The impact of Ulva prolifera green tide on the diatom-dinoflagellate taxa in the Southern Yellow Sea, China

Microalgal taxa distribution trends of diatom dominance, dinoflagellate dominance, and dinoflagellate-to-diatom succession have been observed in the southern, middle, and northern green tide areas, respectively, during Ulva prolifera outbreaks in the Southern Yellow Sea. However, little is known about the interactions between macroalgal and microalgal taxa associated with biogeochemical processes. Focusing on the impact of the Ulva spp. on microalgal communities, we constructed a Nutrient-Ulva-Microalgae-Detritus (NUMAD) model using ship-based mesocosm culture experiments. Culture experiments and the NUMAD model revealed higher dynamic rates of nutrient uptake and growth of U. prolifera than microalgae under oligotrophic conditions, consistent with the R strategy for diatoms with sufficient ambient nutrition. There is an ecological niche overlap between U. prolifera and diatom taxa, which makes the succession from diatom taxa to dinoflagellate taxa in the macro-microalgae co-cultural systems. We demonstrated that diatoms exceeding dinoflagellates with R strategy and ecological niche overlap compete against U. prolifera in the initial stage of the green tide, while dinoflagellates exceeding diatoms with K strategy and ecological niche differences compete against U. prolifera in the expanded stages of the green tide. This study provides new insights into the succession of macro-microalgae and a scientific basis for the mitigation of Ulva spp. green tides in the Yellow Sea.