Zooplankton species diversity was monitored in the Northern Yellow Sea (121°30'-124°00'E、37°30'-40°00'N) in the autumn and winter of 2009. Based on comparisons with historic data 1959 and 1982, the variation of zooplankton diversity was analyzed. The connection between zooplankton diversity and sea currents and global change was also revealed. Zooplankton samples were collected with a plankton net (mesh size 500 μm) in the Northern Yellow Sea (NYS). A total of 28 species belonging to 8 classes (excluding zooplankton larva and juveniles, fish eggs, and fish larvae) were collected. Warm-temperate and eurythermal species constituted the majority of zooplankton in the NYS and only a small number of cold-temperate and subtropical species were found. Based on zooplankton species composition at each station, community classifications were analyzed using CLUSTER analysis and the SIMPROF test (Primer 6.0 software). Three zooplankton communities were distinguished as:the NYS High Salinity Water Community, the NYS Mixed Water Community and the NYS Coastal Water Community.
In the autumn and winter of 2009, the warm water species Doliolum denticulatum and Sagitta enflata were observed in the NYS. The average abundance of D. denticulatum was 81.00 ind/m³ and 0.06 ind/m³ in autumn and winter, respectively. The frequency of occurrence was 57.1% in autumn and 7.1% in winter. The average abundance of S. enflata was 0.24 ind/m³ and 0.37 ind/m³ in autumn and winter, respectively. The frequency of occurrence was 14.3% in autumn and 50.0% in winter. Notably, in autumn of 2009, D. denticulatum became one of the dominant zooplankton species in the NYS. In the same season during 1959, the northward distribution limit of these warm water species did not reach the NYS. The northward expansion of these warm water species might reveal the reinforcement of the Yellow Sea Warm Current (YSWC) from the impacts of climate change.
Calanus sinicus and S. crassa were the main warm-temperate species and dominated the zooplankton in both seasons measured. In autumn and winter of 2009, the abundance of both C. sinicus and S. crassa was higher than those in the same period of 1959. In autumn, the average abundance of C. sinicus in 2009 was 3.7 times that in 1959 and 5.4 times that in 1982. The maximum abundance of C. sinicus in 2009 was 3.5 times that in 1959 and 1.7 times that in 1982. The abundance of the main warm-temperate species has increased significantly in the NYS over the past 50 years.
We hypothesize that climate change is responsible for the northward expansion of warm water species (D. denticulatum and S. enflata) and the increase in abundance of the main warm-temperate species (C. sinicus and S. crassa) in the NYS. The response model of zooplankton to climate change in the NYS (warm-temperate sea) was different from that in the East China Sea (subtropical sea). This research is of scientific interest for studying the response of zooplankton to global climate change.