Optical studies were carried out in waters of the Drake Passage and the South Shetland Islands region from February 14 to March 12, 1981. The total energy of solar radiation reaching the sea surface was continuously recorded over daytime hours. Spectral and energetic characteristics of natural light field in the sea were determined basing on underwater measurements of downwelling irradiance attenuation. The thickness of the euphotic zone and other characteristic optical depths were also evaluated. The investigated waters were conventionally classified into three groups of different optical water types as follows: a) Clear oceanic water, b) Oceanic water of intermediate type and water affected by coastal water, c) Coastal water and water with high biological productivity. The clearest waters were found in the Drake Passage where the average thickness of the euphotic zone was about 100 m. The turbid waters of coastal types were encountered in some areas around the South Shetland Islands. The relatively thin euphotic zone of about 30 m was observed in waters with high biological productivity west of Elephant Island and southwest of Anvers Island.
Following compounds were determined in samples from Bransfield Strait and southern part of Drake Passage (area "A"): dissolved free- and combined amino acids, dissolved mono- and polisaccharides and urea. Concentration of urea in most samples ranges from traces to 1.5 μgat Nurea-1 and total urea content in water column from 10 to 150 m lies between 19.23 and 197.4 mgat Nurea . Dissolved free amino acids concentration ranges from 0 to 0.60μg x 1-1 and total free amino acid content are found to be between 20 and 60 mmol. Concentrations of combined amino acids lay below 7 μmol x 1-11 and integrated value for combined amino acids fluctuates between traces and 450 mmol. Monosaccharides concentrations in most samples do not exceed 2.5 μmol x l-1 and their content in water column lies below 180 mmol. Polysaccharides content in water column ranges from 1.8 to 3.94 mol and concentrations vary between 8 and 32 //mol-1 1. Evident differences in the content of dissolved organic compounds between Bransfield Strait and southern part of Drake Passage were found.
Numbers of saprophytic bacteria were determined by the plate count in samples obtained at 45 oceanographic stations, from six standard depths between 10 and 150 m. Depending on the sampling place, the numbers of bacteria fluctuated between 0.8 x 102 to 4.3 x l 04 x 1-1 and 1.2 x l07 to 1.3 x 108, in a water column under 1 m2 sea surface. Most of saprophytic bacteria were observed at stations located south and south-east of the King George Island, and also north and north-west of the Anvers Island. Fewer numbers were found in areas of large krill swarms in the Bransfield Strait, between 58°30' and 62°30' W, and in the north-western part of the research area, far away from the South Shetland Islands.
Maximum values of the settling volume and dry weight of suspended particulate matter, were found in the open waters of the southern Drake Passage (between 60°8' S and 62° 11' S), and west of the Anvers Island. Minimum respective values were observed in the Bransfield Strait. The distribution of phytoplankton cell numbers and of algal biomass expressed as total cell volumes closely followed the distribution of particulate matter. Diatoms were the major algae of the plankton. Several species of the genera Chaetoceros, Nitzschia and Corethron were dominant and characteristic of the phytoplankton assemblages in different parts of the study area.
Zooplankton samples taken in February and March 1981 in the southern Drake Passage and the Bransfield Strait revealed distinct differences between animal communities inhabiting water masses of different origin and of different physico-chemical properties. The West Wind Drift waters of the Drake Passage were rich in zooplankton; they were characterized by a high abundance of Radiolaria and young Limacina sp., the constant occurrence of Rhincalanus gigas, a significant share of Clausocalanus sp. and Calanoides acutus. On the other hand the above mentioned forms were nearly absent or scarce in the much poorer waters adjacent to South Shetland Islands and especially waters of the Bransfield Strait where such copepods like Metridia gerlachei and Oncaea curvata dominated or at least played a significant role being rare and scarce or absent in the Drake Passage. This picture was especially clear in the upper 100 m water layer, whereas in the deeper layer (300-100 m) these quantitative and qualitative differences were less obvious.
Stocks of krill in the southern part of the Drake Passage and in the Bransfield Strait were estimated by the hydroacoustic method, during the BIOMASS-FIBEX expedition (Febr.-March 1981). Krill stocks in the Drake Passage and Bransfield Strait were assessed at about 1.2 and 2.3 mln tons, respectively. A map of krill distribution in these regions was prepared. The main krill biomass (66%) was found to occur within the Bransfield Strait which accounts for only 13.7% of the total area under survey.
The compilation of experimental data on krill target strength is performed and results compared with the theory. A modification of the Johnson's theory is proposed to fit experiment.
In the investigated area, mass occurence of krill was observed in the vicinity of islands and the Antarctic Peninsula, in the waters above the shelf and shelfs slope. Small quantities of krill were found in the open oceanic waters. Immature individuals dominated close to the Antarctic Peninsula. Large, sexually mature kril dominated farther from the continent. Gravid females contributed little to the total populations. Krill of the largest size occured near the Palmer's Archipelago, and of the smallest size at the Antarctic Peninsula and the Elephant Island. It is likely that krill observed west and north of the Palmer Archipelago had been brought by currents from the Bellingshausen Sea. Krill in the Bransfield Strait originated probably from the mixing of populations carried by currents from the Weddell Sea and the Bellingshausen Sea.
Krill larvae distribution and abundance in waters surrounding South Shetland Islands were studied in February and March 1981. Main concentrations of larvae were noted over great depths near the continental slope. High densities of krill larvae were encountered in stations where phytoplankton was moderately abundant.
The phytogenic food composition in the stomachs of Euphausia superba Dana, caught at 18 sampling stations in the Antarctic part of the Atlantic Ocean, was analysed. The material used was taken from krill catches made from the board of the r/v "Profesor Siedlecki" in the sector "A" of the BIOMASS-FIBEX Programme, in February and March 1981. In the food of Euphausia superba 70 algal taxa were identified, including 68 taxa belonging to Bacillariophyceae and two to Chrvsophyceae. Planctonic diatoms were the main component of the food of Euphausia superba. Single benthic diatoms were found occasionally.
Studies were carried out in the region of southern. Drake Passage and Bransfield Strait in February and March 1981. The relation occurring between the alimentary tract filling (ATF) and the quantity of chlorophyll α integrated within the range of 0-150 m water-layer may be described by Ivlev's equation expressing the amount of the food ration in relation to food concentration. The ATF value increases in large individuals and is proportional to their body weight. The daily rythm of krill feeding, expressed by ATF, depends on the quantities of food in the environment.
Observations on abundance and distribution of juvenile fish within the krill concentrations were made during February—March 1981. Juvenile and scarce postlarval stages belonging to 23 species of the suborder Notothenioidei were recorded in the investigated area. Chionodraco rastrospinosus and Chaenodraco wilsoni were the most frequent and numerous species. Juvenile fish, as well as extensive concentrations of krill, were recorded mainly in the southern part of the Bransfield Strait and in the shelf waters westwards of the Palmer Archipelago. The juveniles were however absent in the open waters of the Drake Passage.
In the region under investigations 30 species of seabirds were observed. In the southern part of Drake Passage Daption capense. and Oceanites oceanicus predominated quantitatively, Macronectes giganteus, M. halli and Pachyptila spp. were subdominant. The vessel was often accompanied by Diomedea exulans. In the Bransfield Strait Fulmarus glacialoides, D. capense and O. oceanicus were predominant. M. giganteus was subdominant. The estimated value of the biomass of seabirds was the highest in the region to the west of Elephant Island, slightly lower in the region of Anvers Island, Smith Island and Livingston Island, and the lowest in the south-eastern part of Bransfield Strait and the middle part of the investigated region of Drake Passage.
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