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China is a country with many lakes, about one-third of which are freshwater mainly distributed in the middle and lower reaches of the Yangtze River. Currently most of the lakes are mesotrophic or eutrophic. Lake eutrophication has become one of the major ecological and environmental problems faced by lakes in China and can lead to a series of abnormal ecosystem responses, including extinction of submerged plants, frequent occurrence of cyanobacterial blooms, increased microbial biomass and productivity, decreased biodiversity, accelerated cycles, and a change in the efficient use of nutrients. With development of eutrophication, the whole lake ecosystem suffers decreased biodiversity, simplification of biotic community structure, instability of the ecosystem, and ultimately the clear-water, macrophyte-dominated ecosystem gradually shifts to a turbid-water, algae-dominated ecosystem. This ecosystem succession mechanism is speculated to be caused by different nutrient utilization efficiencies of macrophytes and phytoplankton. The ultimate ecosystem succession trend of seriously eutrophic lakes is that a phytoplankton-dominated autotrophic lake shifts to a heterotrophic lake dominated by micro-organisms, protozoans. [ABSTRACT FROM AUTHOR]

Abstract: In order to investigate the role of the microbial community in aquatic ecology and nutrient transformations in the development of eutrophication in large shallow freshwater lakes along Yangtze River, the microbial community in the depth-related sediment in Lake Chaohu and Lake Longganhu were compared. Lake Chaohu is one of the three most polluted lakes in China. However, the neighboring Lake Longganhu, a mesotrophic lake, is relatively pristine. The total phosphorous (TP) and total nitrogen (TN) concentration in water was detected at 0.193mgl−1 and 3.035mgl−1 for Lake Chaohu, 0.051mgl−1 and 0.774mgl−1 for Lake Longganhu, respectively. The population of the microorganisms with various ecological nutrient transforming functions (e.g. phosphate solubilizing, denitrifying and cellulose decomposing) and a batch of environmental parameters concerning the nutrient accumulating and transforming (e.g. total organic carbon, total nitrogen, and total phosphorous concentrations) were assayed in the depth-related sediment samples from several defined points in both lakes. The sediment samples from Lake Chaohu showed higher density of actinomycetes (P <0.05) and phosphate-solubilizing bacteria (P <0.001) and less profusion of denitrifying bacteria (P <0.05) and cellulolytic microbes (P <0.001), compared with those of Lake Longganhu. The data suggested that the current microbial community in the sediment of Lake Chaohu is in favor of sustaining or further accelerating the process of the lake eutrophication. A possible positive feedback loop which consists of sustained growth of microorganisms and gradual decline of lake eutrophic status is worth further discussing. [Copyright &y& Elsevier]