Factors regulating epilithic biofilm carbon cycling and release with nutrient enrichment in headwater streams

This study uses the results from in situ C-13-labeling experiments conducted in six streams representing a gradient in nutrient enrichment to explore how nutrient availability, stoichiometry, and the composition of active biofilm phototrophs may regulate C cycling in epilithic biofilms. Carbon cycling was tracked through epilithic biofilm communities by assessing net primary production (NPP) and C-13-labeling of biofilm phospholipids fatty acids (PLFA), and stream water dissolved organic carbon (DOC) within light and dark enclosure incubations. We used generalized linear models coupled with an information-theoretic approach for model selection to assess which factors most influenced C exchange within and DOC release from these biofilms. The ratio of new C incorporated into heterotrophic bacterial PLFA ia15:0 to total polyunsaturated fatty acids (PUFA) indicated that greatest algal-bacterial exchange occurred in the two most nutrient-poor streams. Further, this ratio was best predicted by newC(18:3 omega 3/PUFA) suggesting increased relative activity of some algae relates to reduced algal-bacterial C exchange within these biofilms. Net release of DOC represented 2-45% of NPP with greatest release of DOC having occurred in the two most nutrient-rich streams. Further, the model selection indicated that newC(18:3 omega 3/PLFA) was the only highly plausible explanatory factor for net DOC release, while a combination of NPP and newC(18:3 omega 3/PLFA) was a strong predictor of the quantity of new C released as DOC. The results presented here indicate factors regulating or correlating with the activity of green algae in these biofilms regulated the exchange of C within and DOC release from these biofilms. This suggests increased algal exudation and greater biofilm development with nutrient enrichment may increase DOC release but reduce bacterial use of autochthonous C within these biofilms.