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Many lakes in inner Hanoi have been affected by eutrophication, which commonly leads to uncontrolled growth of algae, generation of cyanobacteria and toxic algae, increased water-treatment costs, and lake shallowing. This study presents an overview of the recent eutrophication status in Hanoi lakes and compares the lakes with Cu Chinh Lake based on a case study performed from April 2017 to March 2018. The eutrophication was mainly assessed by comparing the nutritional parameters (phosphorus and nitrogen) with values of poor nutrition and the concentration of algal biomass with concentrations found in hypereutrophic lakes. In addition, we investigated which nutrients contribute to limiting the growth of algae based on the total nitrogen/total phosphorus ratio, and we identified groups of algae that contained typical species in Cu Chinh Lake. The nutrient levels and algal biomass in many lakes were maintained between the eutrophic and hypereutrophic limits and underwent seasonal changes throughout the year. In Cu Chinh Lake, phosphorus is the predominant limiting nutrient for the development of algae. Thus, some typical algae genera, such as Microcystis, Anabaena of cyanobacteria, and Scenedesmus of green algae, are biological indicators of eutrophication in the lake.

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