A nutrient–microalgae– <i>Ulva prolifera</i> biological model: Key factors and processes in the control of green tides in the Yellow Sea

Abstract The Yellow Sea has experienced the world's largest green tide of Ulva prolifera in each of the last 18 yr. Limited understanding of the mechanisms controlling U. prolifera growth and death complicates mitigation efforts. Focusing on the crucial factors and processes affecting U. prolifera blooms, we constructed a nutrient–microalgae– U. prolifera –detritus (NmiAUD) model based on the results of field microcosm experiments. The NmiAUD model characterized the growth and death processes of U. prolifera and the nitrogen and phosphorus biogeochemical processes in the Yellow Sea with good reliability. Parameter sensitivity, process correlation analysis, and numerical experiments were used to identify the most critical factors and processes. Nutrient concentrations were the most important factors controlling the growth and death of U. prolifera , followed by seawater temperature, initial biomass, and photosynthetically active radiation, with contribution rates of 55.1%, 23.9%, 16.0%, and 5.0%, respectively. Nitrogen was more important than phosphorus, with nitrate‐nitrogen accounting for 29.9%, followed by ammonium‐nitrogen (26.3%), dissolved organic nitrogen (19.9%), phosphate‐phosphorus (17.1%), and dissolved organic phosphorus (6.8%). The key processes comprised nutrient absorption, nutrient assimilation, degradation, detritus generation, dissolved organic matter mineralization, and detritus decomposition. Microalgae, which show high rates of growth, mortality, and nutrient uptake, are indicated to have a competitive advantage in the higher nutrient conditions in the South Yellow Sea, whereas U. prolifera is better adapted to the lower nutrient conditions in the North Yellow Sea. This study provides a scientific basis for the prevention and control of green tides.