Zooplankton assemblages as indicators of seasonal changes in water masses in the boundary waters between the East China Sea and the Taiwan Strait

Hydrology and trophic relationships are frequently reported for inducing changes in mesozooplankton communities. This study investigated the distribution and abundance of mesozooplankton in the boundary waters between the East China Sea and the Taiwan Strait. Samples were collected using a NOR-PAC zooplankton net towed horizontally at a depth of 2 m, at eight stations along a transect, in March (spring) and October (autumn) 2005. The abundance of mesozooplankton was significantly higher during autumn than spring. Densities of many groups (e.g., Noctiluca scintillans, pteropods, copepods, mysids, euphausiids, and other larva) increased in October. During both seasons, copepods represented more than 50% of the total zooplankton abundance. Noctiluca scintillans, appendicularians, and Calanus sinicus were dominant in spring, indicating their association with the cold waters of the East China Coastal Current. Appendicularians and N. scintillans were mainly associated with the coastal waters of Taiwan; whereas C. sinicus was concentrated in the offshore waters along the coast of Mainland China. Chaetognaths, Temora turbinata, Acrocalanus spp., and radiolarians were dominant in autumn, showing their association with the warm waters of the Kuroshio Branch Current. Oncaea venusta was relatively abundant during both seasons. Our study shows that, in addition to the influence of seasonal changes in the water masses, the distribution and composition of mesozooplankton are highly influenced by trophic interactions between zooplankton taxa, in the boundary waters of the Taiwan Strait and the East China Sea.