Zooplankton play a substantial role in marine ecosystems as they serve as a link for energy transfer from primary producers to higher trophic levels and are important contributors to the ocean carbon cycle. Mesoscale eddies are ubiquitous and highly energetic phenomena in the world's oceans, and they are important suppliers of nutrients to the surface waters of oligotrophic gyres, but little is known about the biological response, particularly zooplankton, as major consumers of phytoplankton, to these physical perturbations. These mesoscale eddies not only alter the distribution of zooplankton communities but also modify the energy transfer efficiency among trophic levels through food webs, thereby impacting local carbon cycling. Given the complex mechanisms by which mesoscale eddies affect zooplankton communities and the frequent interactions among environmental factors, a clear understanding of the ecological response mechanisms remains elusive. A pair of seasonal eddies were tracked in the northern South China Sea (16-20°N, 110-118°E) and multi-disciplinary cruise was carried out including hydrological, chemical and biological observations from May 7 to 28, 2023. In this study, we aim to test the effects of the two eddies on the horizontal and vertical pattern of zooplankton and the potential forcing mechanisms from three perspectives: genetic, species, and community levels, by the Multi Plankton Sampler, VPR and molecular method. Environmental parameters showed a significant elevation in temperature, salinity, and chlorophyll a levels at the center of the cyclonic eddy (cold eddy), while the depth of the deep chlorophyll maximum (DCM) is shallower. Conversely, the anticyclonic eddy (warm eddy) center exhibits the opposite trends. The abundance and biomass of copepods at the cold eddy center are markedly higher than those at other sampling stations, whereas both abundance and biomass at the warm eddy center were significantly lower, with large-sized individuals descending to deeper water layers. Notably, Rhincalanus rostrifrons, Lucicutia flavicornis, and Pleuromamma spp. dominate the two eddies region, displaying distinct vertically distribution between the cold and warm eddies. This study extracted dietary DNA from the guts of three indicator species to explore the source and diversity of prey among the different types of eddies and layers based on genetic annotation results. The findings will provide scientific evidence for quantifying the ecological impacts of oceanic mesoscale eddies on multi-disciplinary aspects.
 

Eddy; Zooplankton; Northern South China Sea; Diversity; Copepod diets