Attachment Characteristics And Deposition Region Of Bloom-forming Ulva Prolifera In The Yellow Sea

Green tides increased in both frequency and intensity globally over the last several decades and became an important marine environmental issue.Since 2007,large-scale green tides have been recorded in the southern Yellow Sea of China for 10 consecutive years,and the causative species was identified as Ulva prolifera.Green tides occurred in the Yellow Sea caused severe ecological consequences and huge economic losses in Shandong and Jiangsu province.Previous studies showed that massive green algae originated from Subei Shoal of the southern Yellow Sea,and drifted along a certain route to the Yellow Sea under the impacts of both wind and tidal current.Compared with those in other sea areas,green tides in the southern Yellow Sea have many unique features,such as the long-distance transportation,huge intensity and extensive sea area affected.Therefore,the origin and ecological effects of the green tides are quite complicated,and it is of great significance to study the origin and fate of massive floating green algae of green tides in the southern Yellow Sea.The Porphyra yezoensis aquaculture rafts in the Subei Shoal have been proposed as a major origin of floating green algae in the southern Yellow Sea,because huge amounts of U.prolifera were observed attaching on the rafts.To understand the effects of aquaculture rafts on the green tides in the southern Yellow Sea,various substrates for the attachment and germination of U.prolifera spores and gametes were tested in this study.It was found that most spores and gametes completed attachment within 2-6 h,and the attachment rate was the highest in the first 2 hours.Laboratory experiments showed that the germination rates of U.prolifera microscopic propagules on materials related to aquacultural rafts(plastic,bamboo,jute rope,plastic rope,nylon netting and plastic netting)were much higher than those associated with mudflats(mud,sand and rock).Besides,rubber showed inhibitory effect on the germination of gametes and spores.The results suggest that Porphyra yezoensis aquaculture rafts in Subei Shoal provide important substrates for the microscopic propagules of U.prolifera,which plays an important role in the rapid increase of green algal biomass.The periodic outbreaks of the large-scale green tides formed by U.prolifera in the southern Yellow Sea may cause severe ecological consequences.However,there is little knowledge on the deposition region of massive floating green algae,which limits the understandings on ecological consequences of the green tides.In this study,a method using 28-isofucosterol as a biomarker of U.prolifera to track the deposition area of massive floating green algae was developed.The extraction process of sterols from algae and sediment was optimized,and the method for the determination of sterols was established by using gas chromatography–mass spectrometry(GC–MS).Sterols in several bloom-forming algae in China were analyzed to validate the specificity of 28-isofucosterol as a biomarker for U.prolifera.Based on the simulation experiments in laboratory,it was found that content of 28-isofucosterol in sediment was positively correlated with decomposed biomass of U.prolifera,and content of 28-isofucosterol in sediment could remain stable for at least one month.Based on these findings,samples collected during four field investigations from August 2015 to September 2016 in the Yellow Sea and the Bohai Sea were analyzed.According to the content of 28-isofucosterol in surface sediment,the deposition region of floating green algae was primarily identified in the sea area(122–124°E,36–36.5°N)southeast to Shandong Peninsula.In conclusion,this study focused on the large-scale green tides in the southern Yellow Sea.For the origin of green tides in the Yellow Sea,the attachment characteristics of the gametes and spores of U.prolifera were studied using different substrates,and the potential roles of Porphyra aquaculture rafts in the early formation of green tides was revealed.To track the fate of green tide algae,the content of 28-isofucosterol in surface sediment was analyzed,then the deposition region of massive floating green algae in the Yellow Sea was primarily identified for the first time.The findings in this study will contribute to the understandings of both formation mechanisms and ecological consequences of green tides in the southern Yellow Sea.It will also provide important data to look for alternative materials for aquacultural rafts and to develop prevention and control strategies against green tides.