A Study on Trophic State in Lakes of Dong Da District, Hanoi

Abstract

One of approaches to assessing the condition of lakes is to look at lakes with respect to 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 in lakes of Dong Da district, Hanoi. The tropic states of the lakes were characterized using the Carlson TSI and the Chlorophyll-a concentrations 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 were not limited factor for algal biomass but some factors such as nitrogen limitation, zooplankton grazing or toxics limit algal biomass. The time plot method suggested that transparency were dominated by non-algal factors such as color or turbidity or where very small particles predominated, whereas the difference plot method revealed that large particles were dominated. Consequently, more studies should be done to look for the real limiting factors and conditions of the lakes.