Contributions of temperature and development state to the morphological variation of Ulva prolifera highlight the source management for green tides.

Since 2007, the world's largest transregional green tides caused by Ulva prolifera have periodically occurred in China. The morphology of U. prolifera drastically changes during the drifting period, but the reason for this phenomenon has still been under debate. Our results showed that temperature partly explained the changes in the morphology of U. prolifera, with a determining factor of 0.498. The ratio of highly branched thalli decreased as the temperature increased from around 25 to 30 °C. Because morphological changes and physiological acclimation synergistically occurred in floating macroalgae, we hypothesized that if the morphology of U. prolifera is also determined by its development state, and the U. prolifea thalli with different development states should have distinct morphological and physiological traits even under the same environmental conditions. To test the hypothesis, we investigated the photosynthesis of U. prolifera and found a higher photosynthetic capacity but lower photoprotective capacity in algae that grew in the branched development state compared to those in the unbranched development state. Combined with other field observations and lab experiments, we suggest that both temperature and development state contribute to the morphological changes of U. prolifera. Given the varying trends of temperature during U. prolifera blooms in past eleven years and the initial occurrence of U. prolifera thalli in the branched development state in the source of algal bloom, we emphasize the need for source management of green tides.