Phytoplankton community composition and photosynthetic physiology in the Australian sector of the Southern Ocean during the austral summer of 2010/2011

Phytoplankton population dynamics play an important role in biogeochemical cycles in the Southern Ocean during austral summer. However, the relationship between phytoplankton community composition and primary productivity remains elusive in this region. We investigated the community composition and photosynthetic physiology of surface phytoplankton assemblages in the Australian sector of the Southern Ocean from December 2010 to January 2011. There were significant latitudinal variations in hydrographic and biological parameters along 110°E and 140°E. Surface (5 m) chlorophyll a (chl a) concentrations measured with high-performance liquid chromatography varied between 0.18 and 0.99 mg m^?3. The diatom contribution to the surface chl a biomass increased in the south, as estimated with algal chemotaxonomic pigment markers, while the contributions of haptophytes and chlorophytes decreased. In our photosynthesis–irradiance (P–E) curve experiment, the maximum photosynthetic rate normalized to chl a ( \(P_{ \hbox{max} }^{*}\) ), initial slope (α ^*), the maximum quantum yield of carbon fixation (Φ _{c max}), and the photoinhibition index (β ^*) were higher in the region where diatoms contributed >50 % to the chl a biomass. In addition, there were statistically significant correlations between the diatom contribution to the chl a biomass and the P–E parameters. These results suggested that the changes in the phytoplankton community composition, primarily in diatoms, could strongly affect photosynthetic physiology in the Australian sector of the Southern Ocean.     

Photosynthetic oxygen production and community respiration in the Indian sector of the Antarctic Ocean during austral summer

Photosynthetic oxygen production by phytoplankton and community respiration in the Indian sector of the Antarctic Ocean were estimated from changes in oxygen concentrations in light and dark bottles. Gross production varied between 0.1 and 5.1 µmol O₂ l^-1 day^-1. In the same water, community respiration (the sum of oxygen consumption by heterotrophs and phytoplankton) was 0.4-3.6 µmol O₂ l^-1 day^-1, which accounted for 47-343% of the gross production. Algal and heterotrophic respirations were distinguished using some assumptions. These estimates showed that heterotrophic respiration accounted for most of the community respiration (70-91% depending upon the assumptions), indicating that heterotrophic respiration plays an important role in the mineralization of phytoplankton production in the surveyed sea area. Gross production rate correlated with chlorophyll a concentration, showing that the photosynthetic production rate of oxygen depends on the abundance of phytoplankton. Moreover, there was a significant relationship between gross production and community respiration rates. These regression equations suggested that negative net production occurred under the usually low concentration of chlorophyll observed in the Indian sector of the Antarctic Ocean. Hence, the net exchange of carbon dioxide due to biological processes through the sea surface seemed to be not as large as expected in the Antarctic Ocean, although the number of data were limited at this stage.     

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.     

Microzooplankton grazing in the surface water of the Southern Ocean during an austral summer

Microzooplankton grazing was investigated in surface waters of the Indo-Pacific and Atlantic sectors of the Southern Ocean by the dilution method. Phytoplankton growth varied mainly between 0.1 and 0.4 day⁻¹, and microzooplankton grazing between 0.0 and 0.3 day⁻¹. Great fluctuations in phytoplankton growth rate were observed at one station within 3 weeks and between closely spaced stations. Microzooplankton grazing rates were similar to phytoplankton growth rate despite the variation of phytoplankton growth rates, although in some cases, phytoplankton growth overwhelmed microzooplankton grazing. These observations suggest that microzooplankton are the main consumers of primary producers, and that steady state between phytoplankton growth and microzooplankton grazing is usually established in the Southern Ocean in austral summer. Received: 5 November 1996 / Accepted: 4 March 1997     

Sea surface distribution of coccolithophores in the eastern Pacific sector of the Southern Ocean (Bellingshausen and Amundsen Seas) during the late austral summer of 2001

Horizontal distributions of coccolithophores were observed in sea surface water samples collected on the RV Polarstern between 27 February and 10 April, 2001, in the Pacific sector of the Southern Ocean (Bellingshausen and Amundsen Seas). These samples were analyzed to gain information about the distribution of coccolithophores in relation to the oceanic fronts of the Southern Ocean. A total of fifteen species of coccolithophores were identified, showing cell abundances of up to 67 × 10³ cells/l down to 63°S. Emiliania huxleyi was the most abundant taxon, always accounting for more than 85% of the assemblage. The second most abundant species was Calcidiscus leptoporus, with values lower than 7%. Cell density increases significantly in both the Subantarctic and Polar Fronts (155 and 151 × 10³ cells/l, respectively), decreasing abruptly in the intervening Polar Frontal Zone and to the south of the Polar Front. Although temperature at high latitudes is the main factor controlling the biogeographical distribution of coccolithophores, at the regional level (Southern Ocean) the frontal systems, and consequently nutrient distribution, play a crucial role.     

Composition and community structure of pelagic copepods in the Indian sector of the Antarctic Ocean during the end of the austral summer

The present paper describes latitudinal and vertical changes in the composition, abundance and diversity of copepods in the Indian sector of the Antarctic Ocean, during the end of austral summer along a transect on 66°30′E between 43 and 62°S, within three layers (600–0, 200–0, 100–0 m). Highest copepod densities were noted in the central part of the transect, between the Antarctic Divergence and the Antarctic Convergence, with a maximum in the Antarctic Divergence zone, particularly in the upper levels of the water column. A total number of 80 copepod species were identified over the entire survey area. The south end and the central part of the transect comprised a small number of species. North of the Antarctic Convergence, this number increased markedly with the progressive disappearence of those species characteristic of Antarctic waters and their replacement by temperate and subtropical species. Generally, small copepods, particularly Oithona similis, Oithona frigida and Ctenocalanus citer, dominated in numbers in both Antarctic and sub-Antarctic areas. The contribution of large species to total copepod numbers was much lower, with Calanus simillimus in the central part of the transect, Pleuromamma borealis in the subtropical zone and Calanus propinquus in the southern part. Correspondence analysis showed a marked latitudinal gradient in population structure with four groups of samples and species corresponding to four latitudinal zones. Community structure (species richness, relative dominance index, evenness, Shannon species diversity index) and species abundance patterns (as rank-frequency diagrams) suggested that the maturity and species richness increased gradually from south to north. A low diversity index and evenness were observed in the area of the Antarctic Divergence, whereas the convergence zone showed high diversity and evenness. Conversely, the frontal zone showed high diversity and evenness. Distribution appeared unrelated to chlorophyll concentrations and on the large scale was related to the hydrologic characteristics. Received: 8 May 1996 / Accepted: 27 August 1996     

Mesozooplankton biomass,abundance and community composition in the Ross Sea and the Pacific sector of the Southern Ocean

Polar Biology - Due in part to its remote location, the zooplankton of the Ross Sea and adjacent waters is poorly characterized. Very little depth-integrated information exists for this region,...     

Biogeographic structure of the microphytoplankton assemblages of the south Atlantic and Southern Ocean during austral summer

Microphytoplankton distribution in the Atlantic sector of theSouthern Ocean was investigated along a transect during theSAAMES II cruise undertaken in late austral summer (January/February) 1993. Samples were collected at 60 km intervals between34 and 70°S for the analysis of mineral nutrients, and theidentification and enumeration of microphytoplankton. Peaksin microphytoplankton abundance were recorded in the neriticwaters of Africa and Antarctica, at all major oceanic fronts,and in the marginal ice zone (MIZ). Partial correlation analysisindicated that 45% of the total variance associated with microphytoplanktonabundance could be explained by silicate and phosphate concentrations,while temperature accounted for 65% (P<0.001). Cluster andordination analyses identified two major groups of stations,one north and one south of the Subantarctic Front (SAF). Thisdivision appears to be related to differences in temperatureand silicate concentrations. Each region comprised distinctmicrophytoplankton subgroups associated with specific watermasses or hydrological features. Indicator species could beidentified for some water masses. In the MIZ, microphytoplanktonspecies composition and succession were strongly affected bysea-ice throughout the summer.     

Pelagic fish distribution and ontogenetic vertical migration in common mesopelagic species off Lützow-Holm Bay (Indian Ocean sector,Southern Ocean) during austral summer

The horizontal and vertical distributions of fish were examined off Lützow-Holm Bay in the Indian Ocean sector of the Southern Ocean during midnight sun in January 2005. Fish were sampled from six discrete depth layers (0–2,000 m). The most abundant fish in layers from the surface to 200 m were larval stages of Electrona antarctica and Notolepis coatsi. In layers from 200 to 2,000 m, fish assemblages were relatively uniform among all stations and were dominated by E. antarctica (juvenile–subadult), Cyclothone microdon, and Bathylagus antarcticus. Cluster analysis revealed three epipelagic communities related to water temperature and salinity. An ontogenetic habitat shift to deeper layers was apparent for E. antarctica, N. coatsi, and B. antarcticus. Preferences for warm waters were observed in E. antarctica (larvae) and N. coatsi (preflexion to flexion larvae), although they were distributed across a broad range of temperature and salinity in epipelagic zones.     

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.     

Differences of the protozoan biomass and grazing during spring and summer in the Indian sector of the Southern Ocean

The dynamics of protozoa were investigated during two cruises in the Indian sector of the Southern Ocean: the early spring ANTARES 3 cruise (28 September to 8 November 1995) and the late summer ANTARES 2 cruise (6 February to 8 March 1994). Biomass and feeding activity of protozoa were measured as well as the biomass of their potential prey – bacteria and phototrophic flagellates – along the 62°E meridian. The sampling grid extended from the Polar Frontal region to the Coastal and Continental Shelf Zone in late summer and to the ice edge in spring, crossing the Antarctic Divergence. Protozoan biomass, although low in absolute terms, contributed 30% and 20% to the total microbial biomass (bacteria, phytoplankton and protozoa) in early spring and late summer, respectively. Nanoprotozoa dominated the total protozoan biomass. The geographical and seasonal distribution of protozoan biomass was correlated with that of phototrophic flagellates. However, bacterial and phototrophic flagellate biomass were inversely correlated. Phototrophic flagellates dominated in the Sea Ice Zone whereas bacteria were predominant at the end of summer in the Polar Frontal region and Coastal and Continental Shelf Zone. Furthermore, bacteria were the most important component of the microbial community (57% of the total microbial biomass) in late summer. Phototrophic flagellates were ingested by both nano-and microprotozoa. In contrast, bacteria were only ingested by nanoprotozoa. Protozoa controlled up to 90% of the daily bacterial production over the period examined. The spring daily protozoan ingestion controlled more than 100% of daily phototrophic flagellate production. This control was less strong at the end of summer when protozoan grazing controlled 42% of the daily phototrophic flagellate production. Accepted: 30 October 1999     

Protist community composition during spring in an Arctic flaw lead polynya

The overwintering deployment of an icebreaker during the Canadian Flaw Lead study provided an opportunity to evaluate how protist communities (phytoplankton and other single-celled eukaryotes) respond to changing spring irradiance conditions in flaw lead polynyas, where open water persists between the central pack ice and land fast ice. We combined microscopic analysis of the protist communities (all cell sizes) with clone libraries of 18S rRNA genes and 18S rRNA (from RNA converted to cDNA) of size-fractionated seawater (0.2–3.0 μm) from 10 to 12 m depth in the surface mixed layer. The rRNA gene analysis provided information on the presence of organisms, while the rRNA analysis provided information on the most active members of the community. There was little overlap between the two types of clone libraries, and there were large community shifts over time. Heterotrophic dinoflagellates and ciliates were the most common sequences recovered. The relative proportion of photosynthetic protist sequences increased in March and April, and there was greater representation of Bacillariophyta, Prasinophyta, Haptophyta, and Cryptophyta in the rRNA compared to rRNA gene libraries. Microscopy indicated that large-celled diatoms dominated the community in May, when chlorophyll concentrations were greatest. However, the RNA sequencing showed that heterotrophic and putative parasitic protists were proportionately more active, and the concomitant decrease in nutrients suggested that the spring phytoplankton bloom had begun to decline by this time. These observations provide evidence of substantial changes in protist community structure and function during the spring transition.     

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.     

Phytoplankton dynamics at the ice-edge zone of the Lazarev Sea (Southern Ocean) during the austral summer 1994/1995 drogue study

Phytoplankton dynamics in the open waters of the ice-edge zone of the Lazarev Sea were investigated over 12 consecutive days during a drogue study conducted in austral summer (December/January) 1994/1995. Throughout the study, the upper water column (<30 m) was stratified with a well-defined pycnocline evident. Although a subsurface intrusion of colder, more saline water into the region was recorded on days 5-9 of the experiment, its effect on the water column structure was negligible. Total surface chlorophyll a biomass doubled between days 1 and 5 (from 0.82 to 1.62 mg m^-3), and then showed a tendency to stabilise, while the depth-integrated chlorophyll a standing stock displayed an increasing trend during the entire experiment. All changes in biomass were associated with an increase in microphytoplankton. Flagellates and picoplankton dominated cell counts, while diatoms composed most of the phytoplankton biovolume. Results of the study indicate that, during the period of investigation, average cell abundance decreased. Coupled with this decrease was an increase in the biovolume and average size of the phytoplankton. Phytoplankton succession was observed in the ice edge during the drogue study. Typical ice-associated species of genera Haslea, Fragilariopsis and Chaetoceros, which dominated at the beginning of study, were replaced by open-water species of genera Corethron, Dactyliosolen and Rhizosolenia. The shift in phytoplankton species composition and size can likely be related to high light intensities and grazing by microzooplankton. The intrusion of colder, more saline water on day 5 appeared to modify the diatom succession, indicating extreme variability in phytoplankton dynamics in the near ice-edge zone of the Lazarev Sea.     

Plankton community respiration in Bransfield Strait (Antarctic Ocean) during austral spring

Community respiration (R) was determined in Bransfield Straitfrom oxygen changes in water samples incubated in borosilicatebottles maintained at in situ temperature. The respiratory electrontransport system (ETS) activity of seawater communities wasalso measured from the same samples. Both data sets were relatedby the regression equation: log R (mg O₂ m^–3 day^–1)=0.462+0.730xlogETS activity mg O₂ m^–3 day^–1) (r=0.80, n=23). Fromthis equation and 37 ETS activity depth profiles, we calculatedthe integrated (0–100 m) community respiration as beingin the range 1.2–4.5 g O₂ m^–2 day^–1 (mean=2.2).These values do not differ significantly from other publishedresults for the Arctic and Antarctic Oceans. Assuming a respiratoryquotient of unity, the areal respiration ranges between 0.45and 1.69 g C m^–2 day^–1 (mean=0.8). This would representan important sink for the primary production reported for BransStrait. The spatial distribution of community respiration showedhigher values associated with the warmer and phytoplankton-richwaters outflowing from Gerlache Strait into Bransfield Strait,and with the front that separates Bellingshausen Sea watersfrom Weddell Sea waters. We suggest that this pattern of distributionmay be related to the transport of organic matter by the BransfieldCurrent along the front.     

Species composition and distributional pattern of ichthyoplankton of the Southern Pacific Ocean

Distributional pattern and qualitative composition of ichthyoplankton in waters of the Southern Pacific in September 2002?CJanuary 2003 and in August?CNovember 2009 are analyzed. We noted approximately 70 species of fish pelagic eggs and larvae of fishes belonging to 34 families in this region. Change in distribution of the basic object of commercial fishery of this region, i.e., Peruvian-Chilean jack mackerel Trachurus murphyi, in early ontogenesis are considered in detail. The obtained data on the species composition, distribution, and abundance of ichthyoplankton in this region are compared with the results of investigations in the past years. The spawning biomass of jack mackerel is also estimated in the work by the amount of the eggs spawned by females in the places of their greatest concentrations.     

Size-fractionated primary production in the open Southern Ocean in austral spring

Size-fractionated primary production was measured by carbon-14 uptake incubations on three transects between 47°S and 59°30S along 6°W in October/November 1992. Open Antarctic Circumpolar Current and ice-covered Weddell Gyre water showed comparable low productivity (0.3 gCm^–2 day^–1) and size distribution. Picoplankton (<2 m) was the dominant size fraction, contributing approximately half to the total water column production. The significance of larger (>20 m) phytoplankton was only minor. Productivity in the Polar Front Zone north of 50°S, with higher water column stability, was up to 10 times higher with microplankton (>20 m) being predominant. No ice-edge bloom occurred over the 2 months study period; this is explained by non-favourable hydrographic conditions for blooming and the lack of melt-water lenses upon ice retreat. Picoplankton tended to make higher contributions at lower water column stability, and microplankton to make higher contributions at higher stability. Mixing, together with light climate, are discussed as the driving forces for Antarctic primary production and for its size structure.     

Krill larvae in the Atlantic sector of the Southern Ocean during FIBEX 1981

Summary During the First International BIOMASS Experiment krill larvae were sampled by several research vessels. The present paper compiles data gathered by of Polish, German and Argentinian expeditions undertaken during 19 January–19 March 1981 in the Scotia Sea, Drake Passage and Bransfield Strait regions. In the 1980/81 season the most intense spawing occurred at the end of December and in the beginning of January. In the eastern part of the region development of krill larvae was much more advanced than further to the west. The highest numbers of larvae were observed in the central part of the survey area, in the open ocean and on the shelf slope zones. In the Bransfield Strait larvae were most abundant in the north and north-east. These larvae appeard to have drifted to the north and north-east and occurred outside the region of the main concentrations of adult krill. As such, larval distribution reflected a meandering current at a reference level of 500 db, which was flowing in a general south-west to north-easterly direction. The Weddell Scotia Confluence did not form a border either to the occurrence of krill larvae or of adults.