The variability of trophic cascades on phytoplankton induced by mesozooplankton through in-situ feeding experiments under different temperatures

Mesozooplankton regulate lower trophic levels through direct grazing and trophic cascades, with the balance varying across environmental gradients. Firstly, we conducted seasonal in-situ experiments in Daya Bay in 2015-2017 to quantify mesozooplankton clearance rates and cascading effects on phytoplankton. Results revealed strong seasonality, with cascades peaking in spring/summer and declining in autumn/winter. Size-selective feeding created divergent impacts: large phytoplankton experienced high grazing mortality but weak cascades, while small phytoplankton showed the reverse pattern. Trophic cascades operated through three mechanisms: offsetting direct grazing losses, restructuring phytoplankton communities via size-dependent effects, and reducing ciliate grazing pressure by 14.4±7.8% (while maintaining ~70% of natural ciliate grazing rates). Community composition was the primary driver: cladoceran dominance elevated feeding rates, whereas high omnivorous copepod abundance intensified cascades on small phytoplankton.Building on these findings, we leveraged the thermal discharge from Daya Bay Nuclear Power Plant as a natural warming experiment to directly assess temperature effects. Specifically, we established four distinct temperature stations, ranging from the closest to the farthest from the power plant, to capture the temperature gradient and conducted seasonal in-situ mesozooplankton feeding experiments. Results showed that temperature increases simultaneously enhanced mesozooplankton feeding rates and trophic cascades, with disproportionately stronger effects during low-temperature seasons. In cooler conditions, direct grazing dominated, suppressing phytoplankton biomass. Conversely, under warmer conditions, trophic cascades became dominant, promoting small-sized phytoplankton growth. General Additive Model analysis confirmed that cascade variability was highly dependent on temperature, ciliate abundance, and predator-prey feeding interactions.Our study clarifies how mesozooplankton feeding regulates planktonic communities across temperature gradients and underscores their role in ecosystem stability, providing critical insights for marine ecosystem management under climate change scenarios.