Patterns and drivers of Cladocera community structure in the Black Sea

Cladoceran community dynamics were examined in the Romanian sector of the Black Sea during the warm seasons of 2013– 2020, using Marine Reporting Units (MRUs) to evaluate spatial heterogeneity across transitional, coastal, and marine waters. Analyses of density, biomass, mean individual biomass (MIB), and species composition revealed interannual fluctuations superimposed on strong nearshore–offshore gradients, with higher values in transitional and coastal MRUs. The assemblage was consistently dominated by Pleopis polyphemoides and Penilia avirostris, which together accounted for most of the total abundance and biomass. Multivariate analysis identified significant differences in taxonomic composition across MRUs and years, while correlation analyses highlighted temperature as the primary environmental driver of cladoceran variability. Nutrient influences were more spatially discrete, reflecting local riverine inputs and hydrodynamic mixing. Shifts in MIB indicated alternations between small- and large-bodied dominance, signaling changes in size structure and potential impacts on pelagic trophic transfer. The high sensitivity of cladocerans to thermal and nutrient regimes underscores their value as indicators for ecosystem assessments under the Marine Strategy Framework Directive (MSFD), particularly within Descriptor 4 (Food Webs) and Descriptor 1 (Biodiversity). The present study provides an improved understanding of cladoceran responses to environmental variability in the northwestern Black Sea and supports their integration into regional monitoring frameworks.