Plankton size spectra integrate biomass distributions across size classes and provide a useful indicator of marine food-web structure, trophic transfer efficiency, and ecosystem responses. The Taiwan Strait, located on the southeastern continental shelf of China, is characterized by strong hydrographic gradients driven by monsoon circulation, coastal currents, Kuroshio intrusion, and land-derived inputs. Based on six cruises conducted from 2023 to 2025, this study quantified phytoplankton and zooplankton using FlowCam and ZooScan, constructed normalized biomass size spectra, and examined spatiotemporal variations in the size-spectrum slope β and biomass composition across size classes.
The results showed marked seasonal and spatial variability in β. The highest mean β occurred in spring 2025, with a value of −0.619, indicating a flatter size spectrum, a greater relative contribution of large-sized plankton, and higher trophic transfer efficiency. In contrast, the summer mean β decreased from −0.878 in 2023 to −0.932 in 2024 and −1.080 in 2025, suggesting a progressive shift toward smaller size classes and reduced trophic transfer efficiency during summer. Spatially, nearshore stations generally had lower β values than offshore stations, reflecting steeper spectra and a stronger contribution of small-sized plankton. This pattern was likely related to land-derived nutrient inputs, elevated chlorophyll a, turbidity, and rapid coastal material cycling. Offshore waters, influenced by high-salinity water masses, the Taiwan Warm Current, and Kuroshio intrusion, tended to maintain flatter spectra and a higher contribution of larger plankton. Overall，this study reveals the spatiotemporal variability of plankton food-web trophic transfer efficiency and its environmental drivers, providing insights into energy transfer, structural stability, and ecological resilience in continental shelf ecosystems.

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