Combining assessments of periphyton structure and function to detect subtle anthropogenic impacts to headwater streams in the upper Delaware Basin, USA.

Headwater streams are vital to the health and resiliency of entire watersheds; thus, understanding how human activities impair the structure and function of these vulnerable ecosystems is essential. We combined structural and functional measures of stream periphyton in 46 headwater streams throughout the upper Delaware River Basin, USA, to determine which measures were most helpful in detecting subtle anthropogenic impacts. Stepwise Redundancy analyses run independently with structural and functional variables identified local photosynthetically active radiation and total phosphorus as the most influential environmental variables. Algal biomass and productivity under different light and nutrient concentrations likely drove many subsequent patterns in periphyton structure and function. Generalized additive models indicated possible thresholds with increasing light for algal biomass, bacterial biomass, gross primary productivity, and patterns in carbon-acquiring enzymes. These thresholds were likely driven by algae shifting from light to phosphorus limitation and bacteria shifting from terrestrially derived recalcitrant dissolved organic substrates to more labile algal-derived substrates. Overall, structural and functional measures of stream periphyton were complementary for understanding the mechanisms underlying the impacts of specific stressors and together gave a more complete picture of ecological conditions within each stream and how human activities have influenced the health of these systems. [ABSTRACT FROM AUTHOR]