Continuous modifications of river discharge by dam constructions and its impact on estuarine environment of the Nakdong Estuary, Korea

Abstract

The river discharge of the Eulsuk channels, the main waterway of the Nakdong Estuary, has been modified significantly by dam constructions for the last century. In addition to dam constructions, other anthropogenic alterations would also have changed the estuarine environment. ES-A, a well-dated sediment core, was collected at the Eulsuk tidal flat of the Nakdong Estuary in 2015. Grain size, total organic carbon(TOC), total nitrogen(TN), carbon and nitrogen stable isotope(δ13C, δ15N), and biogenic silica(BSi) of ES-A were analyzed to estimate environmental changes by anthropogenic alterations. Mud, TOC, TN, and BSi content of ES-A increased after each dam installation. These damming effects on ES-A were temporary and the damming effect of Noksan Dam was the largest due to main channel shift from the West Nakdong river to the Eulsuk channels. Though river discharge of the Eulsuk channels significantly modified by Noksan Dam and Nakdong Estuary Dam, TOC/TN ratio(C/N) and δ13C have been in the range of estuarine POM. Moreover, there was no correlation between BSi and δ13C, suggesting changes in freshwater discharge was not a major determinant of diatom production in the Nakdong Estuary. The marine organic carbon proportion in bulk TOC(TOCmarine/TOCbulk), C/N and δ13C were correlated one another indicating that organic matter has been well preserved. BSi/TOCbulk and BSi/TOCmarine were calculated to estimate diatom dominance change. δ13C correlated with BSi/TOCbulk, but not with BSi/TOCmarine which would be more suitable for in-situ diatom dominance by excluding external TOC. This finding suggested that the river discharge has been a controlling factor for TOCterrestrial input to the estuary but not for diatom production and its dominance, so BSi/TOC of ES-A is not a suitable indicator for diatom dominance. Therefore, the Nakdong Estuary has been maintained estuarine condition along with the positive linear correlation between BSi/TOCbulk and δ13C though river discharge has modified by damming. However, after the West gate opening in 2012, there were outliers on BSi/TOCbulk-δ13C correlation due to increased non-diatom bloom in 2013~2015. This suggested that the BSi/TOCbulk-δ13C correlation could indicate environmental changes such as harmful algal blooms.