Current patterns in McMurdo Sound,Antarctica and their relationship to local biotic communities

Summary Current speed and direction measurements collected during summer (January–February) and sping (November–December) of 1984 indicated that currents in McMurdo Sound, Antarctica were dominated by oscillatory flow associated with diurnal tidal components (O1, K1, P1). Net flow was southward in the eastern Sound, mixed in the central Sound, and northward in the western Sound. Short term observations (<5 days) from nearshore stations indicated a similar but more sluggish pattern of tidal and mean flow. Hydrographic data collected during the same period indicated a similar pattern of cold water with low chlorophyll a content flowing northward from under the Ross Ice Shelf in the western Sound and denser, slightly warmer water with higher chlorophyll a content flowing southward in the eastern Sound. Previous studies have shown that productivity is higher in the eastern Sound than in the west, apparently due to the circulation pattern. The western Sound consists of waters from beneath the Ross Ice Shelf which have a lower phytoplankton standing stock than eastern Sound waters which enter from the north. More sluggish current speeds in the western Sound result in even lower particle fluxes past benthic consumers. Finally, more persistent ice cover in the west further inhibits in situ primary productivity.     

Distribution patterns of benthic microalgal standing stock at McMurdo Sound,Antarctica

Summary During the austral summer of 1975–76 and winter of 1977 benthic and water column chlorophyll a and phaeopigments were measured at several sites along the east and west sides of McMurdo Sound, Antarctica. Estimates of in situ primary productivity were made at some McMurdo Sound locations. Additionally, water column samples were collected at 5 stations in the Ross Sea during January, 1976. Standing stock data are analyzed to identify seasonal and spatial patterns. Variability in algal standing stock was related to ambient light levels and appeared to be mediated by ice and snow cover whereby the highest algal standing stock was present under high light conditions (low ice and snow cover, shallow water, summer). Differences in published benthic invertebrate densities appear to be closely allied to differences in benthic primary production, and less so to in situ planktonic ice microalgal production.     

Scavenging and other feeding habits of lysianassid amphipods (Orchomene spp.) from McMurdo Sound,Antarctica

Summary The distribution and scavenging habits of the two most abundant lysianassid amphipods in McMurdo Sound differ markedly. Orchomene plebs lives primarily in deep water (>100 m), where planktonic and benthic food is sparser and scavenging events are less common and predictable than in shallower water. Orchomene plebs is common in shallow areas (<100 m) only under the Ross Ice Shelf and along the western McMurdo Sound. Here Weddell seals frequent tidal cracks in which they discard carrion and defecate; otherwise food is scarce. Orchomene pinguides lives on shallow (<10 m) wave-cut benches that are rich in food along the eastern McMurdo Sound. They, along with other omnivorous invertebrates which scavenge the food-rich eastern sound benches, are rare from shallow water along the western sound. The eastern benches are bathed by dense plankton blooms and harbor a high biomass of benthic diatoms and invertebrates. Scavenging events there were observed throughout the year. Orchomene plebs is larger and more motile, and came to laboratory carrion and baited field traps more rapidly and in greater numbers than O. pinguides. The crop contents of O. plebs contained only amorphous organic matter that suggested a scavenging habit. Crops of O. pinguides contained not only amorphous organic matter but also invertebrate prey, especially planktonic copepods that impact the bottom during winter.     

Composition and abundance of zooplankton under the spring sea-ice of McMurdo Sound,Antarctica

Summary Zooplankton was sampled through holes in the sea-ice of McMurdo Sound from 8 November to 10 December, 1985. Replicated vertical hauls were made to 100 and 300 m off Pram Point in the inner Sound, near the edge of the permanent McMurdo Ice Shelf. The zooplankton was sparse, averaging 2.5 mg/m³ wet weight. The numbers of individual species varied between catches, depths, and occasions. Generally, small copepods, particularly Oithona similis, Ctenocalanus citer and Oncaea curvata, numerically dominated the catches, and higher densities of these were present in the shallower 100 m layer. Deeper hauls contained higher numbers of larger crustaceans, particularly copepods Metridia gerlachei, Calanoides acutus and Euchaeta spp., ostracod Conchoecia belgicae and euphausiid Euphausia crystallorophias. Pteropods Limacina helicina and Clione limacina were also consistently caught, but in equal densities in 100 m and 300 m hauls. Numerous other plankters were caught in low numbers, including amphipods, chaetognaths, medusae, radiolarians, and larval nemerteans, barnacles, shrimps, polychaetes and echinoderms. Comparative samples from 40 km further north, off Cape Royds and near the sea edge of the fast sea-ice in Wohlschlag Bay, and to 100 m deep, contained a similar species diversity to those near the McMurdo Ice Shelf, but always with higher densities of L. helicina. On the last sampling occasions, when microalgae were conspicuous under the ice off Cape Royds, there were increased densities of microcopepods and Paralabidocera antarcticus, indicating different ecosystem processes from the inner Sound location.     

Mesozooplankton beneath the summer sea ice in McMurdo Sound,Antarctica: abundance,species composition,and DMSP content

The summer Phaeocystis antarctica bloom increases under-ice phytoplankton biomass in McMurdo Sound, Antarctica. The magnitude of mesozooplankton grazing on this bloom is unknown, and determines whether this production is available to the pelagic food web. We measured mesozooplankton abundance and body content of dimethylsulfoniopropionate (DMSP) during the McMurdo Sound austral summer (2006 and 2006–2007). Abundance varied from 20 to 4,500 ind. m⁻³ (biomass 0.02–274.0 mg C m⁻³), with peaks in mid-December and late-January/February. Abundance was higher but total zooplankton biomass lower in our study compared to previous reports. Copepods and the pteropod Limacina helicina dominated the zooplankton in both abundance and biomass. DMSP was detected in all zooplankton groups, with highest concentrations in copepod nauplii and L. helicina (95 and 54 nmol mg⁻¹ body C, respectively). Experiments suggested that L. helicina obtains DMSP by directly grazing on P. antarctica, which often accumulates to high biomass under the summer sea ice in McMurdo Sound.     

Seasonal shifts in ice edge phytoplankton blooms in the Barents Sea related to the water column stability

The development of the phytoplankton bloom and its relation to water column stabilisation during the transition from early to high summer (of 1991) in the seasonally ice-covered zone of the Barents Sea were studied from a meridional transect of repeated hydrographic/biological stations. The water column stabilisation is described in detail with the aid of vertical profiles of the Brunt-Väisälä frequency squared (N²). The contributions of seasonal warming and ice melting to stabilisation are elucidated by determining the effects of temperature and salinity on N². The spring bloom in 1991 migrated poleward from June to July by about 400 km, associated with the retreat of the ice edge. The spring bloom culminated with maximum chlorophyll concentrations in the mixed layer about 100–300 km north of the centre of the meltwater lens, at its northern edge, where the ice cover was still substantial. From the distribution of N² it becomes obvious that the bloom starts at the very beginning of stabilisation, which results solely from the release of meltwater. The increase in temperature due to the seasonal warming does not contribute to the onset of vernal blooming; temperature starts to contribute to the stratification later, when the spring bloom has ceased due to the exhaustion of nutrients in the mixed layer. By that time a deep chlorophyll maximum has formed in the seasonal pycnocline, 20–30 m below the base of the mixed layer. The effect of the seasonal ice cover on the mean areal new primary production is discussed.     

Composition and succession of dinoflagellates and chrysophytes in the upper fast ice of Davis Station, East Antarctica

Little is known of the wider Antarctic distribution of the upper fast ice community now comprehensively described from McMurdo Sound. We determined the fast ice protist community at Davis Station, East Antarctica and compared it with that of McMurdo Sound. As at McMurdo Sound, Davis fast ice is characterised by extreme and transitory salinities (96–2.5 psu) and temperatures (−4.5 to −0.1°C) during the spring/summer transition. Both communities are dominated by Polarella glacialis (an autotrophic dinoflagellate), chrysophytes and their life cycle stages. Furthermore, the physical parameters of brine temperature and salinity at which these successions occurred approximated those of McMurdo Sound. The high degree of similarity between the communities from the geographically disparate locations indicates that this community type has a circum-Antarctic distribution. Confirming the areal extent and seasonality of this community type will assist in future predictions of sea ice productivity.     

Summer plankton beneath the McMurdo Ice Shelf at white Island,McMurdo Sound,Antarctica

 The zooplankton of the under-shelf-ice ecosystem at White Island (78^°10′ S, 167^°30′ E), McMurdo Sound, Antarctica was investigated during December 1976 and January 1977. The water column was sampled through a hole in the McMurdo Ice Shelf over a water depth of 67 m. Seawater temperatures under the ice shelf ranged from −1.91 to 1.96^°C. Dissolved oxygen levels ranged from 5.0–6.05 ml l^-1 in early December to 4.65–4.8 ml l^-1 in late January. Current speeds of up to 0.13 m s^-1 were recorded at a depth of 50 m and a predominantly northward flow was detected. Light levels under the shelf ice were low with less than 1% of the incident light being transmitted to a depth of 3 m. No chlorophyll a was detected within the water column throughout the investigation. Mean zooplankton biomass values in the water column ranged from 12 to 447 mg wet weight m^-3 and were similar to values recorded elsewhere from Antarctic inshore waters, but were very much higher than those recorded from under seasonal sea ice in McMurdo Sound. Thirty-two zooplankton species were recorded including 1 ostracod, 21 copepods (10 calanoids, 3 cyclopoids and 8 harpacticoids), 4 amphipods, 2 euphausiids, a chaetognath and 3 pteropods. Larvae of polychaetes and fish were found on some occasions. The species composition in general was similar to that recorded from McMurdo Sound and other Antarctic inshore localities. Among the Copepoda, however, there were a number of species, especially among the Harpacticoidea, that have not been found previously in McMurdo Sound and the Ross Sea, but that are known to be associated with ice in other localities in Antarctica. Two recently described species are known only from White Island. They were present in the water column but were most abundant in the surface water of the tide crack where they were the most abundant zooplankters. The tide crack, which probably is an extension of the under-ice habitat, is apparently a significant nursery area for amphipods and copepod species. Received: 23 November 1994/Accepted 7 May 1995     

Influence of sea ice on the composition of the spring phytoplankton bloom in the northern Baltic Sea

During the late winter and spring of 1994, the influence of sea ice on phytoplankton succession in the water was studied at a coastal station in the northern Baltic Sea. Ice cores were taken together with water samples from the underlying water and analysed for algal composition, chlorophyll a and nutrients. Sediment traps were placed under the ice and near the bottom, and the sedimented material was analysed for algal composition. The highest concentration of ice algae (4.1 mmol C m⁻²) was found shortly before ice break-up in the middle of April, coincidental with the onset of an under-ice phytoplankton bloom. The ice algae were dominated by the diatoms Chaetoceros wighamii Brightwell, Melosira arctica (Ehrenberg) Dickie and Nitzschia frigida Grunow. Under the ice the diatom Achnanthes taeniata Grunow and the dinoflagellate Peridiniella catenata (Levander) Balech were dominant. Calculations of sinking rates and residence times of the dominant ice algal species in the photic water column indicated that only one ice algal species (Chaetoceros wighamii) had a seeding effect on the water column: this diatom dominated the spring phytoplankton bloom in the water together with Achnanthes taeniata and Peridiniella catenata. Received: 9 May 1997 / Accepted: 15 February 1998     

Isolated faecal bacterial communities found for Weddell seals,Leptonychotes weddellii,at White Island,McMurdo Sound,Antarctica

The microbial diversity of faecal communities co-existing with mega fauna is not well understood even though these faecal communities are critical for health and development. Additionally, the transfer of microbial taxa among host animals is little studied. Here, we used 16S sequences obtained from clone libraries to characterise the faecal microbiota of Weddell seals breeding in McMurdo Sound and at White Island, Antarctica. Faecal bacterial communities were dominated by four phyla; Actinobacteria (20 %), Bacteroidetes (13 %), Firmicutes (23 %), and Proteobacteria (13 %). We also used automated ribosomal intergenic spacer analysis to examine the dispersal of bacteria between populations of Weddell seals breeding at White Island and in McMurdo Sound. The Weddell seals at White Island are isolated by the Ross Ice Shelf from the larger population of Weddell seals breeding in McMurdo Sound. We found that the faecal bacteria communities of the seals at White Island had lower diversity and that the community composition was significantly different compared with the seals in the McMurdo Sound area.     

Increased sea ice melt as a driver of enhanced Arctic phytoplankton blooming

Phytoplankton primary production in the Arctic Ocean has been increasing over the last two decades. In 2019, a record spring bloom occurred in Fram Strait, characterized by a peak in chlorophyll that was reached weeks earlier than in other years and was larger than any previously recorded May bloom. Here, we consider the conditions that led to this event and examine drivers of spring phytoplankton blooms in Fram Strait using in situ, remote sensing, and data assimilation methods. From samples collected during the May 2019 bloom, we observe a direct relationship between sea ice meltwater in the upper water column and chlorophyll a pigment concentrations. We place the 2019 spring dynamics in context of the past 20 years, a period marked by rapid change in climatic conditions. Our findings suggest that increased advection of sea ice into the region and warmer surface temperatures led to a rise in meltwater input and stronger near-surface stratification. Over this time period, we identify large-scale spatial correlations in Fram Strait between increased chlorophyll a concentrations and increased freshwater flux from sea ice melt.     

Seasonal Patterns of Planktonic Production in McMurdo Sound, Antarctica

The prolonged periods of continuous darkness and light in polarregions have resulted in a unique seasonal partitioning of primaryand heterotrophic production. In McMurdo Sound for example,the biomass, size distribution and production by phytoplanktonand bacterioplankton undergo distinct seasonal cycles. The seasonalpattern of primary production appeared to be regulated by lightwhereas the three order of magnitude change in phytoplanktonbiomass during mid- to late December was largely controlledby the advection of planktonic algae from the Ross Sea intoMcMurdo Sound. The size distribution of phytoplankton was highlyseasonal; nano- and picoplankton were dominant from August throughNovember while netplankton were more abundant in December andJanuary. Seasonal variations of bacterial biomass and productionwere smaller than those of phytoplankton. During the late australwinter and spring, bacterial biomass and production exceededthose of phytoplankton. This implies that during this period,organic material from allochthonous sources sustained bacterialgrowth. During the late spring and summer, however, the patternwas reversed and autochthonous primary production was sufficientto support concurrent bacterioplankton production. The apparenttemporal disequilibrium in autochthonous bacterioplankton andphytoplankton production was maintained by the seasonal inputof allochthonous organic material into McMurdo Sound. The factsthat a) bacteria were both abundant and highly active, b) bacterivorywas common among many of the endemic protozoa and some planktonicmetazoa and c) these bacterivores consumed >95% of the bacterialproduction strongly suggest that bacteria are a crucial componentin the transfer of energy and material to metazoans in polarregions.     

Diversity within cyanobacterial mat communities in variable salinity meltwater ponds of McMurdo Ice Shelf, Antarctica

This study investigated the diversity of cyanobacterial mat communities of three meltwater ponds--Fresh, Orange and Salt Ponds, south of Bratina Island, McMurdo Ice Shelf, Antarctica. A combined morphological and genetic approach using clone libraries was used to investigate the influence of salinity on cyanobacterial diversity within these ecosystems without prior cultivation or isolation of cyanobacteria. We were able to identify 22 phylotypes belonging to Phormidium sp., Oscillatoria sp. and Lyngbya sp. In addition, we identified Antarctic Nostoc sp., Nodularia sp. and Anabaena sp. from the clone libraries. Fresh (17 phylotypes) and Orange (nine phylotypes) Ponds showed a similar diversity in contrast to that of the hypersaline Salt Pond (five phylotypes), where the diversity within cyanobacterial mats was reduced. Using the comparison of identified phylotypes with existing Antarctic sequence data, it was possible to gain further insight into the different levels of distribution of phylotypes identified in the investigated cyanobacterial mat communities of McMurdo Ice Shelf.     

Testing the Sensitivity of Phytoplankton Communities to Changes in Water Temperature and Nutrient Load, in a Temperate Lake

Freshwater lakes are biologically sensitive to changes in the surrounding environment and the impacts that such changes have on their water quality are of considerable ecological, recreational and economic importance. In this study the phytoplankton community model, PROTECH, was used to experiment with the effects of elevated temperatures and increased nutrient load on phytoplankton succession and productivity. The response of a phytoplankton community to combined incremental changes in these drivers was analysed, in order to elucidate the resulting ecological changes. Annual mean phytoplankton biomass increased with increases in temperature and nutrient loading, although the latter had the larger effect. The phenology of the dominant phytoplankton taxa changed with increasing water temperature; the three spring blooming species all peaked earlier in the year. The simulated summer bloom of Anabaena became earlier in the year and the Chlorella bloom later. The increased phytoplankton biomass was largely dominated by the cyanobacterium Anabaena, which was especially prevalent during the summer bloom. This resulted in a progressive loss of phytoplankton biodiversity with increasing water temperature and nutrient supply. Model experimentation showed that whilst both factors greatly affected the community, the changes to nutrient loading generally had the greater effect and that at low nutrient levels the effect of water temperature change was reduced considerably. Finally, the model predicted that cyanobacteria have the potential to dominate the phytoplankton community, with clear consequences for water quality, and that this dominance was at its greatest when high water temperatures were combined with high nutrient loads.     

Antarctic marine life under the McMurdo Ice Shelf at White Island: A link between nutrient influx and seal population

Summary Throughout 1981, the first routine collections of a variety of fish were made under the McMurdo Ice Shelf near White Island, Antarctica. Estimates of their local biomass were calculated using food consumption patterns of resident seals. Since no significant primary production occurs at White Island, the resident fauna must be maintained by a large nutrient influx from the Ross Sea.     

Physical and Chemical Limnology of a Subsaline Athalassic Lake in West Greenland

Physical and chemical profiles of a shallow (c. 12-m-deep) subsaline (total dissolved solids 2.3-2.8 g l^–1) closed-basin lake in the continental area of southwestern Greenland are described for the first time. Watercolumn data for every 5th consecutive day between April 20 and October 6, 2001, and continuous recordings of lake water level and meteorological conditions are used to infer controls on contemporary lake functioning, sediment formation and climate-lake interactions. Limnological observations demonstrate the importance of lake-ice formation and its role in haline convection and the development of meromixis. Observed lake cycling suggest that the lake at present is in a state of near-meromixis where stagnant bottom waters de-stratify through deep penetration of weak haline convective cells by the end of June. From this study, the primary reasons the shallow Greenlandic low salinity lakes develop meromixis are:(i) lack of an outflow (ii) meltwater dilution and chemical stratification of surface waters, (iii) insubstantial wind mixing, (iv) a weak winter thermohaline convective cell forced by cryoconcentration, and (v) biogeochemically enhanced solute concentrations near the sediment bed. Throughout the open water period the hydrological balance is dominated by evaporative losses. Lake surface water conductivities change from 2110 to 2890 S cm^–1 due to the combined effects of open water evaporation, meltwater dilution, diffusive exchanges over the seasonal pycnocline, and boundary mixing. Freeze-out of salts and resulting deep haline convection increase overall water column salinity during winter. Owing to deep convective mixing, plant nutrients are relatively high in the upper watercolumn with a dominant internal source of phosphorous. Extreme productivity pulses of phytoplankton are observed as soon as sub-ice radiation levels increase and directly after ice-out when sufficient wind mixing can support an intense monospecific diatom bloom of Diatoma spp. leading to the rapid depletion of dissolved silica.     

Adaptations for cryopelagic life in the antarctic notothenioid fish Pagothenia borchgrevinki

Summary The cryopelagic circumantarctic notothenioid fish Pagothenia borchgrevinki lives near the undersurface of the sea ice in McMurdo Sound, Antarctica. When compared to closely related benthic species (especially Trematomus bernacchii), Pagothenia exhibited substantial morphological differences in a variety of organ systems. The values of the fineness ratio and the indices of trunk shape and flatness suggested streamlining and drag reduction, adaptations to life in the water column. Pagothenia also lacked substrate contact adaptations in the pelvic and anal fins. Silvery reflective layers (strata argentea) beneath the skin and in the iris and choroid of the eye provided camouflage when Pagothenia were viewed against a background of platelet ice. The retina had many cones indicating the eye was adapted to both diurnal and nocturnal vision. During the austral spring Pagothenia fed exclusively on nektonic organisms near the ice-water interface. Dietary diversity was low; copepods and amphipods were the most frequently occurring and amphipods were the most frequently occurring taxa. In conclusion, Pagothenia appear specialized for life in the water column.     

Allogromiid foraminifera and gromiids from under the Ross Ice Shelf: morphological and molecular diversity

Allogromiid foraminifera and gromiids constitute an important but poorly known component of the meiobenthos in high-latitude marine settings. Here, we present a first report on the diversity of these protists, together with an account of co-occurring invertebrate taxa, in a sample of sea floor sediment from a water depth of 923 m under the Ross Ice Shelf. Light microscopy of sieved sediment yielded 14 allogromiid foraminifera and three Gromia spp. morphotypes. We also obtained 36 partial small subunit rDNA sequences of allogromiids, including 19 sequences of individual isolates and 17 sequences from environmental DNA samples. Phylogenetic analyses of the obtained sequences identified 18 phylotypes, of which seven were closely related to previously sequenced allogromiids, while 11 represented new lineages. Our results demonstrate that a rich meiofaunal assemblage is present under the Ross Ice Shelf, 12 km from the open ocean. Molecular phylogenetic analyses of these allogromiid foraminifera point to the endemic character of the Ross Ice Shelf assemblage; however, more extensive sampling of the molecular diversity of this group of foraminifera in the Antarctic deep sea is necessary to confirm this conclusion.     

Underwater video observation of the Antarctic toothfish Dissostichus mawsoni (Perciformes: Nototheniidae) in the Ross Sea, Antarctica

A towed benthic camera system captured images of a ca. 66-cm-long Antarctic toothfish partially under a boulder at 76°30'S, 174°59'E (depth 454 m) in the southern Ross Sea. This is noteworthy because, excluding McMurdo Sound, there are only two published records for this species in the Ross Sea and none in an offshore locality. Adult Dissostichus mawsoni are neutrally buoyant and live and feed in the lower reaches of the water column. Benthic perching is unexpected, suggesting that this subadult is not neutrally buoyant.     

Combining a regional climate model with a phytoplankton community model to predict future changes in phytoplankton in lakes

1. Linking a regional climate model (RCM) configured for contemporary atmospheric greenhouse gas concentrations, with a phytoplankton community model (PROTECH) produced realistic simulations of 20 years of recent phytoplankton data from Bassenthwaite Lake, in the North‐West of England. 2. Meteorological drivers were derived from the RCM to represent a future climate scenario involving a 1% per annum compound increase in atmospheric CO₂ concentrations until 2100. Using these drivers, PROTECH was run for another 20 year period representing the last two decades of the 21st century. 3. Comparison of these present and future simulations revealed likely impacts on the current seasonal phytoplankton development. Under future climate conditions, the simulated spring bloom showed an increase in cyanobacteria dominance caused by greater success of Planktothrix. Also, the summer cyanobacteria bloom declined earlier because of nutrient limitation caused by the increased spring growth. Overall productivity in the lake did not change. 4. Analysis showed that these predicted changes were driven by changes in water temperature, which were in turn triggered by the higher air temperatures predicted by the RCM.