RESEARCH ARTICLE Cycling of two carbon substrates of contrasting lability by heterotrophic biofilms across a nutrient gradient of headwater streams Chad S. Lane • David R. Lyon • Susan E. Ziegler Received: 17 May 2012 / Accepted: 19 June 2012 / Published online: 4 July 2012 Ó Springer Basel AG 2012 Abstract Anthropogenic impacts can significantly alter stream nutrient and dissolved organic carbon (DOC) delivery and composition. Nutrient and DOC cycling in headwater streams, however, are linked via a variety of complex feedbacks that are, in part, influenced by DOC composition emphasizing the need to investigate coupled nutrient–DOC interactions. This study assessed differential incorporation and mineralization of 13 C labeled glucose and vanillin by heterotrophic microbes within epilithic biofilm communities in four temperate headwater streams spanning a 100-fold range in total dissolved nitrogen and soluble reactive phosphorous concentrations. The sub- strates were traced via 13 C analyses of DOC, dissolved inorganic carbon, bulk biofilm, and individual biofilm phospholipid fatty acids (PLFA) to assess total incorpora- tion of the substrates and the distribution of substrate use within the heterotrophic community. Results indicate greater nutrient uptake by high nutrient streams with glu- cose additions relative to vanillin additions and support the hypothesis that nutrient retention in high nutrient streams is hampered by a lack of labile C sources. Vanillin-derived C uptake was only detectable in PLFA from the highest nutrient stream and was dominated by eukaryotic organisms, likely including fungi. This suggests biofilms in high nutrient streams are better adapted to access relatively slow turnover substrates perhaps due to their composition and overall structure. PLFA-based glucose use efficiencies were greatest in the lowest nutrient stream supporting the hypothesis that labile DOC sources are used more effi- ciently by heterotrophs in less impacted streams, while biofilms of high nutrient streams are better adapted to utilizing a wider array of DOC sources. This adaption is likely a result of exposure to the lower quality DOC pools in high-nutrient streams resulting from high DOC uptake supported, in part, by fast turnover autochthonous sources of DOC. Nutrient retention in nutrient-rich streams, how- ever, is still likely limited by readily bioavailable DOC leading to lower nutrient retention and downstream nutrient enrichment. Keywords Stream biofilms  Carbon cycling  Mesocosm  Phospholipid fatty acids  Glucose  Vanillin Introduction A large proportion (1.9 Pg C year -1 ) of terrestrially derived carbon (C) enters the aquatic environment via inland waters, but only about half of this C (0.9 Pg C year -1 ) ever reaches the marine environment (Tranvik et al. 2009). Under the framework of the River Continuum Concept (Vannote et al. 1980), much hetero- trophic processing of terrestrially derived carbon occurs in headwater stream biofilms (Hamilton et al. 2001; Sabater et al. 2002; Dodds and Oakes 2008). However, the quantity and quality of DOC available to heterotrophic microbes can vary widely (Volk et al. 1997) and significantly affect C. S. Lane  S. E. Ziegler Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, NL A1B 3X5, Canada C. S. Lane (&) Department of Geography and Geology, University of North Carolina Wilmington, Wilmington, NC 28403, USA e-mail: lanec@uncw.edu D. R. Lyon Environmental Dynamics Program, University of Arkansas, Fayetteville, AR 72701, USA Aquat Sci (2013) 75:235–250 DOI 10.1007/s00027-012-0269-0 Aquatic Sciences 123