Diversity of aquatic organisms in the lowland watercourses: a case study in transboundary rivers, Transbaikalia, Russia

Planktonic communities have important roles in aquatic ecosystems, including rivers. But studies of plankton organisms in lotic systems are infrequent. The present research has been performed in transboundary rivers (River Uldza and River Imalka), flowing through the Daursky State Nature Biosphere Reserve (Transbaikalia, Russia) during the beginning runoff and increasing discharge (2020–2022). Aquatic organisms (algae, cyanobacteria, rotifers, and crustaceans) in the surveyed watercourses have been studied for the first time. We collected phytoplankton and zooplankton samples to explore species and functional diversity and temporal trends in the studied rivers. The total species composition included 134 phytoplankton taxa ranked below the genus level; the order of dominance was Bacillariophyta (49 taxa) > Chlorophyta (47 taxa) > Cyanophyta (18 taxa) > Charophyta (10 taxa) > Chrysophyta (4 taxa) = Euglenophyta > Cryptophyta (1 taxon) = Dinophyta) and 116 zooplankton taxa (Rotifera (64 taxa), Cladocera (35 taxa), and Copepoda (17 taxa). Not determined Bdelloida (Rotifera) and juvenile Copepoda and Anostraca were also encountered in zooplankton. Algal flora and fauna of invertebrates were formed mainly by cosmopolitans, 68.0% and 61.4% of the total composition respectively. Functional diversity of planktonic communities was determined by 19 Functional Groups of phytoplankton (FGph), 7 Morphologically Based Functional Groups of phytoplankton (MBFGph) and 17 Functional Groups of zooplankton (FGzoo). In phytoplankton, codons MP (epiphytic and epilithic species drifted in the plankton that prefer frequently stirred up, inorganically turbid shallow lakes) and X1 (species, which prefer shallow, eu-hypertrophic environments, resistant to stratification, sensitive to nutrients deficiency) and groups IV (organisms of medium size lacking specialised traits) and VI (non-flagellated organisms with siliceous exoskeletons) dominated in species composition and abundance. In zooplankton, littoral and phytophilic species, filter feeders and species with swimming and crawling locomotion were the most diverse and abundant. Plationus polyacanthus (Rotifera) and Megafenestra aurita (Cladocera) have been found for the first time in the fauna of Zabaikalsky Krai. In the River Uldza, the phytoplankton and zooplankton species diversity and quantitative parameters showed seasonal variations. Cyanobacteria, Chlorophyta, and Bacillariophyta dominated phytoplankton biovolume with peaks in Cyanobacteria (Anabaena spp., Coelomoron pusillum, Gloeocapsa minima, G. minor) and Chlorophyta (Monoraphidium griffithii, Willea apiculata, W. irregularis) occurring during summer and peaks in Bacillariophyta (Ulnaria ulna) occurring during spring and autumn. Seasonal patterns in zooplankton structure included peaks in larval stages of Copepoda and Anostraca during spring, Rotifera (Trichocerca elongata, Hexarthra mira, Euchlanis dilatata) during mid-summer and Chydoridae (Chydorus sphaericus, Pleuroxus aduncus, Coronatella rectangula) during autumn. Based on our results, we recommend undertaking a comprehensive study on transboundary rivers in order to research biodiversity and gain a clear understanding of the impact anthropogenic activities (dams) and climatic factors (the intensity and duration of flood) could have on aquatic communities.