Seasonal Variation of Water Quality and Microcystin Concentration in Kandawgyi Lake, Yangon City, Myanmar

Abstract

This study is the first report on seasonal variations in water quality parameters and cyanotoxin microcystin (MC) concentration during one year in Kandawgyi Lake, Yangon City, Myanmar. Water quality parameters were measured and water samples were collected once a month from August 2018 to June 2019 at four observation sites (St-1~4). The seasonal changes of DO, pH and water temperature and turbidity were similar among the four observation sites. The observed DO values were always over the saturation levels due to the high photosynthetic activity in the hypereutrophic lake. However, chlorophyll-a as a surrogate of phytoplankton biomass was correlated negatively with DO. The multivariate linear regression model (linear model) analysis suggested that Kandawgyi lake was in the N-limitation state. Also, the lower N:P ratio (< 10) of lake water supported this fact. The linear model analysis elucidated the dependency on the water temperature for the three MC variants. The highest concentration of MC-YR and MC-LR at a lower temperature of less than 29°C was observed. The higher value of MC-RR in concentration was found at the temperature ranged was 29°C to 31°C than other temperature ranges. In terms of seasonal variation, MC-LR, which is more toxic than MC-RR and MCYR, tended to show higher concentration in cool-season of Yangon city. It is known that the production ratio of MC variants is depending on the species or genetic variants of toxic cyanobacteria producing MCs. Therefore the seasonal variations of MC variants suggested the seasonal succession of toxic cyanobacteria. Moreover, it was thought that the variations of MC variants among the sites could be involved by the low mixing condition of the lake water. Therefore, this result has shown that lake monitoring with sufficient frequency and observation points in lakes with low water mixing is essential for monitoring water safety.