Phytocenosises of the Ob Estuary and Kara Sea Shelf in Late Spring Season
Based on the material obtained in spring-summer season of 2016 composition and quantitative distribution of phytoplankton in Ob estuary and over the Kara Sea shelf were estimated. Latitudinal extent of the region exceed 5 degrees (~560 km) and it embraced areas with different impact of riverine discharge which formed latitudinal zoning in distribution of hydrophysical and hydrochemical conditions. These properties of the environment determined zoning in spatial structure of phytoplankton communities. The estuarine biotope with salinity <1 psu and high concentrations of nitrate, phosphate and silica was populated with freshwater phytocenosis which was stable in species composition and high quantitative characteristics, and dominated by Aulacoseira species. The most spatial and seasonal variability of phytoplankton was peculiar to the area of intensive interaction of fresh and marine waters in the outer part of Ob estuary and over adjacent shallow shelf where freshwater algae species were replaced by marine ones. Abrupt biotopic border separated shallow shelf with depths approximately 30–35 m and outer shelf with 150–200 m depths. Over the outer shelf the lower bound of euphotic zone descends to 25–35 m. At this depth in the conditions of sufficient nutrients concentration plankton algae actively vegetate, and form subsurface phytoplankton maximum.