Temperature-driven competitive dominance of Ulva prolifera in the Yellow Sea green tides revealed by multi-species comparative study

Large-scale green tides dominated by Ulva prolifera occur annually in the Yellow Sea, yet the physiological mechanisms underlying its competitive dominance over other co-occurring Ulva species remain poorly understood. To address this, we conducted a synchronized comparative experiment across four key species (U. prolifera, Ulva linza, Ulva lactuca, and Ulva compressa) under a temperature gradient (5-25 °C) with uniform culture conditions. A suite of growth and physiological variables was evaluated at two time points (days 7 and 13). During early exposure (0-7 days), U. linza outperformed others at ≤15 °C, whereas U. prolifera showed monotonic growth increase with temperature, becoming the fastest-growing species above 20 °C. By day 13, thermal optima shifted leftward and carbon use efficiency (CUE) declined in all species, indicating physiological stress under prolonged cultivation. Nevertheless, U. prolifera maintained higher photosynthetic rates and exhibited minimal CUE decline at high temperatures. In contrast, its light saturation parameter (I_k) and maximal relative electron transport rate (rETR_max) declined markedly at 25 °C by day 13, suggesting vulnerability to combined high light and high temperature stress. These results demonstrate that temperature-driven growth and physiological performance underpin the competitive advantage of U. prolifera during the bloom season. This study provides a physiological basis for its dominance in the Yellow Sea and highlights the need to incorporate multi-species and time-dependent thermal responses into future bloom forecasts under climate change.