Biotic Ligand Model estimation of copper bioavailability and toxicity in the Yacyretá Reservoir on the Paraná River
The Biotic Ligand Model (BLM) was used to assess copper bioavailability, toxicity and water-effect ratios (WER) for the Yacyretá Reservoir on the Paraná River. Copper in the Yacyretá Reservoir watershed is ubiquitous (median dissolved concentration = 4.4 µg L−1), being contributed mainly by non-point surface run-off from citrus fields and urban sewage point sources. BLM simulations indicated that copper bioavailability is negligible, as almost 100% of the dissolved copper is complexed by dissolved organic carbon (DOC). Copper bioavailability in the Paraná River, which has a mean hardness concentration of 16.5 mg CaCO3 L−1, is controlled by the presence of humic substances (≈60% of humic acids) in DOC. BLM estimates of acute toxicity for a cyprinid fish species were of the same order of magnitude as those corresponding to daphnids (EC50 = 63.54 µg L−1). However, zooplankton species were chosen to represent the aquatic biota for water quality regulatory purposes because of their slightly higher sensitivity to copper toxicity. Based on the corresponding minimum WER (14.6) for copper in Yacyretá Reservoir on the Paraná River, a site-specific copper criterion for the protection of the aquatic biota of 24 µg L−1 was derived. All measured dissolved copper concentrations for Yacyretá Reservoir during the 2001–2002 period were in compliance with this criterion. Development and application of the BLM provides a useful tool for managing metal loads in reservoir environments.