| Summary: | Abstract Dynamics of dissolved oxygen in aquatic ecosystems reflect the biological, physical, and chemical processes that regulate ecosystem metabolism. Organic matter that supports ecosystem respiration (ER) is produced both by in situ photosynthesis and via loading from terrestrial ecosystems. Terrestrial‐derived organic matter is relatively recalcitrant and its availability is stable at diel time scales relative to substrates produced through photosynthesis, which are more labile and often show distinct diel changes in availability. Here, we explored whether the contributions of these two sources of organic matter to ecosystem metabolism could be quantified by a process model of photosynthesis and ER fit to high‐frequency observations of oxygen concentration in streams. We found that a two‐stage model of respiration provided a better fit to diel oxygen dynamics in most streams than a model assuming that the substrates supporting ER were one temporally stable pool. Two‐stage models estimated peak daytime respiration rates that were ~2.9× higher on average than nighttime rates, but this increase was variable and ranged from 1.1 to 11.6×. Estimates of gross primary production were 1.35× higher, on average, (range = 1.04–7.91×) compared to estimates generated by a single‐stage model. Streams draining watersheds with less than about 7% gradient exhibited oxygen dynamics that provided comparable statistical support for single‐stage metabolism, likely due to the higher loading of allochthonous organic matter that swamped metabolism based on autochthonous production in these streams. Ecosystem metabolism in streams draining steeper watersheds was best characterized by two‐stage metabolism, reflecting the greater importance of autochthonous contributions to labile organic carbon pools in these ecosystems.
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