Carlson trophic state indices (TSIs) have been frequently employed in the water quality literature. The current study reports the spatial distribution and seasonal variation of trophic status in Danjiangkou Reservoir (DJKR), which is a large water supply reservoir of the South-to-North Water Transfer Project in central China. The calculation of TSICHL indicated a mesotrophic state (40 <= TSICHL < 50) for most regions of the reservoir, whereas a eutrophic state was indicated if the assessment was based on either TSITP (calculated from total phosphorus) or TSISD (calculated from Secchi depth). As indicated by Carlson's two-dimensional deviations, factors other than phosphorus limited algal growth in January and May when non-algal particles affected light attenuation. However, phosphorus was the controlling factor of large algae blooms in July and November in 2007. TSISD had positive correlations with TSICHL and TSITP. It was found that total phosphorus, SiO2-Si and SD were the major factors regulating the trophic state (TSICHL) of DJKR. Light attenuation and SD were mainly affected by non-algal particles.