A quantitative method, Ecosystem Health Index (EHI) for assessing lake ecosystem health has been proposed in this paper, based on data from 30 lakes in Italy. The method were three major steps. Firstly, a numerical EHI in a scale of 0 to 100 was developed. Secondly, in order to calculate the specific and synthetic EHI, phytoplankton biomass (BA) was selected to serve as a basic indicator, while zooplankton biomass (BZ), the ratio of BZ to BA (BZ/BZ), exergy and structural exergy were used as additional indi...