Sensitivity to acidic pH in benthic invertebrate assemblages with different histories of exposure to metals

A number of studies have documented tolerance in aquatic communities exposed to contaminants. but few have examined costs of tolerance and potential community-level consequences. We assessed the effects of metals and acidic pH, a novel stressor, on communities from streams with different histories of metal pollution. Acidic pH was a novel stressor to communities in streams that are consistently circumneutral. Intact benthic invertebrate assemblages collected from metal-polluted sites on the Arkansas River (AR1, low levels of pollution and AR5, greater levels of pollution) and a reference site on the Cache la Poudre River (PR Colorado, were exposed to either metals (Zn, Cu, Cd) or pH 4.5 in stream microcosms. Multivariate analyses on a subset of taxa showed different patterns of response to metals and acidic FH among the 3 sites, which corresponded to exposure histories of the communities: PR assemblages were more sensitive to metals, whereas AR assemblages were more sensitive to acidic pH. These patterns were supported by analyses on specific characteristics of community structure. Exposure to metals significantly reduced abundance of mayfly taxa in PR (p = 0.0041, ANOVA, linear contrast), AR1 (p = 0.0108) and AR5 (p = 0.0329) assemblages and taxa abundance in PR (p = 0.0225) and AR1 (p = 0.0469) assemblages. Total invertebrate abundance also decreased in PR assemblages exposed to metals (p = 0.0274). The results suggested that greater metal tolerance within the Arkansas River communities was a result of assemblage-level differences in community structure and population-level differences in sensitivity among sites. in contrast, Arkansas River assemblages, especially those from AR5, were sensitive to acidic pH, which reduced invertebrate abundance in PR (p = 0.0339), AR1 (p = 0.0284), and AR5 (p = 0.0062) assemblages. Abundances of mayflies and mayfly taxa also were significantly lower in AR1 (p = 0.0247, p = 0.0042, respectively) and AR5 (p < 0.0001, p = 0.0006, respectively) assemblages exposed to acidic pH but were not significantly altered in PR assemblages. These differences among sites in response to acidic pH resulted primarily from differences in community composition. For instance, the AR5 mayfly assemblage was dominated by Baetis spp., which were eliminated from all treatment streams. In contrast, less sensitive mayfly tara (Rhithrogena hageni and Ephemerella infrequens) were abundant at stations AR1 and PR. Our results suggested that chronic metal pollution may have produced communities tolerant of metals but more sensitive to acidic pH.