大型浅水富营养化湖泊中蓝藻水华形成机理的思考

Hypothesis on cyanobacteria bloom-forming mechanism in large shallow eutrophic lakes

湖泊富营养化依然是我国目前以及今后相当长一段时期内的重大水环境问题。研究蓝藻水华的形成机制 ,对于科学预测湖泊中蓝藻水华的产生 ,并采取相应措施减少其带来的影响具有重要的生态和环境意义。为探索富营养化湖泊中蓝藻水华形成机理 ,综述了目前对我国大型浅水湖泊蓝藻水华成因研究现状和对水华形成机理的一般认识。分析了导致蓝藻水华形成的化学、物理和生物等主要环境因素 ,论述了蓝藻 ,尤其是微囊藻成为水华优势种的可能原因。认为对水华的形成需要全面认识 ,营养盐浓度的升高可能仅是蓝藻水华形成、且人们可以加以控制的因素之一 ;在探索水华成因时 ,不能仅仅局限于夏季蓝藻水华发生时环境特征的研究与观察 ,而应该提前关注蓝藻的越冬生理生态特征、春季复苏的生态诱导因子及其阈值以及在复苏后 ,蓝藻如何在生长过程中形成群体 ,并逐步成为湖泊水生生态系统中的优势种乃至形成水华的过程。并需要对蓝藻越冬的生存对策、蓝藻群体的形成的条件、蓝藻在春季复苏的触发条件及其生态阈值、以及蓝藻在与其它藻类种群竞争中取胜的生理生化特征有足够的认识。蓝藻水华的“暴发”是表观现象 ,其前提还是藻类一定的生物量 ,且是一个逐渐形成的过程。

Eutrophication in Chinese lakes is one of the most serious water-pollution problems present and will still be in the near future. It is very important to forecast the occurrence of cyanobacteria bloom in lakes and then find a way to reduce its impact on the environment. Therefore, it is necessary to get comprehensive knowledge about the cyanobacteria bloom-forming mechanism in eutrophic lakes. The aim of this paper is to explore the bloom-forming mechanism based on the past and present research works about the development of water bloom in aquatic ecosystems. We tried to expound the dominant physical, chemical and biological factors that might induce the formation of water bloom. We also discussed the reason why cyanobacteria, especial Microcystis, become the dominant species in phytoplankton community. Our discussion considered that the high nutrient level is just one of the factors that control the water bloom formation in Taihu Lake. We suggested that the future research should deal with not only the cyanobacteria bloom occurring period in summer but also the colony accumulating and settling process to the sediment surface in autumn and the recruiting process in spring. We also suggested the further investigation that should be interested for us: (1) What is the over-wintering survival strategy of Microcystis? (2) What is the colony composition really like? (3) How does the Microcystis turn from small size with only a couple of individual cells into large size with hundreds of cells, then conglutinated together by sheath to form water bloom? (4) How does the alga obtain the threshold valve that induce its recruitment in spring? (5) What is the biochemical and physiological characters that make cyanobacteria dominated in phytoplankton community? We considered that the outbreak of a bloom in short time is just an apparent phenomena. In fact, the bloom is the result of a variation of vertical position of large volume of algal biomass that is gradually developed and accumulated for long time. According to the ecological theory and the in situ observation of the water bloom in Taihu Lake, we raised the four-phase development hypothesis on the process of the cyanobacteria bloom-forming: dormancy in winter, recruitment in spring, growth and float to the water surface in summer and sink to the sediment in autumn. There are different factors that will influence the status of algae in different phases. For example, in the dormancy phases, it is likely to be controlled by low temperature and illumination; and during the recruitment; the main factors may be the temperature and dissolve oxygen on the sediment surface. In addition, the algal growth rate is controlled by the nutrient and energy needed for photosynthesis and cell division. We concluded that (1) the hydrological and meteorological condition would cause the algae to float up to the water surface and then form the water bloom; (2) the further pertinence survey is necessary to control the cyanobacteria bloom before it starts to recruit, develop and bloom.