Seasonal dynamics of copepod functional traits as ecological indicators in the marine ecosystems

Abstract We investigated the ecological indicators and functional traits of copepods across the East Sea (ES), South Sea (SS), Yellow Sea (YS), and East China Sea (ECS) over four seasons in 2020. A total of 128 copepod species were identified and categorized into four functional traits based on variations in body length, feeding type, trophic group, and spawning strategy. In the YS, smaller copepods, primarily omnivores and mixed feeders, dominated spring communities, influenced by cooler temperatures and the Yellow Sea Cold Water Mass. During summer, the SS + ECS regions, affected by the nutrient-rich Kuroshio Current, saw an increase in larger omnivorous-herbivores. Autumn featured a diverse mix of sac-spawners and omnivorous-carnivores across all regions, while winter communities showed varied feeding strategies, with the ES exhibiting stable species composition and the YS and SS + ECS adapting to colder, nutrient-limited conditions. The SS + ECS consistently displayed higher species richness and functional diversity, driven by complex oceanographic processes and monsoonal patterns. Distance-based Redundancy Analysis (dbRDA) revealed significant correlations between multiple environmental variables, with the spatial distribution of indicator species. Threshold Indicator Taxa analysis (TITAN) further indicated species-specific threshold responses to hydrographic changes, emphasizing the role of temperature and salinity in structuring copepod communities across different regions. This study underscores the importance of regional and seasonal variability in marine ecosystems and the utility of copepod functional traits as indicators of environmental change.