Throughout the world, river basins are directly or indirectly affected by human activities, reducing local and global biodiversity and preventing the ecosystem from properly functioning. Our research focused on the Irpin River basin (Ukraine), whose water bodies have experienced various impacts due to human activities, including the unexpected extremes caused by military operations in the catchment area: long-term flooding, disturbance of free flow, significant water level fluctuations, etc. The study hypothesized that the primary factors determining the structural and spatial distribution of quantitative indicators of microalgae and aquatic macrophytes are the result of various hydromorphological changes, that lead to changes in the physical and chemical parameters of the aquatic environment. Very high values of chlorophyll a in the water column (59–106 µg · L⁻¹), an increase in the abundance (number of cells) and biomass of algae (due to the predominance of certain groups in the transformed sections), as well as saprobic index were recorded in the sections of the Irpin River basin that underwent significant hydromorphological changes. Our results revealed a strong correlation between phytoplanktonic (in the water column) chlorophyll a levels and water temperature (r = 0.76, p < 0.001), as well as organic phosphorus and polyphosphate concentrations (r = 0.61, p < 0.01). ANOVA and Monte Carlo permutation tests in a Canonical Correspondence Analysis (CCA) showed that the abundance of different divisions of phytoplankton and phytobenthos were significantly and similarly related to several environmental variables. We observed a positive correlation between the number of cyanobacteria and the concentration of ammonium nitrogen, nitrites, and phosphorus compounds. An increase in dissolved organic matter in the water can explain the increase in the biomass of Dinoflagellata and Euglenophyta. Species richness and the cover values of the macrophytes also clearly reflected changes in vegetation activity in sections of the Irpin River caused by hydromorphological changes. The results indicated that long-term flooding had the most negative impact on macrophyte communities. At some sites, the impact was so severe that the number of macrophyte species was very low. The total number of macrophyte species showed a significant negative correlation with total suspended solids (r = −0.51, p < 0.05) and phytoplankton chlorophyll a concentration (r = −0.73, p < 0.001). Our results provide a scientific basis for predicting changes in riverine microalgal and aquatic macrophyte communities due to extreme hydrological events.

The Sasyk Reservoir is one of the largest seaside reservoirs in Ukraine, artificially maintained as a freshwater area including various floodplain areas. The Reservoir originated from an estuary to now being an almost freshwater reservoir. The diatoms (Bacillariophyta) in benthos were investigated for analyzing the coastal zone diversity of the Black Sea inside the borders of Ukraine. The indicated diatom composition was formed by 97 species (100 infraspecies taxa). Some rare marine and freshwater taxa were noted, as well as three species reported in Ukrainian territory for the first time (Navicula vandamii Schoeman et R.E.M. Archibald, Sellaphora difficillima (Hustedt) C.E. Wetzel, L. Ector et D.G. Mann, and Mastogloia laterostrata Hustedt). The bioindicative characteristics of the diatom composition testified to the freshwater condition of the reservoir. Changes to the salinity level of the Sasyk Reservoir were exposed due to the composition of benthic diatoms. The presence of mesohalobous species (7 taxa) acknowledges conditions that are typical for estuaries and mouths of rivers and reveals the salinity changes in the Sasyk Reservoir. Likewise, the prevailing number of oligohalobous diatoms (61 taxa) reveal presence of freshwater in the reservoir. The implementation of the Polish phytobenthos lake assessment method for the nearby Ukrainian Reservoir was conducted for the first time. The multimetric Diatom Index for lake values varied from 0.39 to 0.76, and revealed alterations in the status over the Sasyk sites, ranging from good, moderate, to poor. Thus, it may be concluded that the current ecological status relating to the composition of benthic diatoms of the reservoir located in the coastal zone of the Black Sea is evaluated as having a moderate status class with some exceptions.

Forty years ago, the transformation of the estuary of the Black Sea to a freshwater reservoir was started by its connection with the Danube River through the Danube–Sasyk Canal. Today, the inflow of the Danube water into the Sasyk is the main component of the water balance, affecting the internal and external water exchange, and it is, in general, responsible for the quality of the aquatic environment. In addition, the channel is a migration route for hydrobionts and the main source of invasion. We describe the changes in the composition of algal communities in Sasyk, which was converted from an estuary to a reservoir during three stages: estuary-lake in 1967–1977 (stage I), the forming of the reservoir from 1980–1990 (stage II), and the reservoir from 2013–2019 (stage III). Average salinity decreased from 7790 to 491 mg L⁻¹ chlorides, and the species richness of algae increased from 259 to 289 taxa during the Sasyk transformation. Analysis of the algal communities revealed that salinity could have an effect on their composition. The species list for the Sasyk, including 586 taxa of algae and cyanobacteria (613 infraspecific taxa), is presented here for the first time. Rare marine and freshwater taxa present in the algal community at the modern stage of the reservoir and their transformation are discussed. An ecological analysis of the different stages of the Sasyk transformation from an estuary into a reservoir was conducted. The increase in species richness for stages II and III along with an increase in desalination was noted. To keep the current ecological status of the reservoir and its hydrological stability, the role of the Danube River should be considered. Bioindicators, statistical analysis of species, and environmental variables’ relationships confirm salinity as a major regulating factor that can be observed by way of long-term monitoring.

The study aimed to conduct a hydromorphological assessment of the main types of surface water bodies (SWB) in the Irpin River basin and to determine the degree of change in the hydromorphological characteristics of water bodies in the areas affected by the war. The assessment was carried out based on the analysis of the results of expeditionary research in 2023–2024 using stock, literary, cartographic data, and space photographs according to the “Methodology of hydromorphological monitoring of surface water bodies of the categories ‘Rivers’ and ‘Lake’ adopted in Ukraine” (2019) and from considering the practice of conducting similar studies on other river basins in Ukraine and the EU. It was established that the studied areas belong to the I–IV class of hydromorphological condition; in particular, the II–III class prevails on small rivers, both I–II and IV classes are observed on medium ones, and large ones (the Irpin River in the lower and middle reaches) I–IV class with a slight predominance of II class. The main factors affecting the deterioration of the hydromorphological state are the regulation of flow by ponds and reservoirs, morphological changes in the riverbed and coastal zone, reclamation of hydrotechnical structures, and using floodplains for economic needs. Hydromorphological indicators before and after the impact of military operations were determined to assess the degree of impact of military operations. Changes in the HM condition were established in the destruction and subsequent restoration of bridges and hydraulic structures. The estuary of the Irpin River, where the HM class changed from II to IV, suffered the greatest military impact. Based on cluster analysis, groups of sites with similar hydromorphological conditions and the degree of military influence were selected. Differences in the HM condition in the main morphological zones of rivers—riverbed, coastal zone, and floodplain were evaluated.