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Although small zooplankton are an important component of both the classical food web and microbial loop, fundamental information about small zooplankton in the Southern Ocean is lacking. This study presents the interannual and horizontal variabilities of small zooplankton and the population structure of the key small copepod species Ctenocalanus citer off Vincennes Bay, East Antarctica. Zooplankton were sampled with a 100 µm mesh net during austral summer from 2016 to 2018. Small copepods numerically dominated all samples, with Oithona spp., Oncaeidae, C. citer and nauplii being particularly abundant (> 75% of total zooplankton abundance). Community structure analysis revealed annual and latitudinal changes in the community, which were driven by the timing of sea-ice retreat and associated with the phytoplankton bloom. The population structure of C. citer showed variability not only with latitude but also between eastern and western areas. The water temperature gradient formed by cyclonic eddies likely influences the developmental period of C. citer, thereby generating the east–west difference in population structure. Spatial differences in the population structure of copepods attributable to cyclonic eddies are likely common during summer in East Antarctica. Our results demonstrate that the development of C. citer population structure is synchronised with short-term fluctuations in phytoplankton size composition following sea-ice retreat, suggesting that the phytoplankton bloom is efficiently used for population growth in the seasonal ice zone. These data suggest that small zooplankton respond rapidly to local environmental changes. [ABSTRACT FROM AUTHOR]

The Antarctic Marginal Ice Zone (MIZ) accounts for 15% of the Southern Ocean's primary production (PP), but limited data has hindered understanding of its variability and connection to carbon export. Using a combination of gliders, biogeochemical Argo floats and satellite observations in the northeast Weddell Sea, we show that years with more sea‐ice formation over winter are followed by more intense phytoplankton blooms (∼15% greater daily PP) and export to 100 m (∼50% higher daily carbon export) the following summer. However, the carbon export beyond the deepest winter mixed layer did not vary in proportion to PP, suggesting different drivers of carbon export at depth compared to surface waters. Furthermore, across the entire MIZ, the response of blooms to sea‐ice volume was spatially variable, indicating the need to consider spatial heterogeneity in the response of the biological carbon pump to future sea‐ice changes. Plain Language Summary: Algae in the ocean surface take up carbon dioxide from the atmosphere through photosynthesis and transfer it to the deep ocean when they die and sink. This process is key to maintaining a habitable planet and is known as the biological carbon pump (BCP). The seasonally ice‐covered ocean around Antarctica is one of the most active areas for algal growth, but also a region of rapid climate change. Because of the difficulty in taking measurements in this remote region, the physical and biological processes that control the growth and sinking of algae and its response to changing sea‐ice remain uncertain. In this study, we use a combination of satellites and autonomous robots to elucidate the role of sea‐ice variability on the BCP. We find that sea‐ice impacts algal growth by its influence on both the light and nutrient conditions needed for photosynthesis. Predicting the amount of algae that subsequently sinks to depth as carbon flux, although influenced by sea‐ice conditions, is more complex and linked to the greater marine ecosystem. Evidence suggests that the species of algae, zooplankton grazing, and the rate at which dead algae breaks down and sinks are important and should be a focus point for further research. Key Points: High‐resolution in‐situ observations are used to characterize multi‐year phytoplankton bloom phenology and amplitude in the Antarctic Marginal Ice ZoneYears with greater sea‐ice volume drive deeper mixing that tend to support higher magnitude blooms in the northeast Weddell SeaCarbon export efficiency is affected by bloom magnitude, community composition and water column stratification [ABSTRACT FROM AUTHOR]

Perciform fish of the suborder Notothenioidei radiated in the cold marine environments of the Southern Ocean around Antarctica where they evolved a diverse array of ecological forms. Despite their relatively high abundance and important roles in the Antarctic and sub-Antarctic ecosystems, many aspects of their life history are still poorly known. In the present study, age, growth and sexual maturity of channel bull blenny Cottoperca trigloides were analysed using the data of 28,229 specimens collected over the last 30 years in the southern part of the Patagonian Shelf in the Southwest Atlantic. It was found that C. trigloides features sexual dimorphism with males attaining larger sizes than females, an unusual characteristic for notothenioid fish. Males also had higher growth rates, older maximum ages and slower maturation. The peak in gonad maturation and spawning in C. trigloides was revealed to take place in austral winter, similar to other notothenioids from the Patagonian Shelf, thus matching larval hatch with the spring bloom of zooplankton. Sexual dimorphism may be advantageous for large territorial males during competitive interactions, particularly in an environment characterised by strong patchiness of rocky areas that are most suitable for spawning. Evolution of life history traits that placed C. trigloides close to the r-end within the r/K strategy continuum evolved by notothenioid fishes, enabled this species to successfully compete with other medium-large and large predators in resources-rich environment of the Patagonian Shelf. [ABSTRACT FROM AUTHOR]

For the Ross Sea, the only Marine Protected Area in Antarctica, available data on the tintinnid ciliates of the marine microzooplankton are mostly limited to nearshore waters near Terra Nova Bay or the vicinity of the McMurdo Sound. Here, we report results from a geographically extensive sampling across the Ross Sea conducted in December 2020. Material from plankton net tows (20 µm mesh), made at 38 stations spanning over 30° of latitude, was examined. We found 11 tintinnid species of varying commonality or rarity, many showing considerable morphological variability that is here documented. We found four forms that had not been previously reported from the Ross Sea. Based on our findings and previous reports, we assembled a species accumulation curve showing the growth in the inventory of tintinnid species recorded from the Ross Sea as a function of sampling effort and time since 1983. Extrapolation of the species accumulation curve, derived from sampling over the last 37 years, indicates that continued sampling will likely provide new species records, suggesting that the Ross Sea is under-sampled at present. This complicates efforts to detect temporal changes in species compositions, at least with regard to tintinnid ciliates. Comparing species accumulation curves for the Ross Sea and the relatively well-studied Weddell Sea, it appears that the Ross Sea may be more species-rich. [ABSTRACT FROM AUTHOR]

Seasonal changes in the zooplankton swimmer (those actively swimming into sediment traps) and faecal pellet contribution to particulate organic carbon (POC) were analysed based on sediment trap samples deployed at 450 m in the polynya region of Prydz Bay during March 2013–February 2014. The zooplankton swimmer flux ranged from 6.18 ind m−2 day−1 to 20.00 ind m−2 day−1 and pteropods (mainly Limacina helicina) were the most abundant group. Part of the L. helicina population could make a large contribution to the organic carbon flux by passive sinking after death during the austral autumn and winter. The potential contribution of sinking L. helicina to the POC flux could reach 15.21% during winter. Four types of faecal pellets were found: cylindrical, round, ellipsoidal and ovoid. The faecal pellet carbon flux (0.01–4.36 mg C m−2 day−1) contributed 5.54%–62.00% of the POC flux. Cylindrical faecal pellets, which had low abundance in winter but higher abundance in the early austral summer, were attributed to the ice krill Euphausia crystallorophias. The faecal pellet carbon flux showed a high value (1.03 mg C m−2 day−1) but made little contribution (9.18%) to the POC flux during early summer (December) in relation to the large amount of ungrazed algae. The high faecal pellet carbon flux (1.98 mg C m−2 day−1) during mid-summer (January) made the greatest contribution (62.00%) to the POC flux. During autumn and winter, the faecal pellet carbon flux was low and ovoid pellets (mostly generated by L. helicina) were the most abundant. The occurrence of intact fresh pellets and faecal fluff (degraded faecal pellets) in the winter samples suggested that zooplankton activities (diel vertical migration and fragmentation of faecal pellets) had an effect on the carbon flux. The results demonstrated that zooplankton could play an important role in the carbon flux in the polynya region of Prydz Bay. • The first report on contribution of zooplankton faecal pellets in the polynya of Prydz Bay, Antarctica. • The potential contribution of sinking Limacina helicina to POC flux could reach 15.21% during winter. • Zooplankton faecal pellets and biological activities could have great role in the carbon flux in the polynya of Prydz Bay. [ABSTRACT FROM AUTHOR]

Understanding how sea ice serves zooplankton species during the food-limited season is crucial information to evaluate the potential responses of pelagic food webs to changes in sea-ice conditions in the Southern Ocean. Stable isotope analyses ( 13 C/ 12 C and 15 N/ 14 N) were used to compare the dietary preferences and trophic relationships of major zooplankton species under pack ice during two winter–spring transitions (2007 and 2012). During sampling, furcilia of Euphausia superba demonstrated dietary plasticity between years, herbivory when feeding on sea-ice biota, and with a more heterotrophic diet when feeding from both the sea ice and the water column. Carbon isotope signatures suggested that the pteropod Limacina helicina , small copepods Oithona spp., ostracods and amphipods relied heavily on sea-ice biota. Post larval E. superba and omnivorous krill Thysanoessa macrura consumed both water column and ice biota, but further investigations are needed to estimate the contribution from each source. Large copepods and chaetognaths overwintered on a water column-based diet. Our study suggests that warm and permeable sea ice is more likely to provide food for zooplankton species under the ice than the colder ice. [ABSTRACT FROM AUTHOR]

Sea ice is an influential feature in Southern Ocean-Antarctic marine environments creating a 2-phase vertical ecosystem. The lack of information on how this system influences community structure during the winter-spring transition, however, is largely lacking. Zooplankton form the link that bridges these environments, with the meiofaunal and algal communities within sea ice directly influencing the epipelagic zooplankton community at the ice-water interface. A combination of methods including sea-ice coring, umbrella net sampling and Laser Optical Plankton Counter were used to describe the vertical structure of zooplankton and meiofaunal communities. The distribution of meiofauna and chlorophyll a both played important roles in structuring the zooplankton community within this dynamic region. Many dominant taxa, including Calanus propinquus and Oithona similis , directly responded to the high availability of algae present within the bottom strata of sea ice. The sea-ice associated species Stephos longipes represented a strong link between this 2-phase ecosystem. Observations of the vertical distribution of biomass obtained from the LOPC suggests that the responses of these species to the sea ice directly influences the vertical structure of zooplankton during the winter-spring transition. [ABSTRACT FROM AUTHOR]

The circum-Antarctic Southern Ocean is an important region for global marine food webs and carbon cycling because of sea-ice formation and its unique plankton ecosystem. However, the mechanisms underlying the installation of this distinct ecosystem and the geological timing of its development remain unknown. Here, we show, on the basis of fossil marine dinoflagellate cyst records, that a major restructuring of the Southern Ocean plankton ecosystem occurred abruptly and concomitant with the first major Antarctic glaciation in the earliest Oligocene (~33.6 million years ago). This turnover marks a regime shift in zooplankton-phytoplankton interactions and community structure, which indicates the appearance of eutrophic and seasonally productive environments on the Antarctic margin. We conclude that earliest Oligocene cooling, ice-sheet expansion, and subsequent sea-ice formation were important drivers of biotic evolution in the Southern Ocean. [ABSTRACT FROM AUTHOR]

Abstract: The kinetics of food processing by zooplankton affects both their energy budgets and the biogeochemical fate of their fecal pellets. We sampled 40 schools of krill across the Scotia Sea during spring, summer and autumn and found that in all 3 seasons, every aspect of their absorption and defecation varied greatly. The C content of fecal pellets varied from 0.85% to 29% of their dry mass (median 9.8%) and C egestion rates varied 75-fold. C:N mass ratios of pellets ranged from 4.9 to 13.2 (median 7.8), higher than values of 3.9 in the krill and 5.4 in their food, pointing to enhanced uptake of N. Pellet sinking rates equated to 27–1218md−1 (median 304md−1), being governed mainly by pellet diameter (80–600μm, mean 183μm) and density (1.038–1.391gcm−3, mean 1.121gcm−3). Pellets showed little loss of C or N in filtered seawater over the first 2 days and were physically robust. When feeding rates were low, slow gut passage time and high absorption efficiency resulted in low egestion rates of pellets that were low in C and N content. These pellets were compact, dense and fast-sinking. Conversely, in good feeding conditions much food tended to pass quickly through the gut and was not efficiently absorbed, producing C and N-rich, slow-sinking pellets. Such “superfluous feeding” probably maximises the absolute rates of nutrient absorption. Food composition was also important: diatom-rich diets depressed the C content of the pellets but increased their sinking rates, likely due to silica ballasting. So depending on how krill process food, their pellets could represent both vehicles for rapid export and slow sinking, C and N-rich food sources for pelagic scavengers. C egestion rates by krill averaged 3.4% of summer primary production (and ingestion rates would be 2–10-fold higher than this) so whatever the fate of the pellets, krill are an important re-packager within the food web. While salp pellets tend to sink faster than those of krill, it is the latter that tend to prevail in sediment traps. We suggest that this is because krill schools are more compact, producing “rain showers” of pellets that exceed the capacity of pelagic scavengers to reprocess them. [Copyright &y& Elsevier]

Abstract: Since the Discovery Investigations of the 1920s and 1930s, seawater temperatures have increased in the Atlantic sector by ∼1°C; greater than the global mean rise. The aims of this paper were first to rescue the Discovery macrozooplankton data, second to provide quantitative “baseline” distribution maps, relating these to indices of temperature and food. Our third aim was to use the relationships we derived between abundance and temperature to project the potential affect of a 1°C warming on the Discovery era distribution patterns. Based on the 1m ringnet data retrieved from 615 stations (Nov–March), four taxa comprised >90% of the Antarctic macrozooplankton abundance: Rhincalanus gigas, Thysanoessa spp., Euphausia superba, and Chaetognaths. Most of the taxa, especially the more abundant ones, were warm water species penetrating into Antarctica and thus total macrozooplankton abundance decreased about 100-fold from 50°S to 70°S. While temperature correlated best with distribution at this large scale, food availability (proxied by a present-day satellite-based Chlorophyll a climatology) had a secondary effect, with the major euphausiids Euphausia superba and Thysanoessa spp. concentrated in high chl a areas. A modelled uniform 1°C temperature rise produced a poleward shift for all taxa, but the Antarctic continent blocked this re-adjustment for the high latitude species, constricting their predicted range. More widespread polar/sub-polar species were predicted to increase their penetration into Antarctica by 4–12° in latitude, whereas the poleward shift in potential range of sub-Antarctic taxa were limited by the steep temperature gradient across the Antarctic Polar Front (APF). However, within the Scotia Sea the relatively warm temperatures of the northern Antarctic Zone, abundant food due to iron fertilisation and intense eddy activity provide a “gateway” for northern species to penetrate south of the APF. Our model predictions, based on measured distributional ranges and observed temperature increases, provide a yardstick with which to compare modern day data compilations and assess the potential effects of future temperature increases. [Copyright &y& Elsevier]

Abstract: Recent warming in the Antarctic has led to increased production of icebergs; however, the ecological effects of icebergs on pelagic communities within the Southern Ocean have not been well-studied. We used a 10m2 MOCNESS to collect macrozooplankton and micronekton in the upper 300m of the water column near free-drifting icebergs in the Atlantic sector of the Southern Ocean during three seasons: December 2005 (late spring), June 2008 (late fall) and March-April 2009 (late summer). Communities were dominated in all three seasons by Antarctic krill (Euphausia superba) and salps (Salpa thompsoni), which collectively comprised 60-95% of the community wet biomass in most cases. During our spring and summer cruises, mean biomass was elevated by 3.1-4.3x at a distance of 0.37km from large icebergs vs. 9.26km away. These differences were not statistically significant, and no trend in biomass with distance was apparent in samples from fall 2008, when total biomass was an order of magnitude lower. Biomass levels near icebergs during Dec 2005 and Mar-Apr 2009 were comparable to values reported from marginal ice zones, suggesting that waters around icebergs support macrozooplankton and micronekton communities comparable in magnitude to those in some of the most productive areas of the Southern Ocean. Sample variance also was significantly higher within 1.85km of icebergs during Dec 2005 and Mar-Apr 2009, reflecting increased patchiness on scales sampled by the MOCNESS (20–40×103 m3 filtered per sample). This pattern was not significant during Jun 2008. Large predatory medusae were observed within 1.85km of icebergs and in Iceberg Alley, an area through which icebergs pass frequently, but were virtually absent in areas remote from icebergs. Small euphausiids showed an inverse distribution, with low densities in areas populated by large medusae. A shift in community composition from a near-iceberg assemblage dominated by herbivores to a carnivore-dominated community in Iceberg Alley may reflect a transition from bottom-up to top-down control with increasing distance and time. Body sizes of dominant species varied seasonally but did not show consistent trends with distance from icebergs. Concentrations of photosynthetic pigments in the guts of E. superba and S. thompsoni corresponded broadly to patterns in surface chlorophyll a concentrations and were comparable to maximum gut pigment concentrations measured in animals collected from highly productive marginal ice zones. Our results suggest that the macrozooplankton and micronekton assemblages near free-drifting icebergs can be quantitatively and qualitatively different from those in surrounding, iceberg-free waters, perhaps due to both bottom-up and top-down processes as well as physical forcing by the passage of a large object through the upper ocean. [Copyright &y& Elsevier]

Abstract: A small ROV was used to collect plankton, make video surveys and take hydrographic measurements in close proximity to six free-drifting, Antarctic icebergs. The icebergs studied ranged in size from <0.5 to >32km in length. Large icebergs have a greater scale of influence than do smaller ones and iceberg-mediated differences in the hydrographic characteristics of their surrounding water depend on the scale sampled. Irrespective of size, temperature generally decreased in close proximity to an iceberg while salinity increased. Chlorophyll a was often lower in the surface waters near the iceberg, relative to the surface waters further away. Tabular icebergs typically had 3 distinct underwater features: shelf, side and bottom. Ablation pockets were a common feature of subsurface ice. The ice itself is a dynamic and seemingly harsh environment with relatively few macrofauna living on it. Those that do inhabit the ice face are either highly specialized or highly mobile. Species composition of zooplankton within 40m of an iceberg did not change relative to distance. However, biomass was generally greater within 5m of an iceberg than it was 15 to 40m distant. [Copyright &y& Elsevier]

Abstract: Free-drifting icebergs in the Weddell Sea are expected to affect the surrounding marine ecosystem. Sampling associated with iceberg C-18a, a large tabular, free-drifting iceberg in the NW Weddell Sea, carried out from 10 March to 7 April 2009, was designed to test the hypothesis that the iceberg''s presence modified phytoplankton composition and abundance. Areas that define a gradient of possible iceberg influence were sampled for phytoplankton: stations close (<1km) and far (18km) from iceberg C-18a, an area with numerous small icebergs, Iceberg Alley, and a control site 74km away. Quantitative samples were obtained from Niskin bottles and counted with an inverted microscope for species abundance. Qualitative samples were collected with nets from the ship''s seawater intake. Taxonomic determinations were performed with light and electron microscopy. Overall, diatoms dominated in the mixed layer (surface-∼40m) and unidentified small flagellated and coccid cells at depth (∼100m). Fragilariopsis nana, a diatom 2.4–15.5μm in length, dominated numerically the phytoplankton and was most abundant at the control area. The iceberg''s effect on phytoplankton composition was consistent with the hypothesis that they facilitate phytoplankton communities enriched in diatoms, as found in other productive areas of Antarctica. Near the iceberg, diatoms were most abundant, principally at depth, while small flagellate concentration diminished. However, total phytoplankton abundance was lowest at Iceberg Alley in the area of highest meltwater contribution, as indicated by low mean temperature in the mixed layer, and highest at the control site. These results suggest that during austral fall, low growth or high zooplankton grazing could be counteracting the positive effect by icebergs on phytoplankton biomass, otherwise observed in summer months. [Copyright &y& Elsevier]

Feedbacks on climate change so far identified are predominantly positive, enhancing the rate of change. Loss of sea-ice, increase in desert areas, water vapour increase, loss of tropical rain forest and the restriction of significant areas of marine productivity to higher latitude (thus smaller geographical zones) all lead to an enhancement of the rate of change. The other major feedback identified, changes in cloud radiation, will produce either a positive feedback, if high level clouds are produced, or a negative feedback if low level clouds are produced. Few significant negative feedbacks have been identified, let alone quantified. Here, we show that the loss of ice shelves and retreat of coastal glaciers around the Antarctic Peninsula in the last 50 years has exposed at least 2.4 × 104 km2 of new open water. We estimate that these new areas of open water have allowed new phytoplankton blooms containing a total standing stock of ∼5.0 × 105 tonnes of carbon to be produced. New marine zooplankton and seabed communities have also been produced, which we estimate contain ∼4.1 × 105 tonnes of carbon. This previously unquantified carbon sink acts as a negative feedback to climate change. New annual productivity, as opposed to standing stock, amounts to 3.5 × 106 tonnes yr−1 of carbon, of which 6.9 × 105 tonnes yr−1 deposits to the seabed. By comparison the total aboveground biomasses of lowland American tropical rainforest is 160–435 tonnes ha−1. Around 50% of this is carbon. On this basis the carbon held in new biomass described here is roughly equivalent to 6000–17 000 ha of tropical rainforest. As ice loss increases in polar regions this feedback will become stronger, and eventually, over thousands to hundreds of thousands of years, over 50 Mtonnes of new carbon could be fixed annually in new coastal phytoplankton blooms and over 10 Mtonnes yr−1 locked in biological standing stock around Antarctica. [ABSTRACT FROM AUTHOR]

The extensive buildup of phytoplankton biomass in the Ross Sea conflicts with the view that high rates of herbivory occur in all regions of the Southern Ocean. Nano and microplanktonic consumers comprise a significant fraction of total plankton biomass; however, the importance of grazing remains uncertain in the Ross Sea. Microzooplankton ingestion of solitary and colonial cells of Phaeocystis antarctica were calculated using a novel live-staining fluorescently-labeled algae method. Different morphotypes of P. antarctica were stained different colors, mixed, and observed inside Euplotes to determine their feeding preference. The blue (7-aminocoumarin) (CMAC) stain was used on the colonies and the green (CMFDA) CellTracker Probe was used on solitary cells. Both morphotypes can be seen inside the food vacuoles of the ciliate, supporting the idea that microzooplankton are capable of ingesting cells within the colonial matrix. This suggests that P. antarctica colonies enter the microbial loop in the Ross Sea before sedimentation. [ABSTRACT FROM AUTHOR]

Here we present a comprehensive review of the diversity revealed by research in limnology and microbial ecology conducted in Byers Peninsula (Livingston Island, South Shetland Islands, Antarctica) during the last two decades. The site constitutes one of the largest ice-free areas within the Antarctic Peninsula region. Since it has a high level of environmental protection, it is less human-impacted compared to other sites within the South Shetland archipelago. The main investigations in Byers Peninsula focused on the physical and chemical limnology of the lakes, ponds, rivers, and wetlands, as well as on the structure of their planktonic and benthic microbial communities, and on the functional ecology of the microbial food webs. Lakes and ponds in Byers range along a productivity gradient that extends from the less productive lakes located upland to the eutrophic coastal lakes. Their planktonic assemblages include viruses, bacteria, a metabolically diverse community of protists (i.e., autotrophs, heterotrophs, and mixotrophs), and a few metazooplankton species. Most of the studies conducted in the site demonstrate the strong influence of the physical environment (i.e., temperature, availability of light, and water) and nutrient availability in structuring these microbial communities. However, top-down biotic processes may occur in summer, when predation by zooplankton can exert a strong influence on the abundance of protists, including flagellates and ciliated protozoa. As a consequence, bacterioplankton could be partly released from the grazing pressure exerted by these protists, and proliferates fueled by external nutrient subsidies from the lake's catchment. As summer temperatures in this region are slightly above the melting point of water, biotic processes, such as those related to the productivity of lakes during ice-free periods, could become even more relevant as warming induced by climate change progresses. The limnological research carried out at the site proves that Byers Peninsula deserves special attention in the framework of the research in extreme environments. Together with nearby sites, such as Signy Island, Byers Peninsula comprises a featuring element of the Maritime Antarctic region that represents a benchmark area relative to the global distribution and diversity of aquatic microorganisms. [ABSTRACT FROM AUTHOR]