Spatio-Temporal Variation of Trophic Status and Water Quality with Water Level Fluctuation in a Reservoir

Water level fluctuation (WLF) is one of the important factors that affect reservoir water quality, habitat, species, and ecosystems. In this study, an independent sample t-test was used to evaluate the trophic status and water quality of the spatial and temporal variations with WLF in Shihmen Reservoir, Taiwan. The results of this study show that the Shihmen Reservoir has the lowest mean water level and higher potential of showing eutrophic status in April and May. This may be attributed to a lower water level, water depth, and transparency in this period. However, although there is no statistically significant difference in mean algal abundance in spring compared with other seasons, seasonal mean algae abundance and the seasonal mean Carlson’s trophic status index (CTSI) show as highly and positively correlated. It means that the increase in the CTSI value may not only be caused by effects on the sediment increase but also by algal proliferation. Mean water depth seems to be one of the important key indexes for reservoir management regarding trophic status since it reflects water quality and can be easy to obtain. This study suggests that reservoir administration can use the water level as a reference threshold for controlling CTSI strategies. In proper hydrological conditions, administration should try to hold a higher water level in a reservoir to downgrade CTSI.