Carbon cycling within epilithic biofilm communities across a nutrient gradient of headwater streams

Primary production (PP), nutrient uptake, and carbon (C) cycling within epilithic biofilm communities were assessed in four headwater streams in the Arkansas Ozarks, U. S. A., representing a hundred-fold range in total dissolved nitrogen (TDN) and soluble reactive phosphorus concentrations. Small enclosures containing stream water and rock substrate were incubated for 8 h with C-13-enriched dissolved inorganic C. C-13 was measured in dissolved organic carbon (DOC), bulk biofilm, and individual biofilm phospholipid fatty acids (PLFA) as biomarkers for C uptake into autotrophic and heterotrophic microorganisms. Biofilm %N, chlorophyll content, C-specific biofilm PP, N-specific TDN uptake, and relative abundance of algal PLFA increased with the nutrient gradient represented by the four study streams. Total net DOC release as a proportion of PP was greatest in both the lowest and highest nutrient streams, while the source of released DOC varied with the nutrient gradient. In the most nutrient-rich stream a greater fraction of DOC was comprised of C fixed during the incubation, but in the most nutrient-poor stream released DOC was mainly derived from older biofilm C. Carbon uptake into PLFA increased with the nutrient gradient for algal but not heterotrophic bacterial biomarkers, indicating lower relative incorporation of new PP by biofilm heterotrophs in higher nutrient streams. Results suggest that tight C flow between biofilm autotrophs and heterotrophs may support biofilm nutrient retention in low-nutrient streams while nutrient enrichment may disrupt this coupling and release more new PP as DOC with potential effects on nutrient export to downstream ecosystems.