Warming affects crustacean grazing pressure on phytoplankton by altering the vertical distribution in a stratified lake.

Zooplankton could efficiently graze algae and thus improve water quality. However, as thermal stratification commonly occurs in deep lakes, the effect of warming on the trophic interactions of plankton in depth profiles is still not clear. To explore the pressure of crustacean grazing on phytoplanktonic responses to enhanced thermal stratification under warming, we evaluated the monthly changes in the Secchi depth (SD), thermocline depth (TD), and the mean residence depth of zooplankton (zp MRD) and chlorophyll a (Chla MRD) in Lake Qiandaohu from January 2015 to December 2018. The thermal-stratification cycle was divided into weakness (from March to June) and formation periods (from July to February). Linear regression analyses showed that during both periods, the zp MRD was more sensitive to Chla MRD than to TD, and TD was negatively related to the difference between the zp MRD and Chla MRD. Structural equation model (SEM) analyses showed that the TD could be decreased by the direct effect of warming and the indirect effect of the decreased SD during weakness periods. A 0.95 °C increase in air temperature and a 0.85 m decrease in the SD between 1987 and 2018 corresponded to a decrease in the TD. Therefore, decreasing TD would weaken the top-down control on phytoplankton by moving phytoplankton far from zooplankton. Future decreasing TDs under climate warming may decouple crustacean grazing pressure on phytoplankton, which may further deteriorate the water quality.