Global warming is increasing the frequency and severity of the marine heat waves, which poses a serious threat to the marine ecosystem. This study analyzes seasonal and interannual dynamics in the abundance and structure of the mesozooplankton community in Sevastopol Bay based on bi-monthly routine observations over 2003–2014. The focus is on the impact of the summer 2010 marine heat wave (MHW2010) on crustaceans belonging to different ecological groups. As a response to the MHW2010, three warm-water species (O. davisae, A. tonsa and P. avirostris) exhibiting the maximum seasonal density in latter summer showed a sharp increase in the annual abundance and their share in the mesozooplankton community. The increase in the annual abundance in 2010 of the eurythermal species P. parvus and P. polyphemoides exhibiting seasonal peaks in spring and autumn is not related to the MHW2010 but can be explained by a rise of temperature in the first part of the year. O. davisae and A. tonsa showed the most pronounced response among the species to the MHW2010, confirming that non-native species exhibited great flexibility as an adaptive response to environmental changes, especially in the case of climate warming. Among crustaceans observed in this study, O. davisae can be considered as an indicator of the environmental conditions associated with the warming of the Black Sea and the Mediterranean basin as a whole.

Plastic pollution has emerged as one of the most pervasive environmental challenges, with microplastics (MPs) widely distributed across marine ecosystems worldwide. This study aimed to assess the uptake of MPs by key fish and invertebrate species from different locations in the coastal zone of the Bulgarian Black Sea. Fish were collected during routine monitoring surveys in September–November 2024, while invertebrates were obtained via scuba diving. The presence of MPs in fish stomachs and invertebrate soft tissues, and their polymer composition, shape and size were analyzed using an Agilent 8700 LDIR Chemical Imaging System. Potential biological effects of ingested MPs were evaluated by an integrated Specific Oxidative Stress (SOS) index. The results revealed MP uptake levels comparable to those reported globally. Small-sized particles (<50 µm) with rounded shapes were most abundant across studied taxa. Polymer composition varied considerably depending on species and sampling region, indicating differences in exposure sources and environmental conditions. Oxidative stress levels in both fish and invertebrates showed substantial interspecific variation, and clear differences between the northern and southern region of the Bulgarian Black Sea. Overall, elevated uptake of MPs appears to contribute to oxidative stress in marine organisms, potentially affecting their health status, resilience, and adaptive capacity, as reflected by increased SOS index values.

Mesozooplankton occupies a central position in the pelagic food web, linking primary producers and the microbial loop with fish larvae, planktivorous fish, and higher trophic levels. Changes in its abundance, biomass, taxonomic composition, and diversity may simultaneously reflect variations in temperature, salinity, food resources, hydrodynamics, eutrophication, and predation pressure. Gelatinous zooplankton is particularly important in the Black Sea, where medusae and ctenophores may periodically reach high abundance and biomass and exert strong control over mesozooplankton and the early life stages of fish. Consequently, interpretation of the system should be based on the combined behaviour of the major groups and on the relative importance of spatial, seasonal, and interannual changes. This study evaluates decadal changes in mesozooplankton and gelatinous zooplankton along a coastal–shelf gradient in the Bulgarian Black Sea during 2012–2023 using data partially collected within National monitoring programme and projects (MISIS, ISMEIMP). Sampling was conducted in Varna Bay and along the Cape Galata transect, including coastal (≤30 m) and shelf (31–200 m) stations. Owing to uneven seasonal sampling, spring and summer data were combined into a warm season, while autumn and winter samples were grouped into a cold season. The specific questions we put forward were: do the coastal and shelf areas differ in total abundance and biomass; which taxonomic groups account for these differences; is there a systematic contrast between the warm and cold seasons; are monotonic changes evident between 2012 and 2023; are changes in mesozooplankton and gelatinous zooplankton synchronous; and (6) do the results justify referring to ecosystem improvement? do the coastal and shelf areas differ in total abundance and biomass; which taxonomic groups account for these differences; is there a systematic contrast between the warm and cold seasons; are monotonic changes evident between 2012 and 2023; are changes in mesozooplankton and gelatinous zooplankton synchronous; and (6) do the results justify referring to ecosystem improvement? Temporal and spatial variability in the abundance and biomass of the total mesozooplankton community and its principal taxonomic groups (Copepoda, Diplostraca (Cladocera), Meroplankton, Chaetognatha, Appendicularia and Noctiluca scintillans ) together with gelatinous zooplankton ( Aurelia aurita, Mnemiopsis leidyi, Beroe ovata and Pleurobrachia pileus ) were analysed. Differences in community composition between habitats and seasons were assessed using PRIMER 6 statistic. The analyses revealed a persistent coastal–shelf gradient, with coastal waters generally supporting higher mesozooplankton abundance (10140 ind/m³, SD±5399) and greater interannual variability than shelf waters (4221 ind/m³ SD±2400). Coastal waters are generally more productive and more strongly influenced by river runoff, local resuspension, and higher nutrient concentrations. At the same time, they are also more dynamic, which may explain the large interannual amplitude. Copepods remained the dominant component of the community throughout the study period, whereas cladocerans, meroplankton and gelatinous zooplankton exhibited marked seasonal and interannual fluctuations. Multivariate analyses demonstrated significant differences in community composition between coastal and shelf habitats, with copepods and cladocerans contributing most to the observed spatial dissimilarity. Overall indicated predominantly stable community organization, interrupted by occasional years characterized by increased dominance of gelatinous zooplankton. Trend analyses suggested that community variability was driven mainly by fluctuations in individual taxonomic groups rather than by directional long-term changes in total mesozooplankton abundance or biomass.