Ecological characteristics, biomass, abundance and dominant complexes phytoplankton of different types of watercourse and waterbasin in the lower Danube

In the context of modern environmental challenges, being under constant anthropogenic pressure, systematic study of the spatial and temporal dynamics of phytoplankton distribution, its quantitative and qualitative indicators, biomass and abundance, and determination of the dominant complex are indicative factors of biodiversity and water productivity. In the context of constant anthropogenic pressure, such research and monitoring of biodiversity and water quality are essential for the functioning of biocenosis. It was found that the diversity of phytoplankton in different hydroecosystems of the lower Danube River is represented by a wide floristic spectrum and amounts to 156 species (158 species) belonging to 8 divisions: Cyanobacteria, Bacillariophyta, Cryptophyta, Miozoa, Ochrophyta, Charophyta, Chlorophyta, Euglenozoa. The research of phytoplankton ecological characteristics showed that planktonic and planktonic-benthic forms are the most common in terms of biotopic habitat. Analyzing the phytoplankton by habitat, indifferent species dominate in all studied hydroecosystems. In relation to pH, most of the representatives belong to indifers, and fewer to alkalifers. The number of phytoplankton cells ranged from 375 thousand cells/m2 to 1320 thousand cells/m2, with Cyanobacteria being the most numerous. The main role in the formation of the biomass of water bodies in summer in the studied areas belongs to the divisions Chlorophyta, Miozoa, and Bacillariophyta and ranges from 0.3361 mg/dm3 to 0.944205 mg/dm3. The analysis of the dynamics of phytoplankton abundance and biomass showed that these indicators vary depending on the location of the algosampling stations. The structure of phytoplankton biomass in the lower Danube is unevenly represented, according to the hydroecosystem, and is formed mainly by the Bacillariophyta-Chlorophyta complex, but in some samples, representatives of the Mioza order dominate in terms of biomass. At the same time, summarizing the values of phytoplankton abundance, it was found that the leading role belongs to Cyanobacteria, with Chlorophyta and Bacillariophyta also widespread. It was noted that in shallow waters and in areas with high dynamics of hydrological processes, namely river-sea transition zones, there is a predominance of the diatom complex, formed mainly by planktonic-benthic and benthic forms. This spatial distribution is typical for areas with high intensity of water mass mixing.