Zooplankton ecocline along salinity gradient in the Pearl River estuary (South China Sea)

Estuarine environment exhibited a marked salinity gradient, with zooplankton species displaying varying salinity tolerances. This led to significant spatial distribution differences and the formation of distinct assemblages across salinity gradient. However, research on the transition of zooplankton assemblages from low to high salinity in the Pearl River estuary was limited. Our study aimed to determine whether the patterns of zooplankton community variation in this region can be classified as ecotone or ecocline. Division of zooplankton community along salinity gradient in the Pearl River Estuary were investigated in spring (1-6 April 2023) and summer (1-9 August 2023). A total of 42 and 34 zooplankton species were identified during spring and summer, respectively. Cluster analysis indicated that the zooplankton can be divided into four assemblages along salinity gradient in both seasons. The spatial distribution of dominant species among different assemblages exhibits significant differences. Additionally, there were different representative species of each assemblage and the representative species of different salinity ranges transitioned gradually. The changes of these four zooplankton assemblages from low-salinity to high-salinity waters exhibited a gradual and alternating transition. This did not align with the abrupt change characteristic of the ecotone model, but it was consistent with the gradual alterations associated with the ecocline concept. Therefore, we concluded that the variation pattern of zooplankton communities in the Pearl River estuary aligned more closely with ecocline.