 day^–1. The increasedproduction rates recorded in the MIZ appeared to result fromincreased water column stability as indicated by a shallow mixed-layerdepth. Within the inter-frontal regions, the existence of adeep mixed layer appeared to limit phytoplankton production.Low silicate concentrations in the waters north of the APF mayalso have limited the growth of large microphytoplankton.     

Biodiversity and structure of the suprabenthic assemblages from South Shetland Islands and Bransfield Strait,Southern Ocean

During the austral summer 1995, suprabenthic samplings were carried out at 24 stations (depth range 45–649 m) located around Livingston Island, within the caldera of Deception Island and in the Bransfield Strait. At each station, the near-bottom motile fauna was simultaneously collected with a multinet Macer-GIROQ sled in three water layers above the bottom. This study presents original data on the occurrence, diversity, vertical distribution and abundance of suprabenthic taxa in this near-bottom environment. The most speciose taxa were amphipods (at least 140 spp.), followed by isopods (66 spp.), pycnogonids (31 spp.) and mysids (19 spp.). Total abundances ranged between 31 ind./100 m² (Bransfield Strait, 361 m depth) and 6817 ind./100 m² (South Livingston Island, 163 m depth). According to stations, the groups numerically dominant and more frequent were amphipods (17 stations) or mysids (seven stations). Four suprabenthic assemblages were discriminated in the study area, apparently more structured by the degree of shelter-exposure and development of sessile epifauna than by water depth or sediment features.     

Diverse trends in shell weight of three Southern Ocean pteropod taxa collected with Polar Frontal Zone sediment traps from 1997 to 2007

The impact of ocean acidification on key ocean calcifiers is predicted to be imminent, particularly in high-latitude ecosystems. Long-term field observations are essential to ground truth predictions of change in regional ecosystems. Here, we report on aragonitic pteropods collected to sediment traps at 800 m depth at 54°S, 140°E in the Polar Frontal Zone (PFZ) of the Southern Ocean from 1997 to 2007. Statistically significant trends were not identified in either mass or number flux from 1997 to 2007; however, differences emerged in decadal trends seen in shell weight for each of the three common taxa collected: Limacina helicina antarctica forma antarctica shells became significantly lighter (P < 0.05), L. retroversa australis shells became significantly heavier (P < 0.05) and L. helicina antarctica forma rangi shells did not change significantly. These results suggest that factors other than ocean acidification affect pteropod population variations on decadal timescales, with the potential to either amplify or counter the impact of decreasing aragonite saturation state, at least in the short term. Comparison to sea surface temperature and chlorophyll biomass did not identify these as significant drivers of the observed changes, and attribution across these multiple variables requires better understanding of pteropod physiology and ecology. Our PFZ pelagic pteropod observations provide a reference for evaluation of southern polar pteropod responses to changing ocean conditions in coming decades. Importantly, these data also raise the issue of taxonomic care when monitoring the region for impacts of ocean acidification on calcifiers.     

Chlorophyll/nutrient characteristics in the water masses to the north of South Georgia, Southern Ocean

Chlorophyll a and nutrient concentrations along with temperature and salinity values were measured at 22 CTD stations along a 735-km transect running to the northwest of the island of South Georgia, Southern Ocean. Measurements were repeated during five summer surveys (January and February 1994, January 1996, December 1996, January 1998) and one spring survey (October 1997). The transect sampled Sub-Antarctic Zone water in the north, Polar Frontal Zone water and Antarctic Zone water in the south. Chlorophyll a concentrations were lowest to the north of the transect and frequently high (up to 17 mg m⁻³) in the deep open ocean of the Antarctic Zone. Sub-surface peaks were measured in all zones and chlorophyll a was detectable to a depth of 150 m. There was a clear latitudinal temperature gradient in the near-surface waters (0–50 m), the warmest water occurring in the north (∼12 °C), and the coolest in the Antarctic Zone (∼2 °C). There was also a well-defined latitudinal gradient in summer near-surface silicate concentrations (∼2, 4, and 10 mmol m⁻³ in the Sub-Antarctic Zone, the Polar Frontal Zone and the Antarctic Zone, respectively), increasing to >20 mmol m⁻³ near South Georgia. Distinct differences in silicate concentrations were also evident in all three zones to a depth of 500 m. Near-surface nitrate and phosphate concentrations were relatively low to the north of the transect (∼14 and 1 mmol m⁻³, respectively) and higher in the Polar Frontal Zone and Antarctic Zone (∼18 and 1.4 mmol m⁻³, respectively). Ammonium and nitrite were restricted to the upper 200 m of the water column, and exhibited sub-surface concentration peaks, the lowest being in the Sub-Antarctic Zone (0.68 and 0.25 mmol m⁻³, respectively) and the highest in the Antarctic Zone (1.72 and 0.29 mmol m⁻³, respectively). Surface (∼6 m) spring nutrient measurements provided an indication of pre-bloom conditions; ammonium and nitrite concentrations were low (∼0.27 and 0.28 mmol m⁻³, respectively), while silicate, nitrate and phosphate concentrations were high and similar to previously measured winter values (e.g. ∼26, 23, 2 mmol m⁻³, respectively in the Antarctic Zone). Although the values measured were very variable, and there was some evidence of a seasonal growth progression, the chlorophyll a and nutrient distribution patterns were dominated by intercruise (interannual) factors. Approximate nutrient depletions (spring minus summer) appeared similar in the Polar Frontal Zone and Antarctic Zone for nitrate and phosphate, while silicate showed a marked latitudinal increase from north to south throughout the transect. Highest chlorophyll a concentrations coincided with the highest apparent silicate depletions over the deep ocean of the Antarctic Zone. In this area, relatively warm, easterly flowing Antarctic Circumpolar Current water meets cooler, westerly flowing water that is influenced by the Weddell-Scotia Confluence and is rich in nutrients, especially silicate. Accepted: 27 November 1999     

Observations of cetaceans in the south-east Atlantic sector of the Southern Ocean,during summer 2008

Knowledge of cetacean species composition and their distribution in the south-east Atlantic sector of the Southern Ocean is scarce. During a survey in February–March 2008, systematic whale sightings were carried out along transect lines following the 5° and 15° E meridians between 35° and 67° S. In total, 67 toothed whales and 126 baleen whales were observed. Both fin whales (four animals) and Antarctic minke whales Balaenoptera bonaerenses (three animals) in addition to 16 individuals of unidentified species were among the observed baleen whales. The dominating baleen whale species in our study was humpback whales Megaptera novaeangliae with 108 individuals observed. They occurred single or in groups up to seven individuals (N _mean = 2.5 ind) and eight of the counts were of calves. The relationship between humpback whale occurrence and environmental variables including Antarctic krill (Euphausia superba) abundance from acoustic recordings, hydrography, bathymetry and production was tested using general additive models. Only temperature increased the predictive power of the model with whale occurrence increasing with the decreasing temperature in more southern areas.     

Biology of Salpa thompsoni in waters adjacent to the Ross Sea, Southern Ocean, during austral summer 2008

Depth-stratified vertical sampling was carried out during the New Zealand International Polar Year cruise to the Ross Sea on board the RV Tangaroa in February–March 2008. The distribution (horizontal and vertical), density and population biology of Salpa thompsoni were investigated. Salps were found at two of the four major sampling locations, e.g. near the continental slope of the Ross Sea and in the vicinity of seamounts to the north of the Ross Sea. Both abundance and biomass of S. thompsoni were highest near the seamounts in the Antarctic Circumpolar Current reaching ~2,500 ind 1,000 m⁻³ and 8.2 g dry wt 1,000 m⁻³ in the water column sampled. The data showed that S. thompsoni populations were able to utilize horizontal and vertical discontinuities in water column structure, in particular the warm Circumpolar Deep Water (CDW), to persist in the high Antarctic. Although salps appeared to continue migrating to the surface colder layers to feed, both aggregate chain and young embryo release seem to be restricted to the CDW. This study for the first time has provided evidence that low Antarctic salp species may successfully reproduce in the hostile high Antarctic realm.     

Trends in Ocean Colour and Chlorophyll Concentration from 1889 to 2000, Worldwide

Marine primary productivity is an important agent in the global cycling of carbon dioxide, a major ‘greenhouse gas’, and variations in the concentration of the ocean''s phytoplankton biomass can therefore explain trends in the global carbon budget. Since the launch of satellite-mounted sensors globe-wide monitoring of chlorophyll, a phytoplankton biomass proxy, became feasible. Just as satellites, the Forel-Ule (FU) scale record (a hardly explored database of ocean colour) has covered all seas and oceans – but already since 1889. We provide evidence that changes of ocean surface chlorophyll can be reconstructed with confidence from this record. The EcoLight radiative transfer numerical model indicates that the FU index is closely related to chlorophyll concentrations in open ocean regions. The most complete FU record is that of the North Atlantic in terms of coverage over space and in time; this dataset has been used to test the validity of colour changes that can be translated to chlorophyll. The FU and FU-derived chlorophyll data were analysed for monotonously increasing or decreasing trends with the non-parametric Mann-Kendall test, a method to establish the presence of a consistent trend. Our analysis has not revealed a globe-wide trend of increase or decrease in chlorophyll concentration during the past century; ocean regions have apparently responded differentially to changes in meteorological, hydrological and biological conditions at the surface, including potential long-term trends related to global warming. Since 1889, chlorophyll concentrations have decreased in the Indian Ocean and in the Pacific; increased in the Atlantic Ocean, the Mediterranean, the Chinese Sea, and in the seas west and north-west of Japan. This suggests that explanations of chlorophyll changes over long periods should focus on hydrographical and biological characteristics typical of single ocean regions, not on those of ‘the’ ocean.     

Genetic structuring of Patagonian toothfish populations in the Southwest Atlantic Ocean: the effect of the Antarctic Polar Front and deep-water troughs as barriers to genetic exchange

The environmental and/or life history factors affecting genetic exchange in marine species with potential for high dispersal are of great interest, not only from an evolutionary standpoint but also with regard to effective management. Previous genetic studies have demonstrated substantial differentiation among populations of the Patagonian toothfish around the Southern Ocean, indicating breakdown of gene flow across large distances between inhabited shelf areas. The present study examined genetic structuring through analysis of microsatellite loci and restriction fragment length polymorphism (RFLP) of the mitochondrial ND2 gene and control region of the toothfish population in the SW Atlantic, allowing examination of the relative effects of the Antarctic Polar Front (APF), deep-water troughs and distance between sites. Mitochondrial DNA (mtDNA) data indicated a sharp genetic division between the Patagonian Shelf/North Scotia Ridge and the Shag Rocks/South Georgia samples, whereas microsatellite data showed much less distinct structuring and an intermediate position of the North Scotia Ridge samples. We suggest these data indicate that the APF, as a barrier to larval dispersal, is the major inhibitor of genetic exchange between toothfish populations, with deep-water troughs and distance between sites contributing to genetic differentiation by inhibiting migration of relatively sedentary adults. We also suggest that differences between mtDNA and nuclear DNA population patterns may reflect either genome population size effects or (putative) male-biased dispersal.     

Distribution and demography of Antarctic krill in the Southeast Atlantic sector of the Southern Ocean during the austral summer 2008

This study documents horizontal distribution and demography of Antarctic krill (Euphausia superba) from the Southern Ocean during January–March 2008. The cruise predominantly occurred in CCAMLR Subarea 48.6, where knowledge about the ecosystem is limited. E. superba were not found north of 52°S. The biomass, estimated from trawl catches, was highest (63.09 g/m²) at a station 680 km southeast of Bouvetøya and at two stations 1,400 and 600 km southeast and southwest of Bouvetøya, 54.67 and 61.38 g/m², respectively. Body length ranged from 19 to 61 mm (N = 8,538), with a mean of 42.0 ± 6.4 mm (SD). The overall sex ratio was 1:1, 46.2% males (13.2% adults and 33.0% subadults), 46.1% females (33.6% adults and 12.5% subadults), while 7.5% were juveniles. Trawl stations dominated by adults were found west and north of Bouvetøya. Stations with high proportions of subadults and juveniles were mainly found southeast of the island. Four cluster groups were differentiated: analyzing data on krill sex proportions, maturity stages, hydrography, nutrients and chlorophyll concentrations. Two groups represented stations located in the northern part of the study area, where E. superba were absent; water temperatures were higher and the nutrient concentrations lower compared to the groups where E. superba were present. This study shows that bathymetric features like the North Weddell Ridge including Bouvetøya are important for concentrating krill probably due to water mass characteristics and advective processes which influence regional krill demography. The southern regions of CCAMLR sector 48.6 are essential for understanding regional krill recruitment and production.     

Biodiversity and function of bacteria in the Southern Ocean

A short overview of the biodiversity of Antarctic marine bacteria is given with respect to morphology and metabolic activity. The importance of spatial and temporal variability is described. The physiological adaptation and ecological function of Antarctic marine bacterioplankton are discussed.     

Palaeoceanographical changes recorded by Cenozoic deep-sea ostracod assemblages from the South Atlantic and the Southern Ocean (ODP Sites 1087 and 1088)

Cenozoic palaeoceanography of the SE Atlantic and Southern Oceans has been investigated using Late Eocene/Early Oligocene to Quaternary ostracod assemblages from 49 samples of ODP Sites 1087 and 1088. Although the overall abundance of ostracods is relatively low (means of 17 and 49 specimens per sample at Sites 1087 and 1088, respectively) and there is an apparently high level of endemism (ranging from 50% to 80% at Sites 1087 and 1088), three major changes in the faunal assemblages are identified at Site 1087 (denoted A, B and C) and two at Site 1088 (denoted B' and C'). The assemblage boundaries, detected on the basis of stepwise changes in the abundance, diversity, dominance, endemism, faunal turnover and relative abundance of common taxa, coincide broadly with previously identified, ostracod-based palaeoceanographical 'events' discussed by Benson and co-workers over the last two decades. The data do not extend sufficiently far back to record the initiation of Assemblage A, but the faunal change between Assemblages A and B, marked by a decline in abundance, species diversity and faunal turnover, occurs within the Middle Miocene (NN5-6). It coincides with a previously documented palaeoceanographical 'event' at 16-14 Ma which, we suggest, may be related to the initiation of North Atlantic Deep Water (NADW) production and/or an expansion of the East Antarctic ice sheet. Assemblage B' is subdivided into the two Sub-assemblages B'¹ and B'² mainly on the basis of an increase in diversity, a peak in faunal turnover and a drop in the relative abundance of the genus Krithe in early Late Miocene time (NN9, c. 10.5 Ma). The B'¹/B'² Sub-assemblage boundary cannot be related to any previously documented faunal change in deep-sea ostracods. Changes associated with the boundaries between Assemblages B and C, and B' and C', which we believe to be synchronous, both include a decrease in diversity and abundance. In addition, two strong turnover peaks occur near the B'/C' boundary at Site 1088. The B/C and B'/C' boundaries coincide with a previously documented midPliocene 'event' (3.5 Ma) (NN15-16) which may be linked to putative closure of the Straits of Panama and increased production of NADW, the latter in turn leading to increased production of Antarctic Intermediate Water (AAIW) and Antarctic Bottom Water (AABW). Alternatively, fiuctuations in size of the Antarctic ice sheet during possible Pliocene warm periods could indirectly be responsible for the observed midPliocene faunal changes.     

Deep-sea pelagic ichthyonekton diversity in the Atlantic Ocean and the adjacent sector of the Southern Ocean

Aim Deep‐sea pelagic diversity is poorly understood. Local (SL) and regional (SR) ichthyonekton species richness are presented and analysed with respect to local and regional environmental factors, and biogeographical processes. Location Sixty‐six stations from the Atlantic Ocean and adjacent sector of the Southern Ocean, 65° N to 57° S. Methods Estimation of SL by means of rarefaction. Stepwise evaluation of SL and SR relationships by means of the second‐order corrected Akaike information criterion (AICc) after locally weighted scatterplot smoothing (LOESS) and linear fitting, analysis of saturation effects by means of slopes of species accumulation curves (log–log plots). Results Latitudinal gradients were present for SL and SR, and were asymmetric between the Northern and Southern hemispheres. Relatively low species richness was encountered for the Southern Ocean. Asymmetry at the regional level by means of higher SR was attributed to area effects in the South Atlantic. Log–log plots indicated saturation of local assemblages and dependence on environmental factors. SL was related to productivity; this relationship was hump‐shaped. SR was positively related to area size and negatively to seasonality of production. Biogeographical effects were indicated in that SR peaks coincided with overlap zones of boreal and tropical faunas as a consequence of historical faunal exchange processes. Main conclusions The stepwise approach allowed for distinction between effects of area size, productivity and biogeographical processes on diversity at local and regional scales. Productivity in particular is important in two ways. At the local scale, the link of productivity to SL is explained by a successional‐functional hypothesis of resource utilization, whereas the seasonality effect for SR reinforces the hypothesis of dependence of deep‐sea fishes on seasonality of production through changes of life‐history traits. The causes of low Antarctic faunal diversity remained unresolved.