ABSTRACT
Zimapán Reservoir is located in the central plateau of Mexico. Its main purpose is the generation of electric energy, and it has a high mean depth (52.40 m). The reservoir has two tributaries: the Tula River, which receives the wastewater of Mexico City, and agricultural run-off; and the San Juan River, with municipal and industrial wastewater. The population growth together with the dry climate are generating pressure on the water resource. The purpose of this study is to characterize the trophic state of Zimapán Reservoir and its trend on a five year study. It is warm with the presence of thermocline between 8 and 14 m. It was found hypereutrophic for the nutrient concentration (average TP and TN of 1,381 and 5,977 microg.L(-1), respectively), and eu-hypereutrophic in case of the Secchi disk (1.94 m) and the chlorophyll concentration (38 microg.L(-1)). The trend analysis showed and increment of nutrients in the period, but a decrement in Secchi disk and chlorophyll results. Nutrients are present in a very high concentration. The reservoir is gradually increasing its trophic state condition.