Changes in algal bloom dynamics in a regulated large river in response to eutrophic status

This study aims to analyze the factors affecting algal growth due to hydrologic and hydrodynamic changes resulting from eight weir constructions along a length of 207 km on the Nakdong River. Nutrients for algal growth, light intensity, water temperature, and hydrodynamic conditions were examined to study their relationships with algal blooms. A three-dimensional hydrodynamic and water quality model was calibrated based on data from 2006 to 2008 and verified against data from 2013 to 2015. Four statistics (i.e., R2, ME, Pbias, and CF) were used for the model performance assessment. Changes in algal bloom were not significant in the upper stream area because the nutrient level for algal growth was insufficient compared to that at other locations. On the other hand, as the nutrient concentration increased in the middle stream area, algal blooms increased significantly. The upper and middle streams showed increases in algal growth after the construction of weirs. However, hypereutrophic downstream area locations showed contrasting results, showing an overall reduction in algal growth after the construction of weirs, except for winter, which showed the largest increase in hydraulic residence time. These results seem to have positive and negative effects on algal blooms due to the construction of weirs. The negative effects seem to be greater in hypereutrophic areas because of the decreased average light effect as a result of increased water depth created through the weir constructions, playing a dominant role over the level of nutrients. The impacts of the construction of weirs on the occurrence of algae in the Nakdong River were as follows: 1) As the hydraulic residence time increased, the algal bloom increased where the nutrients were sufficient; 2) when the nutrients were more than sufficient, the average light availability became the dominant factor for algal growth; 3) the temperature effect was not a significant algal-growth-limiting factor for hydrodynamic changes in the river. However, these results have limitations in terms of the model and data accuracy. In order to use them for water resource management, additional field experiments are required for confirmation and verification.

Automated summary

The study analyzed the factors affecting algal growth due to changes in hydrologic and hydrodynamic conditions caused by weir constructions along the Nakdong River. It found that algal blooms increased in the upper and middle streams post-construction, while decreasing in hypereutrophic downstream areas. The impacts of weirs on algal growth showed both positive and negative effects.