Heavy metals contaminate numerous freshwater streams and rivers worldwide. Previous work by this group demonstrated a relationship between the structure of hyporheic microbial communities and the fluvial deposition of heavy metals along a contamination gradient during the fall season. Seasonal variation has been documented in microbial communities in numerous terrestrial and aquatic environments, including the hyporheic zone. The current study was designed to assess whether relationships between hyporheic microbial community structure and heavy-metal contamination vary seasonally by monitoring community structure along a heavy-metal contamination gradient for more than a year. No relationship between total bacterial abundance and heavy metals was observed (R 2 ؍ 0.02, P ؍ 0.83). However, denaturing gradient gel electrophoresis pattern analysis indicated a strong and consistent linear relationship between the difference in microbial community composition (populations present) and the difference in the heavy metal content of hyporheic sediments throughout the year (R 2 ؍ 0.58, P < 0.001). Correlations between heavy-metal contamination and the abundance of four specific phylogenetic groups (most closely related to the ␣, , and ␥-proteobacteria and cyanobacteria) were apparent only during the fall and early winter, when the majority of organic matter is deposited into regional streams. These seasonal data suggest that the abundance of susceptible populations responds to heavy metals primarily during seasons when the potential for growth is highest.Large-scale mining and other activities have resulted in contamination of many aquatic environments around the world (50). Changes in the geochemical characteristics of heavy-metal-contaminated environments are well documented (for a review, see Moore and Luoma [50]). Heavy-metal contamination can reduce water quality and has been shown to harm numerous organisms (12,45,50,69). Several studies have examined the effects of this type of anthropogenic contamination on aquatic macrobiota (1, 12-15, 33, 47). While heavy-metal effects on the activity and composition of microbial communities in terrestrial ecosystems have been well documented (2,6,7,20,21,34,43,49,54,64,72), relatively little is known about the effects on aquatic microbial communities. In a prior study by our group, heavy-metal contamination was implicated as a structuring factor for hyporheic microbial communities in streambeds (23).The hyporheic zone is the region of heterogeneous sediments beneath and adjacent to the stream channel that is saturated with a mixture of surface and ground water (46), providing connectivity between terrestrial, groundwater, and lotic habitats. As such, this zone is an important component of lotic ecosystems (11,26,35,58,60,68,70,71). The microbial communities in the hyporheic zone play important functional roles in lotic environments (18,31,32,51,52,57,59). For example, transformation of dissolved and particulate nutrients by hyporheic microorganisms can influence the distr...