Interaction between the green macroalga Ulva prolifera and three microalgae under unique various irradiances found in the southern Yellow Sea, China

High turbidity and large concentration of nutrients in the seawater of Subei Shoal are considered to be the primary cause of the green tides in the southern Yellow Sea, China. Interaction between Ulva prolifera and three microalgal species (Skeletonema costatum, Prorocentrum donghaiense, and Nitzschia closterium) under different irradiance was studied. Combined with the field investigation of irradiance at various depths, the depth ranges at which U. prolifera and the three microalgal species can coexist were evaluated. Ulva prolifera is better adapted to a wide range of light intensity than the microalgae. Growth of the microalgae in co-culture was significantly inferior to that in pure culture, and ecophysiological differences were found between these three species. Competition for limited nutrients and allelopathic effects under the various irradiances was the main interaction mechanism for the growth strategy of U. prolifera and microalgae that supported their dominance in the high turbidity seawater of Subei Shoal.