Structure of the Phytoplankton Communities and Primary Production in the Ob River Estuary and over the Adjacent Kara Sea Shelf

The material was collected in the Ob River estuary and over the adjacent shallow Kara Sea shelf between 71 degrees 14'0 and 75 degrees 33'0N at the end of September 2007. Latitudinal zoning in the phytoplankton distribution was demonstrated; this zoning was determined by the changes in the salinity and concentration of nutrients. Characteristic of the phytocenosis in the southern desalinated zone composed of freshwater species of diatom and green algae were the high population density (1.5 x 10(6) cells/l), biomass (210 mu gC/l), chlorophyll concentration (4.5 mu g/l), and uniform distribution in the water column. High primary production (similar to 40 mu gC/l/day) was recorded in the upper 1.5-m layer. The estuarine frontal zone located to the north contained a halocline at a depth of 3-5 m. Freshwater species with low population density (2.5 x 10(5) cells/l), biomass (24 mu gC/l), and chlorophyll concentration (1.5 mu g/l) dominated above the halocline. Marine diatom algae, dinoflagellates, and autotrophic flagellates formed a considerable part of the phytocenosis below the halocline; the community characteristics were twofold lower as compared with the upper layer. The maximal values of the primary production (similar to 10 mu gC/l per day) were recorded in the upper 1.5-m layer. The phytocenosis in the seaward zone was formed by marine alga species and was considerably poorer as compared with the frontal zone. The assimilation numbers at the end of the vegetation season in the overall studied area were low, amounting to 0.4-1.0 mu gC/mu gChl/h in the upper layer and 0.03-0.1 mu gC/mu gChl/h under the pycnocline.