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, Mexi co, between November 1990 and December, 1991. Lake Chapala is the larg est lake in Mexico, and serves as a source of drinking water and an ec onomically important fishery and center for tourism. Historically, Lak e Chapala has received poorly characterized domestic, industrial, and agricultural wastes. As, Cd, Pb, Cr, and Zn displayed peaks in concent ration during the period before the rainy season due, presumably, to e vaporation. Comparison of inflow and outflow concentrations also indic ated a direct source of As to the lake. During this period, the concen trations of both Cd and Pb were above the freshwater chronic criteria values recommended by the United States Environmental Protection Agenc y for protection of aquatic ecosystem health. The metals As, Cr, Zn, N i, and Cu also showed concentration peaks following the rainy season i n August and September, perhaps due to increased input from the waters hed or resuspension of metals in the lake driven by changes in water c irculation patterns. Concentration data for the lake also suggested th at a portion of the inflow water does not immediately mix with the lak e as a whole, creating spatial gradients depending on the concentratio n of metals in the source water. Determination of metals in water hyac inth (Eichornia crassipes) plant tissue showed large partitioning of m etals into root tissue, indicating that plants may be an important tem poral sink for metals. In addition, copper and zinc accumulated to ver y high levels in Tilapia and Carp livers, indicating that some of the metal contaminants are entering the food chain.