Carbon release and cycling within epilithic biofilms in two contrasting headwater streams

摘要

This study investigates the release of dissolved organic carbon (DOC) from epilithic biofilms and how the exchange of C between epilithic algae and bacteria varies between 2 contrasting headwater streams, During 3 seasons, light and dark enclosures were incubated in situ with rock substrate and stream water amended with C-13-labeled bicarbonate to follow newly fixed carbon into DOC, epilithic biofilm, and biofilm phospholipid fatty acids (PLFAs). Net release of DOC only occurred in the light enclosures and was significantly affected by net primary production (NPP). However, the net release of DOC as a proportion of NPP increased with light availability and decreased with dissolved inorganic nitrogen in Huey Hollow, the nutrient-deplete stream, and decreased with dissolved inorganic phosphorus (DIP) in Moore Creek, the nutrient-replete stream. Newly-fixed C, released as DOC, decreased with DIP but represented a greater proportion of net DOC release in Moore Creek (6 to 15 %) relative to that in Huey Hollow (0.7 to 2 %). Congruently, the quantity of newly-fixed C retained in the bulk biofilm and the bacterial PLFA ia15:0 increased together with DIP. Further, the proportion of NPP incorporated into bacterial PLFA in Huey Hollow was 4 times that in the Moore Creek biofilms, suggesting that bacteria accessed relatively more NPP in the low nutrient stream biofilms. Our data indicate that C-cycling differed between the contrasting streams in 2 significant ways. A larger proportion of DOC released from biofilms in the nutrient-rich stream was derived from new photosynthates, reducing the availability of this source of NPP to biofilm bacteria. Concurrently, a greater proportion of NPP was retained in heterotrophic bacterial PLFAs in the more nutrient-deplete stream biofilms. These results suggest that nutrient levels can greatly affect the source and processing of DOC, and emphasize the need to understand the mechanisms responsible for varied DOC cycling and transport in streams.