Linking dissolved carbon dioxide to dissolved organic matter quality in streams

Streams are typically supersaturated with carbon dioxide (CO2) thus playing a pivotal role in the global carbon cycle. To further our knowledge on stream CO2 dynamics, we tested the linkages between dissolved CO2 and dissolved organic matter quality during two time periods (spring and summer). We sampled 48 streams in Ontario, Canada that represent a range of catchment cropland cover (2–80 %), nutrients, dissolved organic carbon concentration (DOC), and dissolved organic matter (DOM) quality. For both sampling periods, DOM composition (quality) was found to be an important predictor of CO2 concentration in streams, more so than DOC concentration. In general, the more complex terrestrially-derived DOM was associated with elevated CO2 levels whereas microbial-like DOM had a poor relationship with CO2. The DOM characteristics associated with increased CO2 levels were related to wetlands rather than croplands. Nutrients (nitrogen and phosphorus) were also important predictors of CO2, however this relationship was not linear. Since the streams in the sampled network were of mixed land use, it appears that CO2 levels were affected by both land use types in a composite non-linear way. Overall, our findings suggest that DOM quality is a better predictor of stream CO2, however the relationship between DOM and CO2 is complicated by the presence of breakpoints in the data associated with non-linearity.