Ecological Indicators (Sep 2023)
Response of zooplankton to warming in a low-salinity, eutrophic bay
Abstract
Warming and eutrophication are universal threats to bay ecosystems. However, the response of zooplankton varies due to diverse environmental settings and species composition. As a long and narrow semi-enclosed bay with low salinity (18–27) in the East China Sea, Xiangshan Bay (XSB) is jointly affected by global warming and the thermal drainage, which makes it an appropriate region for ascertaining the response of zooplankton to warming under low-salinity background. We examined the long-term changes in the abundance, biomass and dominant species of large mesozooplankton (LMZ; >505 μm) over four decades in XSB, and found downward trends in the annual abundance and biomass, and the biomass peak shifting from summer to spring since 2010. In addition, the relative abundance of cold-temperate species decreased, while that of warm-temperate/warm-water species and small-sized zooplankton increased. The decreased LMZ abundance and biomass in XSB were supposed to be related to low gelatinization compared with the other semi-enclosed bays in China, Jiaozhou Bay (JZB) in the Yellow Sea and Daya Bay (DYB) in the South China Sea, where the LMZ biomass and abundance increased or fluctuated mainly linked to the increase in gelatinous zooplankton. Salinity, temperature, and nutrient concentration are important factors affecting gelatinization. By comparing the background salinity and magnitudes of temperature and nutrient increases among the three bays, we speculated that the lower salinity (XSB: 18–27, JZB: 30–32, DYB: 30–34) influenced by freshwater input of the Changjiang River might be a potential factor causing the low gelatinization in XSB, as the similar upward trends in both nutrient concentration and temperature were observed in the three bays. This study suggests that miniaturization with low gelatinization and a decrease in cold-temperate species were the main responses of zooplankton to warming in the low-salinity bay in the East China Sea, which may lead to a specific pelagic ecosystem evolution.