Long‐term monitoring reveals biomass loss and concurrent dominance changes in coastal zooplankton community

Anthropogenically induced changes in environmental conditions have been affecting species communities globally, leading to shifts in ecosystem functioning. Physical drivers like temperature, salinity and acidification are especially important in coastal ecosystems, and high‐resolution time‐series are essential to identify how these variables affect zooplankton community composition due to their importance in marine ecosystems. In this study we analysed a zooplankton monitoring dataset spanning from 1996–2021 to identify community changes and their drivers. We examined long‐term trends in environmental variables, corresponding total zooplankton biomass as well as changes in the biomass of specific taxa using generalised additive models (GAMs). We found a strong decline in total zooplankton biomass during September and October until 2006 and 2004, respectively. Copepod biomass further decreased during the last decade, while rotifer and cladoceran biomass increased, indicating a dominance shift towards species with shorter generation times and less complex ontogeny. Copepod biomass was negatively correlated with salinity, while cladoceran and rotifer biomass was positively correlated with temperature. Our results highlight that multiple climate change‐related environmental variables influence communities in different ways and hence, should be investigated simultaneously. Further, we argue that zooplankton community analyses and monitoring efforts should include small taxa like rotifers.