In Taiwan, a large amount of silts and sands or suspended solids in streams are frequently brought into reservoirs during intensive rain falls to reduce the transparency of reservoir water, which results in overestated Carlson trophic state index (CTSI) value. At such a condition, it is anticiptated that CTSI is failed to accurately evaluate trophic state of reservoir water. Therefore, this study aims to review the suitability of CTSI to reservoir water quality management. The characterization of water quality for reservoirs in high eutrophication potential and the evaluation of alge blooming were carried out simoltenously to formulate a local trophic state index for reservoir management divisions. 　　The results of water quality analysis for six candidates (i.e., Feitsui, Shimen, Mingde, Liyutan, Renyitan and Chengqing Lake reservoirs) showed that total phosphorus (TP) is the limiting factor of algae growth for most of reservoirs. TP is mostly composed of particulate phosphorus instead of dissolved phosphorus, and it is found that lowering the TP/TN (total phosphorus/total nitrogen) ratio has potential to supress blue-green algae growth. On the other hand, the on-site fluorescent probe can be used to chlorophyll a (Chl-a) and water temperature at different water level, which is helpful to determine algae growth state and thermo stratification in reservoirs and their relative percentage of algae species. It is anticipitated that on-site fluorescent probe is effectively used as a early warning technique in water quality management for eutrophicated reservoirs. In addition, the results of big data analysis on water quality of reservoirs showed that CTSI has low suitability to evaluate trophic state of most drinking water reservoirs. The results of big data investigation into water quality database showed that it is necessary to formulate local trophic state index for Taiwan reservoirs for which Chl-a is the most suitable index instead of CTSI. It is suggested that chlorophyll a threshold at different trophic state is determined by the inverse computation of its TSI threshold (i.e., eutrophic: Chl-a>7.2 µg/L, mesotrophic: 2.6≦Chl-a ≦7.2 µg/L, oligotrophic: Chl-a<2.6 µg/L ). Meanwhile, the current CTSI is still used for the long term comparision in results between new and old trophic state index. The meanings of new and old trophic state index should be noted when they are announced to let populace understand their role and effectiveness.

Water quality management index, Carlson trophic state index, Eutrophication