Examination of effects of acid and metals on benthic algae in streams using chemical diffusing substrata

Acid mine drainage environments have multiple stressors, making it difficult to isolate effects of individual contaminants on benthic algal communities in situ in order to develop more specific indicators. Additionally, responses to AMD contaminants may be dependent on the abiotic and biotic conditions within the affected stream. We examined the efficacy of using chemical diffusing substrata (CDS) to determine how acid and metals interact to affect benthic algae in 2 streams with different trophy. CDS containing Mn, Fe, Al, acid, Mn + acid, Fe + acid, and Al + acid were deployed in a flow-through system to determine flux from the liquid media. Flux values for metals ranged from 1.0 to 18.1 µg/cm2/d for 5 days, and 0.5–7.4 µg/cm2/d for 21 days. Flux values for H+ were 467 (5 day) and 212 (21 day) µg/cm2/d. Corresponding water column concentrations that would produce the same flux as CDS metal treatments were estimated to be in the low to mid toxicity range for Al, low range for Fe, and nontoxic range for Mn. Acid treatments were estimated to be equivalent to a stream pH of 2.1–2.5. Five replicates of each CDS treatment, and a water control, were placed into a high-alkalinity, eutrophic stream (Wolf Creek) for 5 days and a low-alkalinity, oligotrophic stream (Silver Creek) for 21 days, respectively. In Silver Creek, all treatments except circumneutral Fe and circumneutral Mn had significantly lower algal cell densities than control treatments. Cell densities were significantly lower on Al + acid and Mn + acid than on the acid-only treatment, indicating synergistic effects. Cell densities were not significantly different among treatments in Wolf Creek. Higher alkalinity, nutrients, and productivity in this stream likely increased neutralization and detoxification processes within the biofilm. However, significantly lower chlorophyll a and higher phaeophytin concentrations of communities on treatments containing acid indicated sublethal effects. Significantly more acid tolerant taxa occurred on treatments with acid in Silver Creek, and significantly more metal-tolerant taxa occurred on metal treatments in Wolf Creek. However, treatment effects on taxonomic composition appeared to be constrained by the species pool within each stream, as these indicator taxa made up less than 4% of any community. Although equating CDS flux of contaminants to water column concentrations requires assumptions, and flux decreases over the exposure period, this study indicates that in situ experiments using CDS can separate out effects of individual toxic components on benthic algae, and determine context-dependent responses.