Annual cycle of heavy metals in a tropical lake - Lake Chapala, Mexico

Dissolved concentrations of seven metals (As, Pb, Cd, Zn, Ni, Cu, Cr) were determined monthly at six sampling stations in Lake Chapala, Mexico, between November 1990 and December, 1991. Lake Chapala is the largest lake in Mexico, and serves as a source of drinking water and an economically important fishery and center for tourism. Historically, Lake Chapala has received poorly characterized domestic, industrial, and agricultural wastes. As, Cd, Pb, Cr, and Zn displayed peaks in concentration during the period before the rainy season due, presumably, to evaporation. Comparison of inflow and outflow concentrations also indicated a direct source of As to the lake. During this period, the concentrations of both Cd and Pb were above the freshwater chronic criteria values recommended by the United States Environmental Protection Agency for protection of aquatic ecosystem health. The metals As, Cr, Zn, Ni, and Cu also showed concentration peaks following the rainy season in August and September, perhaps due to increased input from the watershed or resuspension of metals in the lake driven by changes in water circulation patterns. Concentration data for the lake also suggested that a portion of the inflow water does not immediately mix with the lake as a whole, creating spatial gradients depending on the concentration of metals in the source water. Determination of metals in water hyacinth (Eichornia crassipes) plant tissue showed large partitioning of metals into root tissue, indicating that plants may be an important temporal sink for metals. In addition, copper and zinc accumulated to very high levels in Tilapia and Carp livers, indicating that some of the metal contaminants are entering the food chain.