Nutrient absorption by Ulva prolifera and the growth mechanism leading to green-tides

Nutrient pollution and algal blooms in coastal waters have long been a major concern, and understanding the response of algae to nutrient dynamics is thus essential. The minimum concentration under which Ulva prolifera will not grow, estimated from a nutrient-alga coupling model developed in this study, was 6.5 mu mol L-1 for nitrate and 0.27 mu mol L-1 for phosphate. The model was parameterized with cultural experiments to examine the effects of nitrate and phosphate on the growth of Ulva prolifera and the development of green tide. The cultural experiments showed that flux of nitrate absorbed by Ulva prolifera increased significantly with the concentration of nitrate or phosphate, which followed the Michaelis-Menten equation, while the flux of phosphate linearly increased with its concentrations. Nitrate concentrations at > 25 mu mol L-1 had little influence on the absorption of phosphate, but compensating absorption of phosphate occurred when the nitrate concentration was below 15 mu mol L-1. Phosphate concentrations had a remarkable enhancement to the absorption of nitrate when phosphate concentration was < 1.4 mu mol L-1. The absorption rates of nitrate and phosphate by Ulva prolifera followed the same variation trend as nutrient absorption fluxes. The increase in nitrate absorption rates has a limited impact on the relative growth rate of Ulva prolifera, while the increase in phosphate absorption rates has a marked impact on the relative growth rate. Our results provide new insights into the growth mechanism of Ulva prolifera and the development and evolution of green tides in coastal marine environments. Applications of our model should help governmental agencies in environmental remediation and policy making.