Effects of seasonal and diel variations in thermal stratification on phytoplankton in a regulated river

Abstract

Abstract Thermal stratification in lotic systems has occurred globally and more frequently in recent decades due to global warming and artificial river modification, often with negative ecological effects. However, studies on river stratification have been restricted to rivers below dams or reservoirs affected by their water release and lacked examination of diel stratification and its impact on phytoplankton, in particular. In the present study, we assessed the degree of thermal stratification, its environmental drivers, and the response of water quality and phytoplankton community against stratification in the mid-lower reach of the Nakdong River, whose morphology has been highly modified, including the construction of eight weirs. We implemented vertical temperature profiling at three study sites, both seasonally and diurnally. Then, we calculated three stratification indices: relative water column stability, Schmidt stability, and maximum temperature gradient. Three indices for assessing the degree of stratification showed that most sites experienced diel stratification during summer. Principal component analysis showed that stratification significantly led to seasonal and diel variations in the water environment. Solar radiation and air temperature were positive controllers, while a negative controller (in this case, the river flow rate) existed only for diel variation in the stratification. Higher abundance and surface cell accumulation of cyanobacteria Microcystis were observed at the stratified sites, and the diel variations in its biomass (chlorophyll a) in the surface water were primarily associated with the stratification indices instead of with other temperature and nutrient variables. Overall, the results suggest that the river has summer stratification, which is involved in amplifying cyanobacterial bloom intensity. Without a suppressing factor, summer stratification is expected to be recurrent in the river, and thus mitigating the developed stratification is needed by promptly regulating the river flow.