Water Quality Assessment and Eutrophic Classification of Hanoi Lakes Using Different Indices

The trophic levels in urban lakes are typically based on the forms of nutrition and phytoplankton communities in the lakes. In this study, comparisons between eutrophication indices and the water quality index (VN-WQI) were used to classify the water quality of 20 lakes in Hanoi, Vietnam. The results showed that the water quality ranged from very bad to poor. High levels of the eutrophication phenomenon in terms of total N and P were observed in all sampling sites. Cyanobacteria was the dominate algae phylum making up 65.78% of the total population, whereas Chlorophyta (including 19 genera) was the most diverse phylum. The density of the algae was 5,000-14,000 cells mL-1 and the chlorophyll-a level was 10-40 µg L-1. Based on this information, the water quality levels in the lakes were classified from eutrophic to polytrophic. The Trophic Status Index (TSI) and Trophic Level Index (TLI) values ​​were 66.9-86.0 and 54.4-76.0 points, respectively, corresponding from eutrophic to extreme-trophic. Classifications based on algae community structure indices were from oligotrophic to eutrophic, similar and consistent with the TLI classifications. According to the results in this study, the TLI was found to be more accurate and precise than the other indices. It is recommended that the TLI is an applicable tool to classify eutrophication in urban lakes.