The Plasticity of the Antioxidant Defence System of Coastal Zooplankton Communities

ABSTRACT Zooplankton are expected to enhance their antioxidant defences and activate stress‐related biomarkers in response to warming and reduced salinity or hypoxia and cyanobacteria blooms, reflecting both physiological adaptation to increased oxidative stress and a strategy to maintain cellular stability under changing environmental conditions. This study aims to assess how the antioxidant defence system and stress levels of the mesozooplankton community respond to varying environmental conditions in the changing Baltic Sea. We monitored biomarkers of oxidative stress (lipid peroxidation, LPX) and antioxidants (glutathione‐s‐transferase, GST and catalase, CAT) capacity of mesozooplankton in response to temperature, salinity, chlorophyll a and oxygen concentration at four locations in the northern Baltic Sea, during summer and autumn 2023. We show that antioxidants responded to different environmental conditions, whereas lipid peroxidation remained low and stable independent of environmental conditions, but showed a significant relationship with temperature at 10 m depth. Antioxidant responses in the mesozooplankton community varied across locations, influenced by both environmental conditions and mesozooplankton composition. GST activity was elevated in communities dominated by copepods and showed correlations with temperature, dissolved oxygen, and chlorophyll a . In contrast, CAT activity was higher in cladoceran‐rich assemblages and was significantly associated with salinity and dissolved oxygen. These findings indicate that environmental variables exert an influence on antioxidant enzyme activity, but species composition modulates the specific response patterns. Due to the plasticity of antioxidants, the oxidative damage remained low. The absence of detectable oxidative damage, even under suboptimal environmental conditions, suggests that the mesozooplankton community possesses a highly effective and adaptive antioxidant defence system.