Factors regulating trophic status in a large subtropical reservoir, China

We evaluated a 4-year data set (July 2003 to June 2007) to assess the trophic state and its limiting factors of Three-Gorges Reservoir (TGR), China, a large subtropical reservoir. Based on Carlson-type trophic state index (TSI)(CHL), the trophic state of the system was oligotrophic (TSIS < 40) in most months after the reservoir became operational, although both TSITP and TSITN were higher than the critical value of eutrophic state (TSIS > 50). Using Carlson's (1991) two-dimensional approach, deviations of the TSIS indicated that factors other than phosphorus and nitrogen limited algal growth and that nonalgal particles affected light attenuation. These findings were further supported by the significant correlation among the values of TSICHL - TSISD and nonvolatile suspended solids and water residence time. The logarithmic model showed that an equivalent TSICHL and TSISD could be found at tau = 54 days in the TGR (Fig. 7). Accordingly, nonalgal particulates dominated light attenuation and limited algal biomass of the reservoir when tau < 54 days.