Phytocenoses of the Ob Estuary and Kara Sea Shelf in the Late Spring Season

Based on the material obtained in the spring-summer season of 2016, the composition and quantitative distribution of phytoplankton in the Ob estuary and over the Kara Sea shelf has been estimated. The latitudinal extent of the region exceeded 5 degrees (similar to 560 km), encompassing areas with different river discharge impact, which formed latitudinal zoning in the distribution of hydrophysical and hydrochemical conditions. These environmental properties determined the zoning in the spatial structure of phytoplankton communities. The estuarine biotope with a salinity <1 PSU and high concentrations of nitrate, phosphate, and silica was inhabited by a freshwater phytocenosis stable in species composition with high-quantitative characteristics and dominated by Aulacoseira species. The most spatial and seasonal variability of phytoplankton is characteristic of the area of intense interaction of fresh and marine waters in the outer part of the Ob estuary and over the adjacent shallow shelf, where freshwater algae species are replaced by marine species. A sharp biotopic boundary separates the inner shallow shelf with depths of approximately 30-35 m and outer shelf of the depth from 150 to 200 m. In the area of the outer shelf the lower boundary of the euphotic zone descends to 25-35 m. At theses depths plankton algae actively vegetate, and form subsurface phytoplankton maximum under conditions of sufficient nutrient concentration.