Polish Polar Research

Hydrological conditions in the region between the King George and Elephant Islands were defined on the basis of 24 STD measurements and 20 XBT profiles made in the time period from 26 October to 16 November 1986. In the entire investigated region the surface water temperature was below zero, often close to freezing point. The presence of summer modification of surface water with its characteristic thermal minimum was not found. Between the Scotia Front and the relatively well pronounced stream of geostrophic currents the anticyclonic meander was observed above the edge of the shelf at the depth of 100 m. Its spread was about 10 nM and within this meander the downwelling of surface water was noticed. As a compensating water movement, the upwelling of warmer waters was observed in the Drake Passage at 400 m.

In the examined area three types of waters have been recorded: Surface Waters of winter modification, Warm Deep Waters and East Bransfield Strait Waters. Geographical location of Scotia Front is similar to that observed in previous years. The dynamics of waters within the examined area is high. It is reflected not only in physical and chemical parameters but also in the distribution of chlorophyll α. In the Front its total amount in a water column is greater than outside.

Coarse-scale studies on chlorophyll a distribution in a region covering the Scotia Front zone showed an increased chlorophyll content and its deeper distribution at stations situated in the frontal zone. The sources of chlorophyll α were probably both the phytoplankton released from melting ice as well as spring bloom.

Total count (TC) of bacteria in drifting annual pack-ice in austral spring fluctuated between 2.8-106 and 2,09-109 dm3. TC of bacteria was lowest in the upper layer of a large pack-ice fragment, emersed above water surface and almost completely free of diatoms; it was comparable to TC of bacteria in surrounding sea water, which was very low at this time (1,92- 106 — 5.8-106 dm -3). TC of bacteria increased in the deeper layers of pack-ice, attaining a maximum in the middle layer characterized by a high count of diatoms. TC of bacteria was highest in small pack-ice pieces 10—20 kg in being and densely overgrown with diatoms. Bacterial population in pack-ice was dominated by rods (62%), and it contained filamentous bacteria (2.4%) and prosthecate forms (4,8%), rarely present in deep sea. Mean volume of bacterial cell (0,206/μm3) was small, only slightly exceeding that of cells of free-living bacteria in sea water in summer.

Chlorophyll a content and the density and species composition of algae were determined in drifting sea ice north of the Elephant Island (between 54-56°W and 60°30'—61°00'S) at the end of October 1986. In yellow-brownish pieces of brash ice the amount of chlorophyll α was on average 203.5 ± 149.9 mg m-3 at the density of algal cells of 255.7+137.8-103 in cm3. In not visibly discoloured ice the respective values were about 80 times lower, and in surface water about 700 times lower. 69 algal taxa were recorded in the samples, almost all of which were diatoms. Nitzschia cylindrus dominated in all the samples. A comparison of species composition in the investigated habitats revealed that the highest species similarities occurred between samples collected in discoloured ice, lower in the uncoloured ice and the lowest ones in water.

Low concentrations of phytoplankton (average 2.5 x 104 to б.0 х 105 cells l-1) were found at ten stations surveyed in the region of the Weddell-Scotia Confluence. Phytoflagellates represented mainly by 1—3 μm picoplankton were prevalent among the algae, contributing 65—100% to the total numbers: this group is observed to dominate over diatoms in areas of intensive water mixing. Maximum concentrations of phytoplankton at one station, reaching down to 200 m, were due to a physical aggregation of cells by confluencing and downwelling waters. The average for the water column quantities of the same algal groups were nearly identical at most stations, but peak numbers occurred in the 0—75 m surface layer. Differences in diatom assemblages were associated with the complex hydrography of the WSC region.

Net phytoplankton was studied in 10 stations situated west of Elephant Island along three transects located in the Scotia Front zone and in 2 stations situated in the eastern Bransfield Strait. Altogether 70 algal taxa were identified. Phytoplankton density and seston biomass were the highest in stations neighbouring the Scotia Front from the south. In the Scotia Front region Corethron criophilum was a dominant species. Phytoplankton community of the region is rather uniform, however the closer comparison of the phytoplankton species composition allowed to distinguish 4 stations groups. This division is concordant with the hydrological one. A different phytoplankton community was recorded in the eastern part of the Bransfield Strait.

Zooplankton in the water column from the surface to bottom was studied. Copepods were the dominating organisms. Average zooplankton biomass was about 5 g in 1000 m3 . The highest zooplankton density occurred between 300 and 600 m. The influence of the Scotia Front on the horizontal and vertical distribution of zooplankton is discussed.

The distribution of pelagic Polychaeta in the Scotia Front region is presented. 6 polychaete taxa were recorded in the material with the most abundant Pelagobia longicirrata which constituted 86% of all collected specimens. The mixing of water masses in the frontal zone influence the quantitative distribution of polychaetes in the water column.

Planktonie material was taken in stratified hauls in the water column between King George and Elephant Islands, during the austral spring 1986. The species composition of Copepoda was diversified (abt. 50 taxa). Most frequent and abundant were M. gerlachei, C. acutus, R. gigas, small copepods of the family Pseudocalanidae and Cyclopoida. Interzonal Copepoda did not yet reach the euphotic zone; a comparatively low general copepod abundance and the advanced ontogenetic development in particular populations evidenced for the early spring phase of the planktonie community.

In the planktonie material collected using a Nansen net (vertical hauls) larvae of two euphausiid species were found. The dominant and occurring in all stations were larvae of Thysanoessa macrura. Following larval stages were encountered: nauplius, metanauplius, calyptopis I and calyptopis II. The most numerous and occurring in the widest depth spectrum were calyptopes I. Only twice furcilia VI of Euphausia superba were found. The distribution of euphausiid larvae was influenced by the stratification and circulation of water masses in the investigated area.

Krill population structure was studied in Western Antarctic in austral spring and summer 1986/87. At the end of October and the beginning of November in the waters around Elephant Island the mean krill length was 44 mm and sexually mature specimens dominated. Juveniles were absent. In the Bransfield Strait the mean E. superba length was 43 mm. and males slightly overdominated females. The share of females with spermatophores and of immature females was higher than in the Elephant Island area. Juveniles were also not recorded. Krill was most diversified around Elephant Island in January; juveniles and females with ovaries filled with eggs were recorded. It was found that 84% of krill population was infested by ciliate protozoans (Apostomatida).

Results of hydroacoustic investigations of krill biomass carried out in the South Shetland Island region between October 1986 and January 1987 are presented. A considerable difference in the krill biomass between Antarctic spring and summer was recorded. Initially observations were conducted close to Elephant Island, in the period just after the retreated of compact ice cover. Krill then aggregated only in swarms, the density of which frequently exceeded 100 t nM-2 . In the region of Polygon I (30—31 October 1986) the total estimated biomass was 26899 t, in the region of Polygon II (6—10 November 1986) it was 25827 t. Investigations were repeated in January 1987 obtaining 112372 t in the Bransfield Strait and 390309 t in the region of Elephant Island. The results are presented in tables and maps.

Results of hydroacoustic investigations of krill swarms occurring southwest of Elephant Island carried out between 30 October and 5 November 1986, are presented. Krill swarms of the geometric length of 32 m, mean vertical cross section area of 206 m2 , and mean density of 133 g m-3 were recorded and measured. Biomass distribution is presented in maps. The highest density values amounting to 5001 nM-2 were recorded in the eastern part of the survey area, above the slope of Elephant Island's shelf. On the basis of upper and lower limits of the occurrence of given krill swarms, a scheme of their vertical, diurnal distribution was constructed.

On the basis of hydroacoustic observations it is shown that migrations of krill during spring are stronger than during summer. Migrations of krill are described by the function: H(t) = A + Bcos((2ᴨt/T + φ ) + C c o s ( 2 ᴨt/T + φ ), where: H is depth of the mass center of krill biomass, A — mean depth of krill occurrence, В — amplitude of migrations with period T! = 24 h, С — amplitude of migrations with period T2 = 12 h, (φ1, φ2 — phases of migration process with T, = 24 and T2 = 12 hours. Parameters of the equation are the following: spring — A = 62.2 m, В = 19.5 m, С = 4.6 m, φ1 = 0.1 h, φ2 = 0 . 1 5 h; summer — A = 75.8 m, В = 0.5 m, С = 3.6 m, φ1 = 1.8 h, φ2 = 6.4 h.

On the basis of acoustically registered cross-sections of krill aggregations, regular, irregular and layer forms were distinguished. Regular forms are most frequently observed during spring and in the day time, while irregular forms are most frequent during summer and night hours. The density histograms made for two hour intervals clearly show the day-night difference, but the seasonal (spring, summer) difference is less pronounced. Mean density of swarm is lowest during the night and reaches a maximum in early morning hours. The mean volume backscattering strength values (Sv) for spring and summer are nearly identical. We suggest that regular forms correspond to foraging swarms and irregular forms to feeding swarms as described by Hamner (1984).

In pelagic catches of the Polish BIOMASS III Expedition 24 fish species belonging to 6 families were encountered. Pleuragramma antarcticum, Electrona antarctica and Protomyctophum bolini dominated in the Bransfield Strait. Pagothenia brachysoma was abundant in the region of Elephant Island. The presence of juvenile Channichthyidae was recorded in this area only in January. In comparison with observations in other seasons an increased frequency of Chaenodraco wilsoni and decreased frequency of Chaenocephalus aceratus was noted.

24 species of 8 fish families were found to occur on the shelf of Elephant Island during austral spring and summer of 1986/87 season. Notothenia gibberifrons was a predominant species (78—83% per catch). The presence of Champsocephalus gunnari and Chaenocephalus aceratus (15—20% per catch) almost completed fish composition profile. A pronounced decrease of Notothenia rossi (0.03—0.04% per catch) was noticed. Some specimens of Champsocephalus gunnari were characterized by a spawning-time shifted by almost half a year comparing to the remaining part of its population.

In the region of Bransfield Strait and southern part of Drake Passage the highest amounts of chlorinated hydrocarbons (CHs — compounds of the DDT group, HCH isomers and PCBs) were found in the samples taken at the sampling station where the CHs bottom deposits were released to the upper layers due to the special hydrological situation at this station. Increased amounts of CHs were observed also in phytoplankton sampled close to the melting ice of glacier origin which was considered as a source of pollution. However, phytoplankton sampled from the waters covered with pack-ice exhibited the lowest rate of CHs accumulation. Slightly elevated CHs accumulation was found in sea ice diatoms. All the samples exhibited elevated amount of polichlorinated biphenyls, markedly higher than that of chloroorganic insecticides.