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

KONG Fan-Xiang, GAO Guang(Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China).  
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.
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