Fish assemblages and water quality in pampean streams (Argentina) along an urbanization gradient

The Matanza Riachuelo Basin is one of the most polluted watersheds in the world. We assessed the relationships between land use, stream-water quality, and the structure of the fish assemblages on a subbasin scale from data obtained in 44 sampling sites. A gradient of change from the rural upper sector to the urbanized and industrialized lower basin in conjunction with water-quality impoverishment was recorded. Redundancy analysis revealed that dissolved oxygen concentration, heavy metals, detergents, and Escherichia coli were the most explanatory variables of that gradient. Fish assemblages manifested the highest richness, abundance, and diversity in the rural areas; in the urbanized and industrialized sectors, the fish community became severely modified, represented only by tolerant species. Multivariate analyses indicated that the fish assemblage structure was highly impacted by water-quality deterioration. The combined study of the impacts of land use on water quality and fish assemblages enabled a detailed diagnosis of the consequent effects on a subbasin scale, thus substantiating the usefulness of fish assemblages as a model for that purpose. Field studies over large-scale gradients of environmental impact are essential for understanding the mechanisms of ecological deterioration and biotic responses as a first step in developing remediation policies.