Changes in hydrology, water quality, and algal blooms in a freshwater system impounded with engineered structures in a temperate monsoon river estuary
Yongsik Sin, Hakyoung Lee
Abstract
The Yeongsan River estuary (the Republic of Korea) is turbid and eutrophic under a temperate monsoon climate and has recently undergone weir impoundments in its freshwater zone. We conducted analyses to determine changes in the hydrology, water quality, and algal blooms caused by the impoundment in 2011. Historical records (2004–2016) and sampling data collected along the freshwater channel zones over annual cycles from 2014 to 2015 were analyzed to identify these changes and their underlying mechanisms. The Kruskal–Wallis test, Mann–Whitney U test and non-metric multidimensional scaling (nMDS) were employed for data analyses. Water levels increased after impoundment and the flow velocity significantly decreased, while river discharge remained unchanged. Total suspended solids also significantly decreased. An increase in water transparency combined with more stagnant and thermally stratified water flow during the warm seasons contributed to increases in the magnitude and frequency of algal blooms and dominance of cyanobacteria, including potentially toxic species, rather than diatoms. Seasonal peaks in turbidity and flow velocity during monsoon season, which restricted algal blooms before impoundment, were also sharply reduced. This indicates that the impact of the impoundment on hydrology and harmful algal blooms can peak during monsoon season.