Modification of trophic level index with the contribution of macrophyte and its usage to classify trophic state of shallow lakes

Trophic state index (TSI) only considers the influence of phytoplankton excluding that of macrophytes. It is necessary to combine the contribution of macrophytes into trophic classification systems in waters with extensive growths of macrophytes. A novel trophic level index (TLI_ECa) combined both trophic level index (TLI) and the TSI based on equivalent chlorophyll a (TSI_ECa) with the Chl a of submerged macrophytes as an addition in Chl a was developed to assess the spatial trophic state of 15 lakes and annual trophic state of four lakes in China. TLI_ECa obtained different but significantly correlated results as those of the traditional TLI, concerning the influences of both phytoplankton and macrophytes. The result of TLI_ECa indicated that the trophic state of the 15 lakes varied from mild-eutrophic to moderate-eutrophic. Small particles were the dominant factor for the trophic state of most sampling sites in the 15 lakes. Total phosphorus was the dominant factor for the trophic state for most time of the year in Lake Jinniu. Both small particles and total phosphorus were the dominant factors for the annual trophic state of Lake Taihu, Lake Xuanwu, and Lake Baijia.