What Factors Control the Variations in Abundance, Biomass, and Size of Mesozooplankton in a Subtropical Eutrophic Bay?

To determine the factors controlling variations in the biomass (B), abundance (A), and mean individual biomass (B/A) of mesozooplankton in a subtropical eutrophic bay, a random forest model was used to analyze the effects of physicochemical factors and phytoplankton biomass, size, and taxonomic composition on the variations in B, A, and B/A of mesozooplankton using 100 samples from all four seasons of 2015 in Xiangshan Bay, China. Variations in the B and A of mesozooplankton were related to both physicochemical and phytoplankton factors, while physicochemical factors were the best predictors of variation in the B/A of mesozooplankton. Phytoplankton biomass was too high to limit mesozooplankton growth, leading to a weak relationship between mesozooplankton and phytoplankton biomasses. The mesozooplankton were sensitive to temperature, and an increase in temperature increased mainly the number of small mesozooplankton. Temperature, suspended solids (SS), and nutrients indirectly affected the A and B/A of mesozooplankton by changing the abundance, size, and species composition of phytoplankton. Size mismatches between phytoplankton and mesozooplankton sometimes occur with changes in the physicochemical environment, which might reduce trophic transfer efficiency in planktonic ecosystems. This study implies that we should pay attention to the impact of warming on the planktonic ecosystem in the context of global warming, especially in subtropical bays with intensive human activity.