Comparative Study On The Survival Environment Of Micropropagules Of Ulva Prolifera

Since 2007,there has been a large-scale outbreak of green tide in Yellow Sea waters for several years,with massive proliferation,accumulation and decay and degradation of floating green algae,triggering water hypoxia and death of aquaculture animals,posing a serious threat to marine ecological and environmental safety and sustainable development of marine economy,which has aroused great concern of scientists and governments.The cause species of the Yellow Sea green tide is Ulva prolifera,which originated from the Subei shoal in the process of drifting northward and finally gathered on the coast of Shandong Peninsula.The occurrence of U.prolifera is related to micropropagules（General term for the germ cells and microscopic individuals of U.prolifera）,which occupy an important position in the life history of U.prolifera.The micropropagules of U.prolifera exist in the Subei shoal all year round,while they are only detected during the green tide outbreak in Qingdao waters,and no micropropagules are detected in autumn and winter.To clarify the survival of micropropagules in Qingdao waters,it is important to predict its spread,determine whether Qingdao will become the origin of U.prolifera outbreak in the future,predict the future development trend of green tide in the Yellow Sea,and realize the early warning forecast of green tide.In this paper,we analyze the environmental differences between Qingdao and Subei Shoal from the ecological environment characteristics of the two places,and study the effects of nutrient,salinity,light and temperature on the germination,growth and reproduction process of micropropagules,deepen the role of the above environmental factors in the origin of green tide in the Yellow Sea,and explore the role of life history and reproduction mode of U.prolifera in the formation of green tide,so as to provide data support to reveal the mechanism of green tide outbreak and its prevention and control.The nitrogen and phosphorus nutrients play an important role in the germination and growth process of micropropagules of U.prolifera.The effect of different concentrations of nitrogen and phosphorus on the attachment of micropropagules of U.prolifera was not significant,but the effect on the germination of micropropagules was significant,and phosphorus was the key nutrient factor controlling the germination stage of micropropagules.When nitrogen and phosphorus are sufficient,micropropagules can be fully germinated within a week,when phosphorus concentration is low,it is not conducive to the germination of micropropagules and the germination rate is significantly reduced.During the growth stage,nitrogen and phosphorus had a significant promotion effect on the growth of U.prolifera seedlings,and the higher the nitrogen and phosphorus concentration,the faster the growth rate of U.prolifera seedlings.Phosphorus has an important role in the growth stage of micropropagules,and the lower phosphorus concentration will limit the growth of U.prolifera seedlings.When the nitrogen and phosphorus concentrations are changing,the nitrogen concentration gradually decreases,it is not conducive to the growth of U.prolifera seedlings;when the phosphorus concentration fluctuates,it is more favorable to the growth of U.prolifera seedlings.The nitrogen and phosphorus uptake of the micropropagules of U.prolifera was low at the stage of attachment and germination,and the nitrogen and phosphorus uptake gradually increased during the growth period after complete germination,and the nitrogen and phosphorus uptake was positively correlated.In the early germination stage of micropropagules,PO₄-P was more favorable to the germination and growth of micropropagules than NO₃-N and NH₄-N.Phosphorus was the key environmental factor limiting the germination and growth of micropropagules in Qingdao waters.Different salinities had significant effects on the attachment of micropropagules of U.prolifera,with salinity 5 being unfavorable to the attachment of micropropagules;micropropagules could fully germinate within 7 days in the salinity range of 5-35.There was no effect of salinity variable on the germination of micropropagules.Spores attachment and germination were faster than gametes.At the growth stage,low salinity conditions 5-15 were not favorable for the growth of the seedlings of U.prolifera.During the reproduction stage,the salinity range for the release of micropropagules from U.prolifera is 15-35,at low salinity（5-10）U.prolifera will not carry out reproductive activities,and salinity int the range of 15-28has no significant effect on the release of micropropagules from U.prolifera.In the process of green tide formation,the reproductive strategy of spores is K countermeasure,and the reproductive strategy of gametes is R countermeasure.The salinity was not a key environmental factor limiting the development and reproduction process of the Subei shoal and Qingdao waters.Through indoor culture experiments,the effects of different light conditions and low temperature and dark duration on the germination of micropropagules of U.prolifera were analyzed,and the main conclusions are as follows:light has a significant effect on the attachment and germination of micropropagules of U.prolifera.The micropropagules did not germinate in the absence of light,and the differences between the attachment and germination of micropropagules under 20 and70μmol·m^-2·s^-1 light conditions were not significant.The micropropagules of U.prolifera were highly tolerant to the adverse environment of low temperature and darkness,and the micropropagules could still survive in the low temperature and darkness for 3 months,and could continue to germinate and grow after the temperature and light were restored.Micropropagules are tolerant to harsh environments,light and low temperature are important factors affecting the germination of micropropagule.This thesis investigates the generation and survival of U.prolifera micropropagules in Qingdao waters and the effect of environmental factors on the survival of micropropagules through field enclosure experiments.The enclosure experiments showed that a large number of micropropagules were produced during the extinction period,and the density of micropropagules in the enclosure showed a trend of increasing and then decreasing,but the micropropagules in the enclosure did not grow into seedlings.During the period of extinction and the release of micropropagules,a large amount of nutrients was consumed in the enclosure.Factors such as low nutrient salinity,high temperature and biological competition were not conducive to the germination and survival of micropropagules,which might be the reason for the failure of micropropagules to grow into seedlings.In addition,the results of low-temperature culture experiments on micropropagules collected in the enclosure showed that micropropagules were highly tolerant to low-temperature dark conditions,and they could still germinate and grow into algae when temperature and light were restored after 90 days of low-temperature dark culture.In summary,temperature,light and nutrient play an important role in the germination process of micropropagules.For the reason of short survival time of micropropagules in Qingdao waters,this study suggested that it was caused by the combination of light,temperature and nutrient:after a large amount of U.prolifera floated to Qingdao waters in summer,it would produce a large number of micropropagules,because the water temperature was high and nutrient was low,it was not conducive to micropropagule germination,but still could germinate,and some of the germinated micropropagules were difficult to grow into U.prolifera adults and produce new micropropagules.In the Subei shoal the micropropagules cannot germinate quickly and are buried in the sediment because of the turbid seawater and fast settling rate,and insufficient light despite the suitable temperature and nutrients conditions.In autumn and winter,as the temperature decreases,the micropropagules are not conducive to germination,and the micropropagules can survive the winter and germinate again in the following spring when the environment is suitable,thus completing a life history cycle.Since Qingdao waters do not have low light and high nutrient environment,micropropagules germinate or die.So far from the data obtained from our field survey and laboratory simulation experiments,Qingdao will not be the origin of U.prolifera outbreak.