Phytoplankton assemblages under hydrochemical conditions of the Volga River Delta

In May 2021, comprehensive studies were conducted in the southeastern part of the Volga River avandelta (marine part of the delta). The work aimed to compare and identify the possible interaction of hydrochemical conditions and the structure of the phytoplankton community. Based on field observations, instrument measurements, and statistical analysis, three algaecoenoses were obtained (rapids, periphyton-pelagic, and marine), 4 types of environmental conditions identified by abiotic factors (oxygen-deficient, rapids, creak, and marine), and 2 types of biotopic conditions identified according to production-destructive characteristics (river with a prevalence of production, marine - production shifted towards equilibrium). For the marine part, an discrete phytopelagic community was identified, represented by benthopelagic aggregates with a complex trophic structure based on ribbon-like colonies of araphid diatom (Staurosira spp. и Staurosirella spp.). It is shown that such a community structure may serve as an indicator of water masses with significant destructive processes. Thus, an abiotic oxygen-deficient biotope located in the river part of the study area was characterized by a similar set of species. Both water areas were characterized by high AOU values, an increase in the contribution of mineral phosphorus to the Redfield ratio and a lack of nitrogen. At the same time, the water areas with a pronounced predominance of production processes over destructive ones were characterized by an abundance of centric diatoms. Thus, the selected groups of species can serve as indicators concerning certain biotopic conditions (in terms of production/destruction of organic matter).