Utilization and release of biogenic elements by macroalgae Ulva prolifera: A mesocosm experiment off the coast of Qingdao, China

A mesocosm experiment was conducted on the Qingdao coast to study the effects of nutrient enrichment on the physiological responses of Ulva prolifera and the changes in the entire culture system during decomposition of algal blooms. The decomposition trend of U. prolifera off the Qingdao coast was slightly retarded by nutrient enrichment, and could not be reversed to positive growth. The decomposition of U. prolifera was divided into two stages: the early-stage (0–9 d) and the late-stage (9–19 d). High nutrient levels enhanced the amounts of carbon and nitrogen fixed by U. prolifera and accelerated the subsequent degradation of organic matter. The concentration and composition of amino acids in solution also changed, reducing the bioavailability of dissolved organic matter in seawater. This study evaluated the actual decomposition process of green tides, which will help to assess the impact of green tides on coastal ecosystems and the circulation of biogenic elements.