Tempo-spatial distribution of Ulva spp. micro-propagules in the Yellow Sea during and after green tide in 2019

Large-scale green tides occur frequently in summer in the Yellow Sea in recent decade. Micro-propagules, providing “seeds” for the seaweeds, play an important role in the formation of green tide. We studied the distribution pattern and genetic composition of the Ulva micro-propagules in the Yellow Sea during and after the green tide in 2019. In April, a great amount of Ulva micro-propagules was detected in the Subei Shoal, with the average density of 908 inds./L, being similar to that of previous studies. U. prolifera was found out at a low percentage (4%), and 25% of them were positive when tested by the specific marker for the floating ecotype. The percentage of U. prolifera was obviously lower than that in the same season from 2009 to 2016. Therefore, the quantity of U. prolifera micro-propagules sharply decreased. Significant variation in composition of the micro-propagules in Subei radial sand ridges was revealed in the background of the alternation of macroalgal tides in the Yellow Sea in the latest years. During the green tide in June and July, Ulva micro-propagules were detected from some nearshore sites where the floating seaweeds distributed, and U. prolifera was the dominate species. In September, when the green tide ended, no Ulva micro-propagule was detected from the open sea area of the Yellow Sea. It is indicated that the green algae micro-propagules were unlikely to exist in the open water of the Yellow Sea for a long time according to the large-scale surveys. The mechanism of preserving micro-propagules in Subei radial sand ridges, and the destination of U. prolifera micro-propagules in the open sea area of the Yellow Sea after green tide deserves further study.