Long-term changes in size and community structure of Zooplankton in a semi-enclosed ecosystem

Zooplankton community structure and body size are recognized as sensitive indicators of marine ecosystem responses to environmental variability and human-induced pressures. In this study, we used a two-decade dataset (2002-2022) collected from a coastal station in the Sea of Marmara to investigate long-term changes in zooplankton community components, dominance patterns, and body size dynamics. Copepoda and Cladocera remained the dominant components throughout the study period; however, a notable seasonal expansion of Cladocera dominance and a replacement of key copepod species were observed over time. Two distinct periods (P1: 2002-2009; P2: 2017-2022) were compared to identify structural and functional shifts. Body size declines were recorded in major taxa, particularly in Acartia clausi and Paracalanus parvus, coinciding with seasonal temperature anomalies and major ecological events. Generalized Additive Models were applied to assess the influence of temperature and chlorophyll-a concentration on species-specific dominance and body size variability. Results indicated significant relationships between environmental variables and zooplankton traits, with species showing distinct responses to thermal and productivity shifts. Our findings emphasize the importance of implementing such as body size and trophic role into future assessments of ecosystem resilience under changing environmental conditions.