Tracking the original source of the green tides in the Bohai Sea, China

Abstract

In 2015, a novel green tide began to bloom in Qinhuangdao on the coast of the Bohai Sea, Northern China, and has recurred each late April to late September since. The macroalgal biomass, suspended in the shallow water, washes ashore and accumulates on the beaches. Although previous studies have revealed the blooming dynamics and causative species of the green tides, no convincing evidence of their original source is available to date. In this study, cruise observations in the regions offshore from the areas affected by the green tides showed that the Bohai Sea green tides originated locally. A series of collections of the macroalgae attached to different inshore structures near the affected areas, including naturally-occurring seaweed beds, submerged breakwaters, Lianhua and Hailuo artificial islands, and Argopecten irradians aquaculture facilities, were conducted from late April to late September 2016, and the analyses revealed the species composition and proportion of the attached macroalgae. The green macroalgae species Bryopsis plumosa and Ulva prolifera, which were the dominant species in the second and third stages of the green tides, respectively, were found only on seaweed beds. The chloroplast-encoded rbcL genes of these two species were genetically identical to those of the B. plumosa and U. prolifera suspended in the green tides. Furthermore, the seasonal timing of bloom origin and the community succession trends of these two signature green macroalgae on seaweed beds were highly consistent with those in the green tides. Based on these results, we concluded that the attached macroalgae on seaweed beds are the major original source of the green tide blooms in the Bohai Sea.