Carlson trophic state index (CTSI) has been commonly adopted to assess the eutrophication potential of reservoirs or lakes in water quality management. This study aims to analyze the influential factors of CTSI-based eutrophication by using Pearson correlation analysis and principal component analysis with long-term data from 2008 to 2019 on 21 drinking water reservoirs in Taiwan. The trophic state index (TSI) deviation indicates that most drinking water reservoirs, around 45.5% of statistical data fall within non-algal turbidity with surplus phosphorus,
especially in the spring and winter season. Besides, about 78% of total collected data show that TSI chlorophyll-a (Chl-a) is less than TSI Secchi depth (SD) due to the small particulate predominance. On the other hand, three TSI variables (SD, total phosphorus (TP) and Chl-a) of CTSI exhibits insignificant correlation to each other in most cases. Under such conditions, the probability of eutrophication (TSI > 50) based on TSI (SD) is 63%, while it is low as 20% based on TSI (TP) and TSI (Chl-a). The influencing factors of eutrophication variables by suspended solids (SS)
composition and turbidity have shown that the SD is strongly influenced by non-algal SS. The deviations of three TSI have shown that the highest algae-induced eutrophication potential occurs in summer season. In addition, the TP is the most significant loading factor of algae-induced eutrophication for drinking water reservoirs. It is concluded that the CTSI has limited applicability to identify the trophic state of drinking water reservoirs in Taiwan in the presence of sustainable non-algal turbidity comparative Chl-a that specifically represents the potential of algal growth.
