Phytoplankton of the Ob Estuary (Kara Sea) in the Season Preceding Winter

Abstract—

The material was obtained during cruise 92 of the R/V Akademik Mstislav Keldysh in the area with a latitudinal extent of 72°32.0′ to 73°14.5′ N, including the Ob River estuary and adjacent shallow shelf on October 12–13 in the period preceding seasonal ice formation. The estuarine front was well pronounced in terms of salinity (gradients 0.6 PSU/km) and temperature fields and had a quasi-meridional direction. Sharp boundaries in nutrient distribution typical of earlier seasons were not observed. The abundance and biomass of phytoplankton varied from 22 × 10³ to 75 × 10³ cells/L and 7.3 to 19.0 mg/m³, respectively, which were two to three orders of magnitude lower than the values obtained in July and September. The abundance (30–78%) and biomass (28–80%) of phytoplankton over the entire area were formed by diatoms, among which Fragilariopsis cylindrus dominated. The abundance of freshwater diatoms Aulacoseira was low and the species was represented only by dead cells. A scheme is presented that shows the boundaries between phytoplankton communities of different composition and quantitative characteristics, as well as the specifics of conditions in the eastern area of the wide external part of Ob river estuary.