The fast expansion of Pyropia aquaculture in “Sansha” regions should be mainly responsible for the Ulva blooms in Yellow Sea

- Origin of the macroalgal blooms was located in the Pyropia aquaculture areas, especially in the “Sansha” regions.
- The initial floating Ulva have the similar morphological characters with the attached in the Pyropia aquaculture areas.
- About 4252 tons of attached Ulva seaweeds in “Sansha” regions provided the macroalgal blooms in the Yellow Sea.

Massive Ulva blooms became an environmental disaster in the Yellow Sea from 2007 to 2015. In this study, field shipboard observations indicated that Ulva blooms originated in Pyropia aquaculture area, and the morphology of initial floating Ulva seaweed have the structure of rhizoid, which is similar with the attached Ulva on the Pyropia rafts. The spatial distribution of Ulva microscopic propagules in the southern Yellow Sea also supported that the blooms originated in the Pyropia aquaculture area. Besides, numerical model was used in this study, showing the origin of macroalgal blooms was traced to “Sansha” regions which accounted for almost 70% of the total Pyropia aquaculture area. We conclude that the significant biomass (4252 tons) of Ulva species on the Pyropia rafts during the harvesting season in “Sansha” regions played an important role in the early rapid development of blooms in the Yellow Sea.