Hyporheic Microbial Community Development Is a Sensitive Indicator of Metal Contamination

Accurate natural resource damage assessment necessitates monitoring organisms or communities that respond most sensitively to contaminants. Observational studies have demonstrated a correlation between fluvial heavy metal deposition and hyporheic microbial community structure. To establish a causal relationship between sediment metal content and the structure of colonizing bacterial communities, we performed a controlled field experiment. River sediments of 1.75-2.36 mm in diameter with five different contaminant concentrations were collected from an environmental metal contamination gradient. Sediments were sterilized and then recolonized by incubation in the hyporheic zone of an uncontaminated river (i.e., a common garden experiment was performed). A significant correlation between hyporheic microbial community structure and heavy metal contamination (R-2 = 0.81) was observed. The abundance of two phylogenetic groups was highly correlated with the level of heavy metal contamination (Group I, R-2 = 0.96; Group III, R-2 = 0.96, most closely affiliated with the alpha- and gamma-proteobacteria, respectively). Microbial community structural responses were detected at metal concentratons an order of magnitude lower than those previously reported to impact benthic macroinvertebrate communities. We conclude that hyporheic microbial communities could offer the most sensitive method for assessing natural resource damage in lotic ecosystems in response to fluvial heavy metal deposition.