This study presents an analysis of the trophic state of lakes in Dong Da district, Hanoi. The tropic states of the lakes were characterized using the Carlson TSI and the Chlorophyll-a concentrations and referenced with the thresholds of US. The deviations of the Secchi depth and total phosphorus indices from the Chlorophyll-a index were used to identify limiting factors affecting to the tropic state of the lakes. | VNU Journal of Science: Earth and Environmental Sciences, Vol. 33, No. 4 (2017) 21-30 Study on Trophic State in Lakes of Dong Da District, Hanoi Nguyen Thi The Nguyen* Thuy Loi University, 175 Tay Son, Dong Da, Hanoi, Vietnam Received 30 September 2017 Revised 03 November 2017; Accepted 28 December 2017 Abstract: One of approaches to assessing the water quality of lakes is to look at their primary production or trophic state. Protecting water quality of urban lakes from eutrophication is an important task of all governments. This study presents an analysis of the trophic state of lakes in Dong Da district, Hanoi. The tropic states of the lakes were characterized using the Carlson TSI and the Chlorophyll-a concentrations and referenced with the thresholds of US. The deviations of the Secchi depth and total phosphorus indices from the Chlorophyll-a index were used to identify limiting factors affecting to the tropic state of the lakes. It comes to a conclusion that most of the lakes in study area were hypereutrophic or eutrophic in August 2017 and eutrophic or oligotropic in March 2017. It also reveals that phosphorus was not a limited factor for algal biomass while other factors such as nitrogen showed certain effects. The time plot method suggested that transparency was dominated by non-algal factors such as color and size,of suspended substances/ particles, whereas the difference plot method revealed that large particles were dominated in the studied lakes. Consequently, further studies should be done to find out the real causes of the lakes’ eutrophication. Keywords: Tropic state, lake, time pots, different pots. 1. Introduction concentrations of nutrients, low rates of productivity and generally low biomass are termed oligotrophic. Lakes that fall in between are mesotrophic, and those on the extreme ends of the scale are termed hypereutrophic or ultraoligotrophic. Lakes exist across all trophic categories; however hypereutrophic lakes are usually the result
