Seasonality shapes community structure and functional group dynamics of zooplankton in Changjiang River estuary and its adjacent waters

Aim: Functional trait-based approach links the zooplankton community to the marine ecosystem functioning. It provides a comprehensive insight into the diversity of zooplankton ecological strategies but has rarely been used to characterize the zooplankton community in estuarine ecosystem. Here, we investigated the seasonality of mesozooplankton functional structure with respect to environmental changes in Changjiang River estuary (CRE) and its adjacent waters. Location: Changjiang River estuary and its adjacent waters. Methods: Samples were collected in three seasons of 2019. Morphologically identified species were assigned functional traits including body length, feeding strategy, trophic group, reproduction mode and habitat type, and clustered into functional groups. Taxonomic and functional diversity indices were calculated. The relative importance of the environmental variables in explaining the seasonality of functional structures was assessed. Results: Ten functional groups were identified, and functional structure showed strong seasonal dynamics. Current- and mixed-feeding omnivore-herbivores were the predominant group over seasons. The contribution of carnivorous, detritivorous and omnivorous groups increased from winter-spring to autumn. The seasonality of zooplankton functional structure was mainly explained by the effect of temperature, fluvial discharge and warm current. Low temperature and riverine input of nutrient in winter-spring favoured the small-sized herbivores. Subsequently, the rising temperature and run-off in summer shifted the zooplankton community to larger herbivores and carnivores. Moderate temperature and relatively enhanced effect of warm current promoted the proliferation of carnivores and detritivores in autumn. Functional diversity also varied over seasons. Main conclusions: The functional dynamics of mesozooplankton in CRE across estuarine gradient and seasonality is shaped by the seasonal variation of temperature, fluvial pulse and warm current intrusion. Our findings pave the way for further understanding estuarine biodiversity and have implications in conservation strategies.