The effects of fluvially-deposited heavy metals on hyporheic zone microbial community structure

The impacts on streambed geochemistry resulting from mining activity are well{09} documented. Effects on local biota resulting from the altered geochemistry and increased metal concentrations have traditionally been determined by comparing changes in fish and invertebrate populations between contaminated and uncontaminated streams. Studies monitoring these two macrofaunal populations completely overlook the effects of increased metal loads on the local microbial communities. Increased metal concentrations in soil have produced negative effects on soil microbial communities, influencing both community structure and function. Microbial communities in streams are influenced by, and intricately involved with, the local geochemistry, and thus may be useful as indicators of heavy metal contamination in aquatic environments. A suite of molecular tools were applied (16S DNA phylogeny, denaturing gradient gel electrophoresis pattern analysis, and real-time quantitative PCR) to a combination of observational field studies, field experiments and laboratory experiments in order to understand how hyporheic microbial communities are affected by heavy metal contamination. These studies characterized the structure and seasonal dynamics of microbial communities inhabiting the 1.7--2.36 mm sediment fraction in pristine streams and along a heavy metal contamination gradient in western Montana. A causal link between fluvially deposited heavy metal contamination and changes in hyporheic microbial community structure was established. In addition we determined the level of contamination and rate at which these changes can be detected. A conceptual model based on these studies was developed to integrate the effects of increased metal loads on the microbial component of hyporheic ecosystems. This body of research is the first to describe the effects of fluvially deposited heavy metals on hyporheic microbial community structure. Potential implications of the approaches and conclusions presented here on the regulation of hard rock mining are discussed.