The Causes Of Green Tides In The Yellow Sea, And Ecophysiological And Phylogenetic Analysis Of The Bloom-forming Alga, Ulva Prolifera

The region of Qingdao, China, experienced the world largest green tide from May to July 2008. More than one million tons of fresh biomass of the green alga identified as Ulva prolifera was harvested. The original source of this seaweed was suspected to be from the south of the Yellow Sea as revealed by satellite images. The floating biomass drifted with water current northward and flourished in nearshore waters around Qingdao. However, it is unclear whether the bloom forming alga would have left the microscopic propagules which could survive in the Qingdao local coastal environment and pose future danger of potential bloom. Systematic and seasonal sampling of waters in the intertidal zone at six collection sites along the Qingdao coast was conducted from December 2008 to April 2009. Forty eight water samples were analyzed. From these, nine different morphotypes of Ulva were grown in the laboratory under standard temperature and light regimes. Growth of Ulva was observed in all water samples. However, morphological and molecular phylogenetic analyses revealed that the bloom forming alga was absent in all the water derived cultures during the sampling period. These results provide evidence that the bloom forming alga was not native and it was unlikely able to survive the coastal waters of Qingdao, even though all the sampling locations were heavily covered by this alga in June 2008.Qingdao experienced the world largest drifting macroalgal bloom composed of the filamentous macroalga Ulva prolifera in 2008. No convincing biological evidence regarding the algal source is available so far. A series of field collections of both Ulva samples and water samples in various sites along Jiangsu coasts were conducted in April to May of 2009. Density of microscopic Ulva germlings in the waters sampled from different sites ranged from 7 to 3140 individuals L?1, indicating the wide spreading and long term existence of the algae in the investigated region. Morphological and ITS nrDNA and rbcL gene comparisons revealed that the algae collected from land based animal aquaculture ponds was identical to the dominant bloom forming alga in 2008. Mismatch of Porphyra farming period with the occurrence of the green tide bloom, as well as the negative identification results of the sampled green algae from the Porphyra rafts eliminated Porphyra rafts as the principal and original source of the bloom forming alga.In the Yellow Sea of China, large scale green tides have broken out for three consecutive years from 2007 to 2009. As part of the efforts to localize the algal source, five open sea cruises were conducted in the period of the bloom in 2009. Drifting algal samples and the water samples were collected. Results showed the ubiquitous existence of the Ulva microscopic propagules in the Yellow Sea. The number of the Ulva individuals decreased with the increase of the water depth. In August 2009, the average number of Ulva microscopic stage individuals in 1 L seawater was more than that in May 2009. This quantitative method can be expected to predict the potential biomass of the green tide Ulva species in seawater and thus the scale of the potential drifting biomass. The drifting Ulva and those derived from the water samples were analyzed by morphological and molecular systematic comparisons. For phylogenetic analyses, the ITS nrDNA and rbcL gene were used. Our molecular and morphological data indicated that the dominant free floating Ulva species in 2008 and 2009 belonged to the same strain of the U. linza-procera-prolifera (LPP) clade. Microscopic individuals of the dominant Ulva strain were detected in eight stations, revealing that spore dispersal in the water helped to enlarge biomass in the water during the outbreak stage of green tide in the Yellow Sea.In November 2009, the potential recruitment and development of Ulva species from propagule banks in the muddy or sandy sediment of Jiangsu coasts were investigated. The results indicated that numerous Ulva microscopic propagules existed in the intertidal muddy sediment and the Ulva propagules could tolerate the low temperature and dark in the intertidal for at least three months. Morphological and molecular comparisons revealed that the bloom forming alga was not found in the four investigated locations. However, the stardand method to evaluate the number of Ulva propagules was proved to be effective and with significant value in the study of tracking the origin of bloom.