Diel vertical migration of mesozooplankton in the northern Yellow Sea

The Yellow Sea Cold Water Mass (YSCWM), one of the most vital hydrological features of the Yellow Sea, causes a seasonal thermocline from spring to autumn. The diel vertical migration (DVM) of zooplankton is crucial to structural pelagic communities and food webs, and its patterns can be affected by thermocline depth and strength. Hence, we investigated zooplankton community succession and seasonal changes in zooplankton DVM at a fixed station in the YSCWM. Annual zooplankton community succession was affected by the forming and fading of the YSCWM. A total of 37 mesozooplankton taxa were recorded. The highest and lowest species numbers in autumn and spring were detected. The highest and lowest total densities were observed in autumn (14 464.1 inds./m3) and winter (3 115.4 inds./m3), respectively. The DVM of the dominant species showed obvious seasonal variations. When the YSCWM was weak in spring and autumn, most species (e.g. Paracalanus parvus, Oithona similis, and Acartia bifilosa) stayed above the thermocline and vertically migrated into the upper layer. Calanus sinicus and Aidanosagitta crassa crossed the thermocline and vertically migrated. No species migrated through the stratification in summer, and all of the species were limited above (P. parvus and A. crassa) or below (C. sinicus and Centropages abdominalis) the thermocline. The YSCWM disappeared in winter, and zooplankton species were found throughout the water column. Thus, the existence of thermocline influenced the migration patterns of zooplankton. Cluster analyses showed that the existence of YSCWM resulted in significant differences between zooplankton communities above and below the thermocline.