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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 (¹³C_DIC). We added ¹³C_DIC to North Fork French Creek, Wyoming, USA during two days in August. We monitored changes in ¹³C pools, fluxes, and storage for 44 d after the addition. Two‐compartment flux models were used to quantify net release of …