Abiotic Conditions for the Formation of Pelagic Phytoplankton Community in the Volga River Delta

This paper presents the results of an expedition conducted from May 28 to 30, 2022, in the avandelta of the Volga River and within the Astrakhan State Biosphere Reserve. The study encompassed various biotopes of the avandelta, including river channels, Gryaznukha Bay, the Gandurinsky and Babushkin Channels (intrazonal section of the avandelta), and the Chistaya Banka area in the shallow waters of the Northern Caspian Sea near Zhemchuzhny Island. Measurements in the aquatic study area included key nutrients (mineral nitrogen species, phosphates, silica) and oxygen; Apparent Oxygen Utilization (AOU) was calculated, and samples were collected for analysis of phytoplankton communities. Two microalgae groups, conditionally “marine” and conditionally “riverine”, were identified for phytoplankton with a high level of probability (Global R = 0.873, p = 0.1%). Cyanoprokaryotes dominated both assemblages. The “riverine” group was dominated by Kovacikia cf. muscicola and Cyanodictyon cf. filiforme, while the “marine” group was dominated by Aphanothece clathrata, Kovacikia cf. muscicola, and Aphanothece floccosa. Biotopic conditions characterizing the “riverine” group exhibited higher nutrient concentrations compared to those of the “marine” group. The phytoplankton communities demonstrated significant differences in the composition of dominant complexes, structure, and life forms from the communities identified in the same area during the comparable seasonal period of the previous year (May 18-20, 2021), which were dominated by diatoms.