Bioaccumulation of selenium (Se) in the Cienega de Santa Clara wetland, Sonora, Mexico

The Cienega de Santa Clara, on the east side of the Colorado River delta, is a brackish wetland supported by agricultural drainage water from the United States that provides habitat for endangered fish and bird species. Bioaccumulation of selenium has created toxicity problems for wildlife in similar wetlands in the United States. This is the first selenium survey in the Cienega de Santa Clara. Ten sites were selected to collect water (dissolved), sediments (total), plants, invertebrates, and fish. Samples were collected from October 1996 to March 1997. Selenium was detected in all samples. Concentrations in water ranged from 5 to 19 microg/L and increased along a salinity gradient. Although water levels of selenium exceeded EPA criterion for protection of wildlife, levels in sediments (0.8-1.8 mg/kg), aquatic plants (0.03-0.17 mg/kg), and fish (2.5-5.1 mg/kg whole body, dry wt) did not exceed USFWS recommended levels. It is concluded from this study that the levels of selenium in water did not affect the overall health of the fish sampled. Therefore, it is important to maintain or improve the water quality entering this wetland to continue to have normal levels of Se in the food chain components.