The effects of stream substratum texture on interstitial conditions and bacterial biofilms: Methodological strategies

Hyporheic substrates play a key role in aquatic ecosystems, and increasing loads of fine sediment are considered one of the major threats to stream ecosystems. Knowledge concerning the interaction of stream substratum properties with habitat quality and microbial community structure is essential for a comprehensive understanding of the functionality of the hyporheic zone. To date, there is a lack of optimal field sampling methods for hyporheic microbial communities in streams. We systematically tested the effects of defined substratum textures on the physicochemical properties of interstitial water and on bacterial communities utilising T-RFLP fingerprinting. We also tested the representativeness of different methodological approaches of investigating bacterial diversity comparing sampling of substratum, interstitial water, and exposed coupons made of granite, carbonate and glass. The temporal development of physicochemical habitat characteristics in the interstitial zone, especially of fish-toxic nitrogen compounds and oxygen supply, significantly depended on substratum texture and was strongly correlated with bacterial community composition. Multivariate analyses of bacterial community data indicated strong differences in community composition between direct (substratum samples) and indirect (interstitial water samples, coupons) sampling strategies. Substratum samples yielded highest richness of operational taxonomic units (OTUs) and the most pronounced temporal dynamics of bacterial community composition. Consequently, this technique appears most representative for assessing bacterial community structure and diversity in hyporheic habitats. The observed couplings between substratum texture, physicochemical habitat conditions and bacterial community structure expand current knowledge of previously described negative effects of fine sediments on taxa from higher levels of biological organization.