Spatial and seasonal variability of Arctic phytoplankton communities in conditions of interaction with Atlantic origin waters in the St. Anna Trough (Kara Sea)

Abstract

For the first time, seasonal changes in phytoplankton communities were described in the Kara Sea region, which is influenced by modified Atlantic water through the St. Anna Trough. The study focused on June, when the sea ice began to retreat, July after three weeks without sea ice, and September, when Siberian river discharge had a widespread surface impact. In all seasons, the structure of the phytoplankton community differed significantly on the outer shelf of the Kara Sea and the slope of the St. Anna Trough. This boundary was caused by changes in the hydrophysical structure and was located approximately at 76.5 N, with a width of about 20 km. The highest phytoplankton abundance, biomass and photosynthetic activity were observed on the outer Kara Sea shelf in the cold winter layer with temperature ranging from −1.7 to −0.5 °C and salinity ranging from 32.5 to 33.7. In June and July, the layer was located in conditions with sufficient underwater irradiance and nutrient supply. The integral abundance and biomass of algae differed 10–20 times between the outer shelf of the Kara Sea and the slope of the St. Anna Trough, where this layer was not present. In September, stratification on the Kara Sea shelf increased under the influence of pronounced river runoff, and the cold winter layer became thinner and deeper, with a significant decrease in phytoplankton abundance. Despite the high similarity (61–64 %) in species composition between the outer shelf and slope of St. Anna, there were differences in dominant species between the areas each month. In June the spring diatoms Chaetoceros socialis, Fragilariopsis spp. (F. cylindrus, F. oceanica) and Navicula pelagica mass developed on the Kara Sea outer shelf, while Phaeocystis pouchetii did on the slope of the St. Anna Trough. In July, resting spores of Chaetoceros socialis, as well as Apedinella radiance, and Choanoflagellates prevailed on the shelf, but Dicrateria sp. was the most abundant in the St. Anna Trough. In September Leptocylindrus danicus, Peridiniella catenata and Thalassionema nitzschioides were the base of community on the outer shelf, however Thalassiosira cf. levanderii, Leptocylindrus minimus, Nephroselmis sp. dominated on the slope. The “Atlantification” through the St. Anna Trough in June and July had a similar effect on the phytoplankton in the Kara Sea's outer shelf as it did on the phytoplankton communities in the Polar Front area of the Barents Sea during April–May.