Microeukaryotes are more vulnerable than picoeukaryotes to reservoir salinization: Evidence from multi-year high-frequency monitoring.

In freshwater ecosystems, rising salinity driven by factors such as climate change and urbanization can severely threaten ecosystem services. Eukaryotic plankton, a vital component of freshwater ecosystems, spans a size range from picoeukaryotic plankton (0.2-3 μm) to microeukaryotic plankton (3-200 μm), each exhibiting unique ecological functions and environmental response traits. Changes in the composition of eukaryotic plankton communities may have a substantial effect on material transfer and energy flow within freshwater reservoir ecosystems. However, the differential effects of salinization on timescales on the diversity, assembly, and stability of pico- and micro-eukaryotic plankton remain to be elucidated. We utilized a four-year high-frequency monitoring dataset comprising 892 samples to disentangle the responses of pico- and micro-eukaryotic plankton to salinity fluctuations. Results showed that as salinity increased, the α-diversity of microeukaryotic plankton declined more sharply than that of picoeukaryotes. The β-diversity of microeukaryotes remained consistently lower than that of picoeukaryotes. During the initial phase of salinity increase, deterministic processes became significantly more important in microeukaryotic community assembly. In contrast, picoeukaryotic community assembly was jointly influenced by stochastic and deterministic processes throughout the salinity gradient. Most notably, the stability of the microeukaryotic community decreased with rising salinity, whereas that of the picoeukaryotic community increased. These findings highlight the differential stability responses of different size fractions of eukaryotic plankton to salinity stress and offer a scientific basis for predictive modeling of ecological responses to freshwater salinization.