Hydrological conditions are responsible for the changes in lateral connectivity between the main river channel and the floodplain lakes, a factor controlling zooplankton abundance and diversity. We tested the hypothesis that the degree of connectivity between the aquatic habitats and the river channel governs the zooplankton densities and community structure. Abundances, community composition and species diversity of zooplankton were analysed against the gradient of lakes’ connectivity and the water quality parameters under a natural flood pulse in the Biebrza River (North-Eastern Poland). Our findings revealed that the water level fluctuations directly affect the availability of nutrients, aeration, what in turn controls the densities and biovolumes of zooplankton communities. Along with the increase in the lake isolation, the taxonomic diversity of zooplankton decreased, while the eudomination of taxa indicative of advanced trophy (Rotifera) was observed. Qualitative parameters, as number of species, diversity and richness, were significantly higher at mean water levels, which supports the intermediate disturbance hypothesis. The sensitivity of the zooplankton community to variable hydrological conditions and lateral connectivity gradient demonstrates its potential as an unexploited indicator of any habitat changes in the aquatic ecosystems.

The response of bacterioplankton structure to salinity level in coastal lakes (n = 9) along the southern Baltic Sea coastline was studied. In terms of mean salinity levels (0.2–5.2 PSU), the lakes represented freshwater, transitional, and brackish types. Results showed that salinity determines the spatial and seasonal distribution patterns of microorganisms in costal lakes. Increased salinity contributed to a significant decline in total bacterial numbers (TBN). The TBN was lowest in brackish lakes in autumn (4 × 10⁶ cells/mL) and highest in freshwater lakes in summer (7.11 × 10⁶ cells/mL). The groups of Proteobacteria are appropriate bioindicators in any classifications of coastal ecosystems, particularly at low-haline stress. Alpha- and Gamma- subclasses of Proteobacteria are identifiers for brackish habitats, while Betaproteobacteria, due to their intolerance to haline stress, prefer freshwater habitats. Counts of euryhaline Actinobacteria, the dominant group of bacterioplankton (31.8%), decreased significantly with increased salinity. Actinobacteria and Deltaproteobacteria were identifiers of transitional lakes. Cytophaga-Flavobacteria showed affinity with freshwater ecosystems, but this relation was not statistically significant (p > 0.05). The bacteria groups correlated with other physico-chemical parameters of water, such as oxygenation (Actinobacteria) or organic carbon (Betaproteobacteria, Deltaproteobacteria). The impact of hydrological connectivity and salt-water interference on the microbiota structure and biogeochemistry of coastal waters should be considered in the assessment of the ecological status of coastal lakes.

We evaluated the influence of habitat connectivity and local environmental factors on the distribution and abundance of functional fish groups in 10 floodplain lakes in the Biebrza River, northeastern Poland. Fish were sampled by electrofishing, and 15 physico-chemical parameters were recorded at three sampling sites at each lake in the period of 2011–2013. A total of 18,399 specimens, belonging to 23 species and six families, were captured. The relationships between environmental factors and fish communities were explored with the use of canonical correspondence analysis (CCA). Sampling sites were grouped based on fish communities using a hierarchical cluster analysis (HCA). Along a lateral connectivity gradient from lotic to lentic habitats (parapotamic–plesiopotamic–paleopotamic), the proportions of rheophilic species were determined as 10:5:1, whereas the proportion of limnophilic species was determined as 1:2:5. The predominant species were the roach (Rutilus rutilus), and pike (Esox lucius) in parapotamic lakes, rudd (Scardinius erythropthalmus) and pike in plesiopotamic lakes, and sunbleak (Leucaspius delineates) and Prussian carp (Carassius auratus gibelio) in paleopotamic lakes. The findings indicated that the composition and abundance of fish communities are determined by lake isolation gradient, physico-chemical parameters and water stage. Although intact riverine ecosystems may promote fish biodiversity, our findings suggest that lateral connectivity between the main channel and floodplain lakes is of utmost importance. Thus, the conservation of fish biodiversity requires the preservation of this connectivity.

Microbiological activity is an important parameter for understanding the functioning of different environments. Therefore, the purpose of this study was to estimate the quantity and contribution of metabolically active at the single-cell level bacteria in the microbial community in peat-bog lakes. To determine different aspects of the metabolic activity of bacteria, four fluorescent staining methods (Dehydrogenase/Electron Transport System Activity -CTC+, Nucleoid Containing Cells- NuCC+, Active Bacteria with Intact Ribosome Structures- RIB+ and Active Bacteria With an Intact Membrane—MEM+) were applied. We identified four natural peat-bog lakes in Northern Europe to determine which factors—community (bacterial factors) or environment (hydrochemical and physical factors)—have a significant influence on the quantitative dynamics of metabolically active microorganisms, in terms of seasonal and habitat changes. The results show that change in the amount of abiotic components such as DOC, TN, and TOC can result in stress, which may limit a function but does not lead to losing all other metabolic functions in the community-forming bacteria. In nutrient-poor peat bog lakes, nutrients and organic carbon are factors which regulate the overall activity of the community.

The effect of pesticide contamination of the littoral zone on the population of bacteria and fungi was analyzed using the example of a eutrophic water reservoir exposed for >30 years to the influence of expired crop-protection chemicals, mainly DDT. For three consecutive years, quantity analyses of bacteria and fungi were conducted and the composition of the microorganism population analyzed against seasonal dynamics. Mold and yeast-like fungi were also isolated and identified. Within the Bacteria domain, in addition to the large groups of microorganisms (Alphaprotobacteria, Betaprobacteria, and Gammaproteobacteria, Actinobacteria, and Cytophaga-Flavobacterium), the analysis also involved the presence of bacteria predisposed to degraded pesticides in natural environments: Pseudomonas spp. and Alcaligenes spp. The quantity dynamics of aquatic microorganisms indicated that bacteria and fungi under the influence of long-term exposure to DDT can adapt to the presence of this pesticide in water. No modifying effect of DDT was observed on the quantity of microorganisms or the pattern of seasonal relationships in the eutrophic lake. Changes were shown in the percentage share of large groups of bacteria in the community of microorganisms as was an effect of contamination on the species diversity of fungi. The data show the effectiveness of aquatic microorganism–community analyses as a tool for indicating changes in the water environment caused by pesticide contamination.