Nutrient-driven biomass variation in Ulva prolifera: Experimental insights for predicting Yellow Sea green tide magnitude

Abstract

The green tide in the Yellow Sea, one of the largest globally, is primarily driven by the biomass of Ulva prolifera, which is influenced by seawater nutrient levels, particularly inorganic nitrogen and phosphorus. This study aimed to clarify the relationship between U. prolifera growth and seawater nitrogen and phosphorus concentrations. Experiments were conducted in the northern Jiangsu coastal waters using three nutrient concentration gradients (high, medium, and low) to assess their impact on U. prolifera biomass. Additionally, the proportional relationship between nutrient uptake by algal tissue and nutrient depletion from the water was analyzed. Results showed a significant positive correlation between nitrogen and phosphorus concentrations and U. prolifera growth, with Spearman's correlation coefficients exceeding 0.4 and p-values <0.05. Increased nitrogen and phosphorus levels promoted faster growth and higher biomass, with NH₄⁺ having the most pronounced effect, leading to a maximum weight gain of 107.4 %. Nutrient uptake measurements indicated that for each gram of nitrogen absorbed, U. prolifera biomass increased by 277.9 g, and for each gram of phosphorus absorbed, biomass increased by 4056.5 g. In conclusion, nutrient enrichment accelerates U. prolifera growth, and limiting nitrogen and phosphorus inputs could reduce biomass accumulation in the early stages of green tide formation. These findings provide valuable insights for developing strategies to manage and control green tide events and protect marine ecosystems.