Whole‐stream 13 C tracer addition reveals distinct fates of newly fixed carbon

Many estimates of freshwater carbon (C) fluxes focus on inputs, processing, and storage of terrestrial C; yet inland waters have high rates of internally fixed (autochthonous) C production. Some fraction of newly fixed C may be released as biologically available, dissolved organic C (DOC) and stimulate microbial‐driven biogeochemical cycles soon after fixation, but the fate of autochthonous C is difficult to measure directly. Tracing newly fixed C can increase our understanding of fluxes and fate of autochthonous C in the context of freshwater food webs and C cycling. We traced autochthonous C fixation and fate using a dissolved inorganic C stable isotope addition ( 13 C DIC ). We added 13 C DIC to North Fork French Creek, Wyoming, USA during two days in August. We monitored changes in 13 C pools, fluxes, and storage for 44 d after the addition. Two‐compartment flux models were used to quantify net release of newly fixed 13 C DOC and 13 C DIC into the water column. We compared net 13 C fixation with tracer 13 C DIC removal and gross primary production (GPP) to account for the mass of tracer fixed, released, lost to the atmosphere, and exported downstream. Much of the fixed C turned over rapidly and did not enter longer‐term storage pools. Net C fixed was 70% of GPP measured with O 2 . Algae likely released the remaining 30% via 13 C DOC exudation and respiration of newly fixed C. Primary producers released 13 C DOC at rates of up to 16% per day during the 13 C addition, but exudation of new labile C declined to near zero by day 6. DIC production from newly fixed C accounted for 21% of ecosystem respiration the day after the 13 C addition. All measured organic C (OC) pools were enriched with 13 C 1 d after the tracer addition. 20% of fixed 13 C remained in benthic OC by day 44, and average residence time of autochthonous C in benthic OC was 62 d. Newly fixed C had two distinct fates: short‐term (<1 week) exudation and respiration or longer‐term storage and downstream export. Autochthonous C in streams likely fuels short‐term microbial production and biogeochemical cycling, in addition to providing a longer‐term resource for consumers.