Biofilm-specific uptake does not explain differences in whole-stream DOC tracer uptake between a forest and an agricultural stream

Benthic biofilmsare often assumed to control terrestrially-derived dissolved organic carbon(tDOC) uptake in streams. We tested this by comparing 13C-enriched ryegrass leachate uptake in an agricultural and a forest stream, hypothesizing that a greater abundance of autotrophic biofilmsin the agricultural streamwould cause its whole- streamtDOC uptake to be comparatively low. We measured whole- streamand biofilmtDOC tracer uptake, metabolism, bacterial and algal diversity, and nutrient status of benthic epilithic biofilms, and assessed whole- streamhydromorphology. Whole- streamuptake of tDOC was six times lower in the agricultural (3.0 mg m−2 day−1) than in the forest (19.0 mg m−2 day−1) stream, and tDOC uptake velocity indicated lower tDOC demand in the agricultural (1.2 mm min−1) than in the forest (1.9 mm min−1) stream. The agricultural streamdiffered from the forest streamby slightly lower transient storage capacity and higher benthic biofilmbacterial abundance and production, lower biofilmbiomass and lower biofilmmolar C:N, C:P, and N:P ratios. Changes in epilithic biofilmscontributed little to the differences in whole- streamtDOC tracer uptake between streams, as biofilmtDOC uptake only amounted to 4% and 13% of whole- streamuptake in the forest and agricultural stream, respectively. This comparison of a forest and an agricultural streamsuggests that agricultural stressors have the potential to diminish both whole- streamtDOC uptake and uptake efficiency. Furthermore, the weak link between biofilmand whole- streamtDOC uptake implies that benthic biofilmscharacteristics are poor predictors for human impacts on tDOC uptake in agricultural streamsand that hot spots of tDOC uptake are likely situated in the hyporheic zoneor in the streamwater column.