Stream metabolism sources a large fraction of carbon dioxide to the atmosphere in two hydrologically contrasting headwater streams

Headwater streams are control points for carbon dioxide (CO${}_2$) emissions to the atmosphere, with relative contributions to CO${}_2$ emission fluxes from lateral groundwater inputs widely assumed to overwhelm those from in-stream metabolic processes. We analyzed continuous measurements of stream dissolved CO${}_2$ and oxygen (CO${}_2$) concentrations during spring and early summer in two Mediterranean headwater streams from which we evaluated the contribution of in-stream net ecosystem production (NEP) to CO${}_2$ emission. The two streams exhibited contrasting hydrological regimes: one was non-perennial with relatively small groundwater inflows, while the other was perennial and received significant lateral groundwater inputs. The non-perennial stream exhibited strong inverse coupling between instantaneous and daily CO${}_2$ and CO${}_2$ concentrations, and a strong correlation between aerobic ecosystem respiration (ER) and gross primary production (GPP) despite persistent negative NEP. At the perennial stream, the CO${}_2$–O${}_2$ relationship varied largely over time, ER and GPP were uncorrelated, and NEP, which was consistently negative, increased with increasing temperature. ... mehrMean NEP contribution to CO${}_2$ emission was 51% and 57% at the non-perennial and perennial stream, respectively. Although these proportions varied with assumptions about metabolic stoichiometry and groundwater CO${}_2$ concentration, in-stream CO${}_2$ production consistently and substantially contributed to total atmospheric CO${}_2$ flux in both streams. We conclude that in-stream metabolism can be more important for driving C cycling in some headwater streams than previously assumed.