Estimating the potential occurrence probability of Ulva prolifera blooming in the South Yellow Sea using a multi-factor coupled growth model of large floating algae

To gain a deeper understanding of the limitations, stages, and mutual influences of ecological factors during the green tide in the southern Yellow Sea, this study established a relationship between nutrient concentration and light intensity for macroalgal growth based on a multi-factor coupled growth model of large macroalgae. Using this relationship, the compensation depth and critical depth for the growth of large floating algae in a given sea area was calculated. The potential occurrence probability of macroalgal blooms, such as Ulva prolifera, was assessed by comparing the thickness of equivalent suspension depth with critical depth. The results indicate that the increase in both nutrient concentration and light intensity in the study area increase the growth rate of U. prolifera, demonstrating a clear complementary ecological effect. The critical depth increases with the rise of nutrient concentration but decreases with increasing light extinction coefficient (λ) in the seawater. The northern Jiangsu Shoal exhibits relatively high nutrient concentration, but the critical depth for U. prolifera growth is less than 0.20 m, which is smaller than the equivalent suspension depth (about 0.35 m). Consequently, the potential for large-scale green tides in this area and its adjacent waters is very low (less than 0.05), classifying it as a low-risk zone for green tide outbreaks. In contrast, the areas to the north and east of the northern Jiangsu Shoal show a decrease in nutrient concentration but an increase in critical depth, which is greater than equivalent suspension depth, leading to a gradual increase in the potential occurrence probability, marking these regions as the medium risk zone for green tide outbreaks. Further north, in the southern waters of the Shandong Peninsula (north of 35°N) , nutrient concentrations is low, but critical depth exceeds 2.00 m, which is far greater than equivalent suspension depth, leading to the significant increases of the potential occurrence probability and classifying this area as a high-risk area for green tide outbreaks. Based on the spatial distribution of critical depth and the probability of green tide occurrence, it is recommended to continue strengthening the salvage efforts in the tidal ditch area. These efforts should aim not only to remove all floating U. prolifera but also to intercept and salvage suspended U. prolifera within tidal channels to prevent its potential escape.