Phosphorus balance in Lake Chapala (Mexico)

Lake Chapala is the largest and most important lake in Mexico and the third largest lake in Latin America. It is the main water supply for Guadalajara , whose population is close to 5 million inhabitants. The primary tributary to the lake is the Lerma River. Large quantities of domestic and industria l sewage and agricultural runoff from the entire Lerma-Chapala basin still flow largely untreated into the lake. Starting in the 1970s, the lake has u ndergone significant changes in hydrology, resulting in an increase in the hydraulic residence time of the Lerma River (inlet waters) from a value of less than 10 years to one of more than 40 years. There are no previous stud ies establishing the total phosphorus balance in the lake. The focus of thi s work is to determine an historical phosphorus balance in Lake Chapala by quantifying the main pint and non-point sources of total P to the lake. Usi ng water quality data recorded over a 24 year period (1974 to 1997), the ma ss balance shows an average total phosphorus accumulation rate in the lake of about 11 to 683 metric tons/year. The total P input to the lake is about 626 to 910 metric tons/year, of which the Lerma River contributes more tha n 90%. In the period of study, Lake Chapala has maintained a consistent eut rophic status, with an average annual external P load of 0.67 +/- 0.49 g/m( 2). The results for three different periods show a trend to increasing year ly P loads per square meter of surface water.