Impacts of dam discharge on river environment and phytoplankton communities in a regulated river system, the lower Han River, South Korea

Abstract

To investigate the effects of fluctuations in dam discharge by rainfall on river environment and phytoplankton communities, we monitored such communities and environments biweekly, from February 2001 to February 2002 and from February 2004 to February 2005, in the lower Han River (LHR), South Korea. Phytoplankton communities in the LHR can be categorized into four key temporal stages, related to changes in water inflow by dam discharge. In the first stage, towards the end of winter, small centric and pennate diatoms, as well as cyanobacteria, develop in response to increased nutrient availability, light intensity, and water temperature, as well as slow flow of the water. In the second stage, associated with dam discharge by summer rainfall, the crop of phytoplankton is swept away by the high-velocity flow of the river. At the same time, concentrations of suspended solids, dissolved inorganic nitrogen, and dissolved silica are increased rapidly by run-off from the land. In the third stage, after the rainy season, phytoplankton crops for autumn start to accumulate. Among them, Aulacoseria granulata becomes increasingly abundant as autumn progresses. In the fourth stage, associated with a reduction of light energy and high nutrient availability, Stephanodiscus hantzschii comes into bloom. This blooming leads to a depletion of Si in the dry season that occurs during winter. The results of our monitoring show that the phytoplankton communities were damaged more severely by the high-velocity flow of water introduced by dam discharge than by nutrients or other environmental factors. Therefore, to conserve and promote aquatic organisms in regulated rivers, an ecological conservation strategy is required that involves the construction of infrastructure to control the water velocity and an increase in the water retention time.