Weddell-Scotia Confluence Research Articles

The Weddell-Scotia marginal ice zone: Physical oceanographic conditions, geographical and seasonal variability

Physical oceanographic conditions were measured in the Weddell and Scotia Sea marginal ice zones (MIZ's) during 1983, 1986 and 1988. The field work encompassed spring, autumn and mid-winter periods and included retreating, advancing and steady-state ice edges. Observed upper ocean structures, which typify MIZ's and reflect input of low salinity water from melting ice, included low salinity upper layers, lenses and fronts. An upper mixed layer was always present and was generally more fully developed in autumn and winter than at other times of year. Conditions in the deeper waters reflected regional oceanographic processes and significant differences were present between the Weddell and Scotia seas. Weddell Sea Water is a major source of water for the southern Scotia Sea, however, the upper Scotia Sea was dominated by warmer, less saline waters from Drake Passage. The colder, denser Weddell water appeared to have mixed isopycnally with deeper water in the Scotia Sea, present there at depths exceeding 500 m. The Scotia Sea was dominated by strong gradients and energetic mesoscale features, with currents exceeding 50 cm/s. The northwestern Weddell Sea had, in contrast, current speeds well below about 5 cm/s and small to negligible lateral water property gradients. Our observations suggest that the Weddell western boundary current was weaker than has been estimated in the past. In addition, we found scant evidence of deep winter convection in the Scotia Sea, a process which has been hypothesized in the past to contribute to deep water formation. No evidence was found during winter 1988 in the Scotia Sea of the modified water known as Weddell-Scotia Confluence water.

Does prey preference affect habitat choice in Antarctic seabirds?

Diet composition of the members of 2 seabird species assemblages in the Scotia-Weddell Confluence region, Antarctica, was investigated during 3 seasons/years: spring 1983, autumn 1986, and winter 1988. One assemblage frequented the pack ice and the other was present in adjacent open waters; most members of the latter species assemblage vacated the Antarctic during winter. We sought answers to 2 questions: (1) Did the 2 species assemblages depend on food webs that differed substantially; and (2) Was there a trophic basis to explain why the pack-ice assemblage did not expand into the open water left vacant by the other during winter? To test an a priori hypothesis that diet was affected by habitat, diet samples were obtained from birds encountered in 3 habitats: open water, sparse concentrations of ice, and heavy ice cover. Cluster analysis showed broad overlap in seabird diet regardless of species, habitat (ice/water mass) or year. Seabirds exploited prey largely according to ranked availability, although they appeared to choose the larger fish and crustaceans over smaller crustaceans. Myctophids in particular, but also krill and squid, were the main prey groups. Diets did not separate on the basis of predator/prey size to any appreciable degree even though a lOOO-fold difference in predator size existed. Feeding success, as indicated by fullness of stomach, of the members of the 2 species assemblages was highest when in their respective usual habitats. We conclude that open-water seabirds lack the specialized foraging behaviors required to exploit the pack-ice environ­ ment and that open waters during the winter offer much poorer feeding conditions than those of the pack ice.

The distribution of biogenic silica and the composition of particulate organic matter in the Weddell—Scotia Sea during spring 1988

During EPOS 2 (European Polarstern Study, leg 2, 26 November 1988–5 January 1989 the distribution of biogenic silica (BSi) was determined from the surface to 600 m along two transects (49°W and 47°W) spanning the Scotia Sea, the Weddell—Scotia Confluence (WSC) and the marginal ice zone (MIZ) of the Weddell Sea. Particulate organic carbon (POC), particulate organic nitrogen (PON) and chlorophyll a concentrations were determined in parallel in surface (10m) samples. In the 0–100 m layer, the distribution of particulate biogenic silica (BSi) showed little correlation with the meltwater field in late November. High BSi concentrations were measured at the end of November in the Weddell—Scotia Confluence (maximum 4.8 μmol l −1), and early in January in the Antarctic Circumpolar Current (maximum at 8.2 μmol l −1) where the wind-mixed layer was only 35 m, creating favourable conditions for phytoplankton growth. However, maximum and mean biogenic silica values remained fairly low as compared with those reported for the Ross Sea. This relatively low silica content of the 0–100 m layer appears to be mainly due to intense grazing which resulted in rapid exportation of particulate material towards the deeper layers. The mean POC/PON molar ratio was 5.5, i.e. significantly lower than the usual Redfield ratio. BSi/POC molar ratios ranged from 0.01 to 0.61, the highest values being found in the Scotia Sea, where siliceous phytoplankton was dominant. The high BSi/POC ratios confirm that diatoms in Antarctic Ocean are able to incorporate unusually high amounts of Si relative to carbon, as compared with other ecosystems.

Dissolved aluminium in the Weddell-Scotia Confluence and effect of Al on the dissolution kinetics of biogenic silica

In the Scotia and Weddell Seas the concentration of dissolved Al was 1–1.5 nM in ice-free surface waters, up to 3 nM in ice-covered waters and about 2.6 nM in bottom waters. The solubility and the dissolution rate of diatomaceous silica, obtained from net samples and from incubations in the presence or absence of dissolved Al, were inversely related to solid-phase Al/Si ratios. At in situ temperatures, dissolution rates for Antarctic diatoms are higher than those for diatoms from temperate regions. Effects on distribution patterns of silicic acid in the Weddell Sea are discussed.

Suspended barite as a tracer of biological activity in the Southern Ocean

During two cruises in the Southern Ocean (INDIGO 3, Indian sector and EPOS 2, Scotia-Weddell Confluence) suspended matter samples were collected for analysis of several biogenic elements including Ba, Si and particulate organic carbon (POC). The good agreement between particulate barium in subsurface water with oxygen minimum concentrations as well as with nitrate depletions suggests that it reflects the intensity of new production over the past season. As a consequence of new production in the Southern Ocean being performed mainly by diatoms, the occurrences of barite and diatoms are related in the water column. In areas characterized by diatom-poor phytoplankton, barite does occur, but in lower concentrations.

Total organic carbon in the sea—ice zone between Elephant Island and the South Orkney Islands at the start of the austral summer (1988–89)

During an Antarctic survey carried out on board the R/V “Professor Siedlecki” (26 December 1988–18 January 1989) seawater samples were collected from nine depths, from the surface to 150 m, at 29 stations near the pack-ice in a zone of the Weddell—Scotia Confluence between Elephant Island and the South Orkney Islands. The samples were analyzed for total organic carbon (TOC) and silicate concentrations. The two main water masses in the sampling area, coming from the Drake Passage and Weddell Sea, are well-defined using silicate as a tracer. The concentrations of TOC are higher than 100 μM and are associated with low silicate concentrations. However, in the deep water influenced by the Weddell Sea the concentrations of TOC are about 30 μM.

Biomass and composition of size fractionated phytoplankton in the Weddell-Scotia Confluence area

The measurement of Chl a, Chl b and Chl c contents in four size fractions (Nuclepore filters of 10 μm, 3μm, 1 μm and 0.2 μm pore-size) together with microscopic examination illustrate the structure and the relative importance of the micro-, nano and pico-phytoplankton in the production system in the Weddell/Scotia Confluence area. In the Scotia Sea, large diatoms were prevalent and their biomass increased during the six week cruise period, exceeding 1 mg Chl a m−3 at the beginning of January. In contrast, in the Marginal Ice Zone of the Weddell Sea, the biomass remained low, up to 0.3 mg Chl a m−3. A diversified nanoplankton community accounted for more than 90% of this biomass: small diatoms, naked dinoflagellates, cryptophyceans, prymnesiophytes and green flagellates which increased the Chl b/Chl a ratio to values >0.20. An important trend affected the Confluence area, where a high biomass net-plankton community (4 mg Chl a m−3) rapidly changed towards a uniform nanoplankton system of the same kind as in the Weddell Sea. At times, autotrophic cryptophyceans were almost dominating (>4.106 cells/l), with a biomass up to 2 mg Chl a m−3 and a low phaeopytin ratio (<10%). This situation probably arises because of a grazing pressure by krill. However, due to the geographic and oceanographic peculiarities of this area, it is not possible to extrapolate these observations concerning the size structure of the primary producers to the Southern Ocean in general.

Production of biogenic silica in the Weddell-Scotia Seas measured with 32 Si

During leg 2 (November 1988–January 1989) of the European Polarstern study (EPOS) in the Weddell-Scotia Confluence zone, we implemented a new, simple, rapid, and sensitive method based on 32Si (a β-emitter in equilibrium with 32P) to determine biogenic silica (BSi) production. This method provides simultaneous determination of the production of organic P. Rates of BSi production ranged from 0.09 to 0.95 mmol Si m‒2 h‒1, and the BSi content of the photic layer ranged from 7 to 220 mmol m‒2. The Si: C production molar ratios varied between 0.08 (small cell density of diatoms in communities dominated by cryptophyceans and dinophyceans) and 0.81 (diatom-dominated populations), while the C: P production ratios varied between 47 and 151 with an average of 105, in agreement with the Redfield ratio. We give here the first direct evidence that in the well-mixed waters of the Scotia Sea highly silicified diatom blooms may occur and that unlike earlier descriptions, because of the intense grazing pressure of krill in the marginal ice zone (MIZ) of the Weddell Sea, the remaining siliceous phytoplankton can grow with only moderate Si production rates. We conclude that the contribution of the MIZ to the total BSi production of the Southern Ocean must not be overstated.

The structure of upper level pelagic food webs in the Antarctic: Effect of phytoplankton distribution

We investigated diet composition and diversity and the diet overlap between species within the seabird community of the Scotia-Weddell Confluence region, Antarctica, during spring, autumn and winter. Seasonal changes in diet characteristics and overlap among species were viewed in the context of changes in prey availability as a function of vertical distribution in the water column. The latter was in turn affected by where phytoplankton and associated grazers were located. When primary productivity was confined to the pack ice during winter, thus bringing grazers and their predators closer to the surface, micronekton were much more vulnerable to avian predators and the latter were able to select prey on the basis of energetic value. As a consequence the upper food web was much less diffuse than during the other seasons when prey occurred throughout the upper portion of the water column and predators had to be opportunistic. The mobility of micronekton and nekton bring some structure to the upper portion of marine food webs, thereby introducing seasonal variability of trophic relationships within Antarctic food webs.

Oceanic micronektonic/macrozooplanktonic community structure and feeding in ice covered Antarctic waters during the winter (AMERIEZ 1988)

Fifty-seven species of oceanic micronekton and macrozooplankton were collected under pack ice during the winter in the vicinity of the Weddell-Scotia Confluence with a modified opening-closing Tucker trawl. The majority of the 57 species did not vertically migrate and lived deeper during the winter than during the spring or fall. However, despite the short day length, several of the most common mesopelagic fish and crustaceans did migrate. Fish moved into shallower depths at night but apparently most did not continue into the near-freezing upper mixed layer, leaving that zone to the migratory crustaceans. In the upper 1000 m, the dominant species were, in order of decreasing biomass, Euphausia superba, the cnidarian Atolla wyvillei, the ctenophore Beroe sp., and the mesopelagic fish Electrona antarctica, Bathylagus antarcticus and Gymnoscopelus braueri. Thysanoessa macrura and Salpa thompsoni were biomass subdominants. The majority of the dominant species showed little seasonal differences in biomass. However, the biomass of gelatinous species varied considerably with A. wyvillei and Beroe sp. being most abundant and S. thompsoni least abundant during the winter. Incidence of food in the stomachs in several important species was low, suggesting a low impact on their Zooplankton prey. Specimens of S. thompsoni had high quantities of food in their guts but this species was uncommon so its net impact would also have been low. Euphausia superba and the three common mesopelagic fish had significantly lower stomach fullness ratings during the winter than during the fall, suggesting an overall decrease in feeding activity of dominant species during the winter.

Ammonium regeneration in the Scotia-Weddell Confluence area during spring 1988

The presence of large ammonium stocks and the related enhanced quantities of microheterotrophs in the Scotia-Weddell Confluence area indicate the importance of ammonium remineralization processes there. Maximal ammonium concentrations in the marginal ice zone amounted to more than 2-mu-mol N l-1 and remineralization rates in this zone were consistently higher than those measured in the adjacent open ocean and permanently ice-covered zones. At one station, characterized by a large kill swarm, the remineralization rate was 18 nmol N l-1 h-1; rates in the marginal ice zone generally ranged between 5 and 9 nmol N l-1 h-1. Protozoan activity was of predominant importance for the remineralization process; only after the krill event was a very high bacterial activity observed. The presence of krill does not contribute in a direct way to the ammonium pool, but probably initiates the bacterial breakdown activity in algal debris and faecal pellets. The increased remineralization leads to enhanced availability of ammonium in the water and to decreased f-ratios for phytoplanktonic nitrate assimilation. Nevertheless, nitrate uptake represented at least 30% of inorganic nitrogen assimilation, significantly higher than values measured for oligotrophic systems.

Spatiotemporal variability of inorganic nitrogen stocks and uptake fluxes in the Scotia-Weddell Confluence area during November and December 1988

Distributions of nitrate and ammonium and corresponding uptake fluxes were investigated in the Scotia-Weddell Confluence area during EPOS LEG 2 (November and December 1988) on board RV 'Polarstern'. In this area a central low-salinity and low-nitrate compartment extends southward from the Scotia Front over 1 to 1.5-degrees-latitude. High ammonium concentrations and marked nitrate depletions indicate an enhanced biological activity in this part of the ocean. By means of the uptake rates, obtained from N-15 incubations, and of nitrate depletion data it is proved that this marginal ice zone ecosystem shifts from predominantly new production based (highest f-ratio 0.83) to regenerated production based (lowest f-ratio 0.30) after some 3 wk. This transition corresponds with the change from a higher diatom presence to a higher flagellate presence in the phytoplankton community. Moreover, it is in agreement with the described sudden termination of export fluxes in the Weddell Sea.

Algal pigment patterns in different watermasses of the Atlantic sector of the Southern Ocean during fall 1987

During the autumn of 1987 a survey was carried out in the Atlantic sector of the Southern Ocean in order to study phytoplankton community structure in relation to hydrological features. The positions of the boundary zones, determined by means of hydrological and chemical properties (especially silicic acid) match with previous studies. The phytoplankton community structure was studied by means of algal pigment fingerprints. A cluster analysis of the main phytoplankton pigments revealed the presence of four distinctive phytoplankton communities in the area. In three cluster groups phytoflagellate pigments of different taxonomical groups were found which showed different relative abundance between the cluster groups. In between the Polar Front Zone and the Weddell Scotia Confluence a fourth group was found which was rich in diatoms as compared to the other cluster groups. High concentrations of the fucoxanthin related 19′-hexanoyloxyfucoxanthin indicated the relative importance of Prymnesiophyceae during fall in this area. The relative contribution of each taxonomical group to total phytoplankton biomass was estimated by using specific pigment ratios. This calculation showed that in this time of the year phytoflagellate biomass (especially Prymnesiophytes) surpasses diatom biomass.

On iron limitation of the Southern Ocean: experimental observations in the Weddell and Scotia Seas

Shipboard studies were performed for testing the classical hypothesis that Antarctic phytoplankton suffers from a deficiency of Fe. In a suite of 5 experiments over 8 to 12 d periods and encompassing different water masses (Weddell Sea water proper, Weddell-Scotia Confluence water, Scotia Sea water), and various plankton communities, biomass and dynamic spring/summer (ice) conditions, we always observed Fe to stimulate chlorophyll a synthesis and nutrient assimilation. In 3 out of 5 experiments there was an immediate response to added Fe, while in the other 2 expenments an effect was observed after 3 to 6 d. In 4 out of 5 experiments final particulate organic carbon (POC) levels were also higher in Fe-enriched cultures compared to controls. However the controls were also found to outgrow steadily typical chlorophyll a and POC levels found in ambient waters. This strongly suggests that the in situ Fe concentration in itself does not hamper build-up of high biomass levels. Extrapolation to the in situ ecosystem therefore suggests that, despite enhancement of phytoplankton growth, Fe is not the major factor controlling phytoplankton in the Weddell/Scotia Seas. Marginal sediments appear to supply adequate dissolved Fe for supporting at least minimum growth of phytoplankton. More remote sectors of the Southern Ocean might be more likely candidates for occasional limitation by Fe alone.

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

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.

Upper layer thermal structure in the Atlantic sector of the Southern Ocean

By means of 7 XBT* sections between South America resp. South Africa and Antarctica the variability of the Subtropical Convergence (STC), the Polar Frontal Zone (PFZ) and the Weddell-Scotia Confluence (WSC) are particularly discussed. Between 45°W and 56°W, the STC is shifted to the South by 2.3 degrees of latitude confining a tongue of subtropical surface water near the continental slope of the Patagonian shelf. Further South, the PFZ is displaced to the North by 7 degrees of latitude between 58°W and 39°W. An intense Subantarctic Front has been observed northwestwards of South Georgia. Even East of the South Sandwich Islands the WSC can be detected by a positive temperature anomaly.

The Weddell-Scotia Confluence

Abstract The Weddell-Scotia Confluence is the zone separating the waters of the Weddell Sea from those of the Scotia Sea. Available historical hydrographic station data were examined to improve the description of this boundary zone from its western limit near the northern tip of the Antarctic Peninsula (∼55°W) to 20°W. Vertical sections of potential temperature, salinity, oxygen and density, as well as horizontal property distributions, confirm that the waters in the western part of the Weddell-Scotia Confluence are more homogeneous in the vertical than the surrounding waters and that, at depth, this zone is characterized by relatively low temperatures and salinities and high oxygen values. Near the sea surface, on the other hand, the confluence is characterized by relatively high salinities and low oxygen values. The distinguishing characteristics attenuate toward the east. Previous speculation has been that the relative homogeneity of this zone is due to vertical convection driven during winter by ice f...

Antarctic Polar Front Zone in the Western Scotia Sea—Summer 1975

Abstract The component of the FDRAKE-75 data obtained by the R/V Conrad in the western Scotia Sea reveals a definite sequence of thermohaline stratification zones encountered on passing from Antarctic to Sub-antarctic waters. A Polar Front Zone, displaying multiple temperature minima, separates the Antarctic Zone, characterized by a single intense T-min above 200 m, from the Subantarctic Zone with its nearly isohaline layer from 100 m to over 400 m. The Antarctic Zones of the Weddell and the Scotia Seas aye separated by a cold, relatively homogeneous zone situated in the southern Scotia Sea called the Weddell-Scotia Confluence. The boundaries of the Polar Front Zone are highly meandered and isolated eddies of Subantarctic water may occur within the zone. The main axis of the Antarctic Circumpolar Current apparently lies close to the Subantarctic boundary of the, Polar Front Zone, while a secondary axis is associated with the southern limit of the Polar Front Zone. Inspection of the Islas Orcadas and Melvi...