Analysis of the impact of urban effluent discharge on malacofauna in a river of Southern Patagonia (Santa Cruz, Argentina), through field studies and laboratory bioassays

Abstract

Continental water bodies are usually receivers of anthropogenic effluents, which cause environmental disturbances that can manifest with changes in the composition of biological communities. Mollusks can be considered excellent indicators of the health of aquatic ecosystems, due to their sensitivity to environmental changes. The aim of this study was to examine the impact of urban effluent discharge on mollusk assemblage in the Chico River of Santa Cruz, Argentina, through field studies and laboratory bioassays. Four separate sampling surveys were conducted between March 2017 and March 2019, in seasons with different flow regimes. Six sampling stations were designated, two reference stations upstream of Gobernador Gregores town and four downstream at increasing distances from the town. At each sampling station, physicochemical variables were measured, and four benthic samples were collected using a manual Surber net with a 500-µm mesh. The environmental variables and mollusk abundance data were analyzed using multivariate statistical methods. Additionally, a direct toxicity assessment was performed in laboratory setting to evaluate the effect on growth and survival of Pectinidens diaphanus (Lymnaeidae). The response of mollusk species in terms of distribution and abundance, exhibited a strong relationship with the different environmental conditions at the sampling stations, as determined by their degree of sensitivity or tolerance. The observed spatial changes in the composition of the mollusk assemblage are indicative of a response to a combination of point and diffuse sources that introduce pollutants to the river. The bioassay permitted the isolated evaluation of the punctual effect of urban sewage effluent, which was found to negatively affect the survival of the studied species.