Variability in mentum deformities of Tanytarsus larvae (Diptera, Chironomidae) in a metal rich basin in Northern Mexico

Mentum deformities in chironomids have been commonly used as indicators of metal contamination in freshwater ecosystems. Incidence of these deformities suggests sublethal effects and can provide early signals of environmental deterioration. While anthropogenic metal pollution is known to lead to an increase in deformities in chironomids, natural background deformity incidence information is key to their effective use as biomonitoring tools. Here we explore the incidence of deformities from 5000 Tanytarsus (Diptera, Chironomidae) menta in relation to water physicochemical, and sediment metal (Al, As, Cu, Fe, Mn, Pb, Zn) concentration data from numerous sites in the naturally metal-rich Sonora River (NW Mexico) obtained over a two-year period. Higher metal concentrations were found in the upper basin. Higher salinity, total suspended solids and conductivity were found in the lower basin. Only As and Cu were occasionally found above published threshold effect levels (TEL). The proportion of deformities (% D) was low (2.1%) and a generalized linear model only explained 25.36% of % D variability among samples; this model included several physicochemical parameters. The only metal significantly related to deformities was Cu. Despite significant spatial and temporal variability in all metal concentrations and other physicochemical parameters in the basin, we were unable to explain a strong relationship between metal concentration and deformity incidence. Our results, based on consistent % D quantification from 5000 samples from a single taxon, suggest that natural variation in metal concentrations, even when statistically significant, is not associated with notable variations in the incidence of deformities.