Marine phytoplankton at the Weddell Sea ice edge: Seasonal changes at the specific level

Summary The Antarctic ice edge acts as a dynamic frontal system on the phytoplankton in the water column. Austral spring and autumn cruises to the Weddell Sea ice edge provided the opportunity to compare phytoplankton at the beginning of biological spring and at the end of biological autumn. The USCGC icebreakers Westwind (1983) and Glacier (1986) went into the sea ice, and the RV Melville (1983 and 1986) completed the transects in the adjacent open ocean. Field samples were observed alive on board ship to record different lifestages near the ice edge. In both seasons cell numbers were low under the ice, and single cells or short chains were the common growth habit. In spring in the open ocean, long chains of vegetative cells with large vacuoles and gelatinous colonies of diatoms and of prymnesiophytes dominated; in autumn in the open ocean close to the accreting ice edge, short chains, single cells, and resting spores were mostly packed with storage products. Enlarged cell diameters and auxospores also occurred near the ice cover in the autumn. Species from the following genera are included: the diatoms Leptocylindrus, Stellarima, Thalassiosira, Eucampia, Corethron, and Chaetoceros, the prymnesiophyte Phaeocystis, and the chrysophyte Distephanus.     

The zooplankton community in the vicinity of the ice edge,western Weddell Sea,March 1986

Summary The zooplankton community in the vicinity of the ice edge in the west central Weddell Sea was investigated in the late austral summer (March 1986). Sampling was done with two ships operating concurrently, one in the pack ice and the other in the adjcent open sea. Metazoan microzooplankton (<1 mm) was most abundant in the epipelagic zone. It consisted mostly of copepod nauplii and copepods of the genera Oithona, Oncaea, Ctenocalanus and Microcalanus. While species composition was similar in both areas, vertical patterns differed in that the microzooplankton had sparse populations in the upper 50 m under the ice. This may have been related to water temperature which in the upper 50 m under the ice was more than 1°C cooler than in the open sea. Zooplankton in the 1–20 mm size range was dominated by the calanoid copepods Metridia gerlachei, Calanus propinquus and Calanoides acutus which constituted half the biomass in the upper 1000 m. Their populations had highest densities in the upper 150 m, though much of the C. acutus population resided below 300 m. Metridia gerlachei and C. propinquus underwent diel vertical migrations in both areas whereas C. acutus did not migrate. Species diversity in the epipelagic zone was moderate and the fauna was characterized by species typical of the oceanic east wind drift. Diversity increased with depth and was due primarily to the appearance of circumpolar mesopelagic copepods in Weddell Warm Deep Water. Biomass of 1–20 mm zooplankton in the 0–1000 m zone was low (1.1–1.3 gDWm^-2) compared to other Southern Ocean areas investigated with comparable methods. It is suggested that this is related to Weddell circulation patterns and the resulting low annual primary production in the central Weddell Sea.     

Respiratory electron transport activity of microplankton in the Weddell Sea during early spring: influence of the ice cover and the ice edge

Summary The activity of the respiratory electron transport system (ETS) of the microplankton (<240 m size) was measured in the Northern Weddell Sea during EPOS 1, in the Close Pack Ice (CPI), and in the ice edge (Outer and Inner Marginal Zones, OMIZ and IMIZ). During early spring the activity increased with time and in the pack ice-open water direction. The temporal trend was more obvious than the spatial one. ETS activity ranged from 0.01 to 1.25 ml O₂ m^–3 h^–1 under the ice and from 0.1 to 1.6 ml O₂ m^–3 h^–1 in the open water at the ice edge. Depth-integrated ETS activity in the upper 300 m ranged from 13 to 130 ml O₂ m^–2h^–1. 60% to 80% of the activity took place above 100 m in the OMIZ in the prebloom conditions at the end of the cruise. ETS/Chl a ratios showed the importance of microheterotrophs under the ice, versus a greater phytoplankton dominance in the ice edge-open water zone. The carbon-specific activity reached a maximum (0.43 day^–1) in the innermost zone of the CPI where bacteria dominated. Respiratory activity under the ice is important in producing the oxygen deficit observed, due to the negative balance between photosynthesis and respiration. The ETS activity was at the lower range of that found in the region in summer and is comparable to that measured in other oligotrophic, stratified systems in oceanic areas.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Copepods in summer platelet ice in the eastern Weddell Sea,Antarctica

Copepods in platelet-ice layers underlying fast ice and in the water column below were studied at Drescher Inlet, eastern Weddell Sea in February 1998. Three copepod species were found: Drescheriella glacialis and Paralabidocera antarctica occurred in platelet-ice layers, while Stephos longipes was only present in the water column. The distribution of all species varied considerably between station and depth. D. glacialis dominated the platelet-ice community and occurred at all five platelet-ice sampling sites, except one, with numbers of up to 26 ind. l^–1. In contrast, P. antarctica was only found in low numbers (up to 2 ind. l^–1) at one site. The total copepod abundance in the platelet ice was not associated with algal biomass, although it was strongly correlated with high ammonium concentrations (up to 9 M) in the interstitial water between the platelets. This is the first indirect evidence to support the hypothesis that zooplankton excretion can partly account for the high ammonium values often found in platelet-ice layers.     

Effects of a rapidly receding ice edge on the abundance,age structure and feeding of three dominant calanoid copepods in the Weddell Sea,Antarctica

Open-water, marginal-ice and in-ice zones were sampled in the Weddell Sea during November and December, 1993 in an effort to examine the influence of the early spring bloom on the diet and population structure of the three biomass dominant copepods: Metridia gerlachei, Calanus propinquus, and Calanoides acutus. The abundance of all three species in the upper 200 m was highest at stations in the open water, but individually, each species displayed a unique trend. M. gerlachei, which showed the least variability, was significantly more abundant in open water than in the marginal-ice zone. The abundance of Calanus propinquus was higher in open water than in the marginal-ice zone or in the ice. Calanoides acutus displayed the highest variability, with significant differences between all three ice-cover zones. Diet analysis revealed no significant differences in the number of food items within each ice-cover zone and diatoms were the most numerous item identified in the guts of all three species. However, M. gerlachei and Calanus propinquus also contained metazoan material, while Calanoides acutus did not. There were dramatic differences in the age composition of the species between the zones. Early copepodite stages of all three species predominated at the ice edge and in open water. Numbers of M. gerlachei adult females were roughly equivalent in all three zones while Calanoides acutus and Calanus propinquus adult females composed a higher fraction of the total population within the ice. These results compare well with life-history data compiled by other authors and reinforce the importance of the ice edge to bloom-dependent Antarctic zooplankton. Accepted: 5 April 1999     

Biogeochemical conditions and ice algal photosynthetic parameters in Weddell Sea ice during early spring

Physical, biogeochemical and photosynthetic parameters were measured in sea ice brine and ice core bottom samples in the north-western Weddell Sea during early spring 2006. Sea ice brines collected from sackholes were characterised by cold temperatures (range −7.4 to −3.8°C), high salinities (range 61.4–118.0), and partly elevated dissolved oxygen concentrations (range 159–413 μmol kg⁻¹) when compared to surface seawater. Nitrate (range 0.5–76.3 μmol kg⁻¹), dissolved inorganic phosphate (range 0.2–7.0 μmol kg⁻¹) and silicic acid (range 74–285 μmol kg⁻¹) concentrations in sea ice brines were depleted when compared to surface seawater. In contrast, NH₄ ⁺ (range 0.3–23.0 μmol kg⁻¹) and dissolved organic carbon (range 140–707 μmol kg⁻¹) were enriched in the sea ice brines. Ice core bottom samples exhibited moderate temperatures and brine salinities, but high algal biomass (4.9–435.5 μg Chl a l⁻¹ brine) and silicic acid depletion. Pulse amplitude modulated fluorometry was used for the determination of the photosynthetic parameters F _v/F _m, α, rETR_max and E _k. The maximum quantum yield of photosystem II, F _v/F _m, ranged from 0.101 to 0.500 (average 0.284 ± 0.132) and 0.235 to 0.595 (average 0.368 ± 0.127) in the sea ice internal and bottom communities, respectively. The fluorometric measurements indicated medium ice algal photosynthetic activity both in the internal and bottom communities of the sea ice. An observed lack of correlation between biogeochemical and photosynthetic parameters was most likely due to temporally and spatially decoupled physical and biological processes in the sea ice brine channel system, and was also influenced by the temporal and spatial resolution of applied sampling techniques.     

Interactions in the microbial community of the marginal ice zone of the northwestern Weddell Sea through size distribution analysis

Summary Enumeration and identification of planktonic microorganisms (phytoplankton, bacteria, protozoa) were carried out for 16 stations sampled in the marginal ice zone of the northwestern Weddell Sea during sea-ice retreat in 1988 (EPOS Leg 2). From these data, carbon biomass distribution among various classes, chosen according to size and trophic mode, has been determined. This analysis reveals the general dominance of nano-phytoplankton (74 %), mainly Cryptomonas sp.. In two stations only, significant microphytoplanktonic biomass occurred. Bacterioplankton biomass was 16 % of the phytoplanktonic biomass. Protozooplankton appeared as a significant group whose biomass represented an average of 23 % of the total microbial biomass. Maximum phytoplankton and protozooplankton biomass was reached at about 100–150 km north of the receding ice edge whilst bacteria did not show marked spatial variations. From these results, indirect evidence for close relationships between protozoa and bacteria, as well as protozoa and autotrophs, is given. The size range of autotrophic prey and predators overlaps (equivalent spherical diameter range = 6 to 11 m). This size overlapping increases the complexity of the trophic organization of the microbial community. Our results thus support the idea of a flux of energy not always oriented towards an increasing particle size range. Potential ingestion rate, calculated from a mean clearance rate in the literature, indicated that protozooplankton might ingest as high as 48 % of the daily phytoplankton production in the marginal ice zone.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

The effect of the receding ice edge on the condition of copepods in the northwestern Weddell Sea: results from biochemical assays

We compared six biochemical measures of nutritional condition: citrate synthase activity (CS), malate and lactate dehydrogenase activity (MDH and LDH), RNA:DNA ratio, and percent body protein and lipid. Adult females of five species of calanoid copepod (Calanoides acutus, Calanus propinquus, Metridia gerlachei, Rhincalanus gigas and Paraeuchaeta antarctica) were collected in the marginal ice zone of the northwestern Weddell Sea at the time of the annual phytoplankton bloom that occurs in association with the receding ice edge during austral spring. Three zones within the marginal ice zone were sampled: heavy-ice-cover pre-bloom, ice-edge bloom and low-ice-cover post-bloom. Lipid generally increased greatly from ice-covered to open water zones, and its importance in the life of polar copepods cannot be overstated. Increases in protein from ice-covered to open water were also observed, but were of less significance. Each species exhibited significant changes in at least one enzyme activity level. Citrate synthase activity in C. acutus, C. propinquus and R. gigas, all herbivores, increased between pre- and post-bloom stations. C. propinquus and M. gerlachei, which feed during winter, had large increases in LDH activity between pre- and post-bloom stations. Rhincalanus gigas and P. antarctica, the two largest species studied, showed variations in MDH activity, with peak enzyme activity occurring in post-bloom stations. RNA:DNA ratio did not change in any species. The effects of size, shipboard handling and freezer storage were easily corrected statistically, and did not alter any conclusions. The patterns observed in copepod nutrition at the Antarctic ice edge were consistent with existing models of life history for each species. The observations reported here, in conjunction with previously reported data, suggested that measurement of metabolic enzyme activity, especially in concert with lipid, enables estimation of nutritional condition in adult copepods. Additional studies comparing metabolic activity and ecology of common species should yield more information on the ecology of rarer species.     

Biogeochemistry of platelet ice: its influence on particle flux under fast ice in the Weddell Sea, Antarctica

An array of four sediment traps and one current meter was deployed under a well-developed platelet layer for 15 days in the Drescher Inlet in the Riiser Larsen ice shelf, in February 1998. Traps were deployed at 10 m (just under the platelet layer), 112 m (above the thermocline), 230 m (below thermocline) and 360 m (close to sea floor). There was a substantial flux of particulate organic material out of the platelet layer, although higher amounts were collected in the traps either side of the thermocline. Material collected was predominantly composed of faecal pellets containing diatom species growing within the platelet layer. The size classes of these pellets suggest they derive from protists grazing rather than from larger metazoans. Sediment trap material was analysed for particulate organic carbon/nitrogen/phosphorus (POC/PON/POP) and '¹³C_POC (carbon isotopic composition of POC). These were compared with organic matter in the overlying platelet layer and the water column. In turn, the biogeochemistry of the platelet layer and water column was investigated and the organic matter characteristics related to inorganic nutrients (nitrate, nitrite, ammonium, silicate, phosphate), dissolved organic carbon/nitrogen (DOC/DON), pH, dissolved inorganic carbon (DIC), oxygen and '¹³C_DIC (carbon isotopic composition dissolved inorganic carbon).     

Isolation and properties of psychrotrophic and psychrophilic, pectolytic strains of Clostridium

Fourteen strains of pectolytic clostridia have been isolated that were capable of growth at 5–10°C in 7 d; two strains were psychrophiles and failed to grow at 20°C in 14 d and the remainder were psychrotrophs. The bacteria formed pectate lyase enzymes and were capable of degrading potato tissue; they are therefore a potential cause of soft rot of potatoes stored at low temperatures. Doubling times for representative strains were 15–19 h at 10°C and 21–53 h at 5°C. Twelve strains were classified with Group I Clostridium species and two strains with Group II. In the case of one strain the mature spores were not released from the sporangium. Electron microscopy of ultrathin sections of this strain showed the presence of disorganized lamellar structures associated with the spore coat.     

Spin-labeling studies on the lipids of psychrophilic, psychrotrophic, and mesophilic clostridia.

Spin-labeling studies were conducted to elucidate the viscosity and phase transition temperatures of lipids isolated from psychrophilic, psychrotrophic, and mesophilic clostridia. Electron spin resonance spectroscopy indicated that the lipids, for all the growth temperatures tested, were in a fluid state and from 13 to 24 C higher than the corresponding lipid transition temperatures. When the organisms were grown at different temperatures, a psychrotropic and two mesophilic clostridia were shown to be able to adjust their lipid-phase transition temperature to the growth temperature. A psychrophilic Clostridium strain, when grown at different temperatures, synthesized lipids that had the same phase transition temperature. It is suggested that this lack of growth temperature-inducible regulation of lipid-phase transition temperature may be a molecular determinant for the psychrophily of this organism. It is proposed that the growth temperature range of an organism is dependent upon the ability of the organism to regulate its lipid fluidity within a specific range.     

Winter aggregations of marine mammals and birds in the north-eastern Weddell Sea pack ice

Summary A seabird and mammal census was carried out in the north-eastern Weddell Sea during the austral winter of 1986. The German research icebreaker Polarstern operated in heavy pack ice along the Greenwich Meridian between the northern sea ice boundary and the Antarctic coast. Crabeater seals (Lobodon carcinophagus), minke whales (Balaenoptera acutorostrata), Adélie penguins (Pygoscelis adeliae), Antarctic petrels (Thalassoica antarctica) and snow petrels (Pagodroma nivea) were found to be more abundant in the vicinity of the submarine Maud Rise, about 700 km north of the continental margin, than in other areas of substantial ice cover traversed during that cruise. The aggregations of birds and mammals are expected to reflect aggregations of their principal food, krill (Euphausia superba) wintering underneath the ice cover. The distribution pattern of krill predators coincides with the course of a warm water belt upwelling near Maud Rise. This upwelling could induce local ice melting which in turn may result in an increased release of sea ice algae.     

Phytoplankton distribution in relation to sea ice,hydrography and nutrients in the northwestern Weddell Sea in early spring 1988 during EPOS

Summary Phytoplankton biomass and distribution of major phytoplankton groups were investigated in relation to sea ice conditions, hydrography and nutrients along three north-south transects in the north western Weddell Sea in early spring 1988 during the EPOS Study (European Polarstern Study), Leg 1. Three different zones along the transects could be distinguished: 1) the Open Water Zone (OWZ) from 58° to 60°S with high chlorophyll a concentrations up to 3.5 g l^–1; 2) the Marginal Ice Zone (MIZ) from 60° to about 62.5° with chlorophyll a concentrations between 0.1 and 0.3 g l^–1, and 3) the closed pack-ice zone (CPI) from 62.5° to 63.2°S with chlorophyll a concentrations below 0.1 gl^–1. Nutrient concentrations increased towards the south showing winter values under the closed pack-ice. Centric diatoms such as Thalassiosira gravida and Chaetoceros neglectum forming large colonies dominated the phytoplankton assemblage in terms of biomass in open water together with large, long chain forming, pennate diatoms, whereas small pennate diatoms such as Nitzschia spp., and nanoflagellates prevailed in ice covered areas. Fairly low concentrations of phytoplankton cells were encountered at the southernmost stations and many empty diatom frustules were found in the samples. The enhanced phytoplankton biomass in the Weddell-Scotia-Confluence area is achieved through sea ice melting in the frontal zone of two different water masses, the Weddell and the Scotia Sea surface waters.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Fluxes of gases in an ice related ecosystem in the north-western Weddell Sea

Summary During the European Polarstern Study (EPOS leg 1 and leg 2) measurements of temperature, salinity, inorganic nutrients, chlorophyll-a, oxygen and total inorganic carbon dioxide were performed from October to January 1988–1989 in north-south sections at 47–49 °E in the NW Weddell Sea from approximately 58 °S to 63 °S (Hempel 1989; Hempel et al. 1989). In order to explain parts of the obtained data, a time-dependent ecological model was constructed by Svansson (1991). He found that a moderate mixing with a constant diffusion coefficient from sea surface downwards resulted in good agreement between computed and measured chlorophyll. In this paper we introduce the gas fluxes, mainly oxygen but also carbon dioxide, into the model work. It turns out that air-sea fluxes are necessary to explain the vertical oxygen distribution. The annual development of chlorophyll, phosphate, oxygen and total inorganic carbon dioxide are computed. Hours of day-light, losses and the eddy diffusion coefficient are allowed to vary during the year with the condition that the mean total chlorophyll at 14 selected leg 1 stations was nearly double the magnitude of that of 18 selected leg 2 stations. This yields variations consistent with the observations. Different steady-state solutions after 91 days are also tested to show effects of one selected variation at a time, for example the eddy diffusion coefficient or the loss rate. The oxygen air-sea flux, of about 90 mmol m^–2 day^–1 in the time variable model computation, is compared to estimated fluxes by a gas transfer formula. The formula used gives a flux which is about 5 times smaller than the model flux. Some of the 91 days solutions give results of fluxes which are less than 90 mmol m^–2 day^–1 but still higher than the transfer formula result. Fluxes of total inorganic carbon dioxide in the model computation are always directed from air to sea.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Temporal dynamics of grouse populations at the southern edge of their distribution

Studies of grouse conducted at northern latitudes have shown that tetraonids frequently exhibit cyclic fluctuations in abundance but little is known about the dynamics of grouse species at the southerly edge of their range. Hunting statistics from four species of grouse based on 30 yr of data collected from 210 hunting areas were examined from the Dolomitic Alps in the province of Trentino. These data were summed to represent 18 time series from discrete mountain groups. Analyses identified cycles of ca 5 yr in the minority of rock ptarmigan Lagopus mutus and hazel grouse Bonasa bonasia populations. These cycles only showed significant negative autocorrelation at half the cycle period and were classified as phase-forgetting quasi-cycles. Cycles were not found in time series of black grouse Tetrao terix or capereaillie Tetrao urogallus. Correcting time series for hunting effort or hunting restrictions tended to increase the proportion of populations that exhibited cycles but no difference in the strength of second order density dependence, A linear first order density-dependent autoregressive model described the dynamics of most of the populations with the exception of a proportion of rock ptarmigan and black grouse populations where a non linear first order model provided the best tit. We compare the findings with studies conducted in Finland and suggest possible reasons for the reduced tendency to cycle in the populations of southern Europe.     

Vertical distribution and abundance of mesoplanktonic medusae and siphonophores from the Weddell Sea,Antarctica

The composition, abundance and vertical distribution of mesoplanktonic cnidarians collected along a transect across the Weddell Sea have been analysed. The transect was characterized by a thermocline, approximately between 200 and 100 m, which deepened significantly towards the shelf edges. In total, 10 species of medusae and 18 species of siphonophores were identified. The most abundant medusae were Pantachogon scotti (up to 11,671 specimens/1,000 m³) and Arctapodema ampla (up to 960 specimens/1000 m³). The most abundant siphonophores were Muggiaea bargmannae (up to 1,172 nectophores/1,000 m³) and Dimophyes arctica (up to 230 nectophores/1,000 m³). Five assemblages of planktonic cnidarians were distinguished: (a) epipelagic species located in and above the thermocline; (b) epi- and upper mesopelagic species located in, above and just below the thermocline; (c) epi- and mesopelagic species located in and below the thermocline; (d) mesopelagic species; (e) lower mesopelagic species. Differences in the depth distribution of the various species gave rise to a clear partitioning of the mesoplanktonic cnidarian population throughout the water column. This vertical partitioning was related to the existence of a thermocline, the structure of the water column and the vertical distribution of prey.     

Abundance,horizontal and vertical distribution of fish in eastern Weddell Sea micronekton

Summary The spatial distribution and species composition of high-Antarctic ichthyonekton was investigated during the EPOS 3 cruise by RV Polarstern in the eastern Weddell Sea during January–February 1989. A multiple rectangular midwater trawl was used to collect samples from the surface to near the sea floor at 11 stations along a 245 kra transect off Halley Bay. Early larval stages of 18 species, representing about 24% of the known Weddell Sea ichthyofauna, were present in the water column. The Antarctic silver-fish, Pleuragramma antarcticum, over-whelmingly dominated the catches comprising 84.5% of the 5022 specimens caught. The abundance of this species markedly increased towards the offshore end of the transect with the highest numbers occurring near the shelf-break front associated with the westerly current of the southern limb of the Weddell Gyre. The increased abundance of P. antarcticum in continental slope waters was attributed to deflection of the East Weddell Coastal Current beyond the shelf/slope break by fringing ice shelves. Most larval and juvenile fish were found in the seasonally warmed upper 0–70 m layer of the Antarctic Surface Water where conditions occurred that appeared to be favourable to both feeding and growth. Cluster analysis indicated that inner-, central-and outer-shelf assemblages were represented and that the species composition was most effectively described by reference to water mass and depth.     

Protozooplankton in the Weddell Sea,Antarctica: Abundance and distribution in the ice-edge zone

Summary Protozooplankton were sampled in the iceedge zone of the Weddell Sea during the austral spring of 1983 and the austral autumn of 1986. Protozooplankton biomass was dominated by flagellates and ciliates. Other protozoa and micrometazoa contributed a relatively small fraction to the heterotrophic biomass. During both cruises protozoan biomass, chlorophyll a concentrations, phytoplankton production and bacterial biomass and production were low at ice covered stations. During the spring cruise, protozooplankton, phytoplankton, and bacterioplankton reached high concentrations in a welldeveloped ice edge bloom 100 km north of the receding ice edge. During the autumn cruise, the highest concentrations of biomass were in open water well-separated from the ice edge. Integrated protozoan biomass was <12% of the biomass of phytoplankton during the spring cruise and in the autumn the percentages at some stations were >20%. Bacterial biomass exceeded protozooplankton biomass at ice covered stations but in open water stations during the fall cruise, protozooplankton biomass reached twice that of bacteria in the upper 100m of the water column. The biomass of different protozoan groups was positively correlated with primary production, chlorophyll a concentrations and bacterial production and biomass, suggesting that the protozoan abundances were largely controlled by prey availability and productivity. Population grazing rates calculated from clearance rates in the literature indicated that protozooplankton were capable of consuming significant portions of the daily phyto- and bacterioplankton production.