Predictable shifts in diversity and ecosystem function in phytoplankton and zooplankton communities along thermocline stratification intensity continua

Thermocline stratification is a global threat to water quality in drinking water reservoirs, though its underlying mechanisms are not clear. The impacts of thermocline stratification intensity on biodiversity-ecosystem relationships were assessed using phytoplankton and zooplankton indicators from four stages of stratification in Lake Qiandaohu. There were significant differences in biomass, species diversity, and resource use efficiency (RUEpp = phytoplankton biomass/total phosphorus) for phytoplankton between continuous stratification and mixing periods, but only in FDis and RaoQ diversity indices for zooplankton. Phytoplankton species diversity and RUE were higher in the formative and stable periods, while zooplankton species diversity and zooplankton biomass/phytoplankton biomass (RUEzp) were lower. When combining the data from the four periods, a negative linear pattern was found between phytoplankton Simpson's, functional dispersion (FDis), and Rao's Quadratic (RaoQ) diversity indices, and thermocline depth (TD). Only zooplankton FDis and RaoQ diversity indices were significantly positively related to TD. Phytoplankton RUE was significantly negatively related only to its Pielou's evenness (J) diversity, while zooplankton RUE was significantly negatively related to its J, FDis, and RaoQ diversity indices. The results of structural equation models (SEMs) showed that the R2 of RUE for phytoplankton was much higher than that for zooplankton. Thermocline stratification intensity exerted an indirect positive effect on phytoplankton RUE by affecting species diversity but had a negative effect on zooplankton RUE. These findings underscore the negative influence of thermocline stratification resulting in various biodiversity changes in freshwater ecosystems.