The relative importance of seasonality versus regional and network-specific properties in determining the variability of fluvial CO2, CH4 and dissolved organic carbon across boreal Quebec

Streams and rivers are known to be significant processors and emitters of carbon (C) to the atmosphere. There have been many large-scale estimates of fluvial C emissions, yet many lack quantification of temporal variability. Here, we compared the relative importance of spatial versus seasonal variability of CO2 and CH4 concentrations and fluxes, and DOC concentration in 23 streams and rivers spanning seven Strahler stream orders in two regions with contrasting watershed features, geomorphology and climate in boreal Quebec. Much of the overall variance in pCO(2), pCH(4) and DOC concentration could be explained by region and site-specific properties rather than season. Region was the main driver of pCO(2), pCH(4), and DOC concentration followed by network-specific variability. pCO(2) had the strongest seasonal patterns, yet season accounted only for 16% of the total variation, whereas intrinsic network and landscape properties accounted for 60%. Seasonality explained even less of the overall variability for pCH(4) (5%) and DOC (6%) once the region and network features were accounted for. Despite significant spatial and temporal variation in pCO(2) and pCH(4) point fluxes were not significantly different between regions and seasons, because of a modulating interplay between concentration and gas exchange at the whole network scale. The declining trend of pCO(2) and pCH(4) with flow distance across all watersheds and seasons (even under ice and snow) suggests a predominance of network degassing independent of season, and together with the relative constancy of fluxes across regions and time further suggests that seasonality may be dampened when considering fluvial emissions at large spatial scales and especially when integrating these emissions at the whole network scale.

Automated summary

Streams and rivers play a significant role in emitting carbon (C) into the atmosphere, with region being the main driver of CO2 and CH4 concentrations and fluxes, and DOC concentration. While pCO(2) and pCH(4) show spatial and temporal variation, the overall fluxes remain relatively constant across regions and seasons due to the interplay between concentration and gas exchange at the network scale. This suggests that seasonality may be dampened when considering fluvial emissions at large spatial scales.