Long‐Term Trends in pCO2 in Lake Surface Water Following Rebrowning

Abstract Over the last 40 years, dissolved organic carbon (DOC) has been increasing in freshwater across many boreal regions. The extent to which these long‐term changes have affected lake CO 2 dynamics is unclear. We have studied the temporal trends in DOC and pressure of CO 2 (pCO 2 ) over 40 years in 15 lakes in two regions of Ontario (Canada) subjected to browning and the abatement of SO 2 deposition. Whereas from 1980 to 1999 there were no significant trends in pCO 2 in any of the lakes, a significant increase was observed from 2000 to 2017. Although DOC concentrations increased during the same period, pCO 2 and DOC were only weakly coupled, and we hypothesize the existence of DOC thresholds that determine this coupling. The recent increases in pCO 2 , linked to shifts in the C balance of lakes, may be contributing to the observed declines in lake pH, leading to a re‐acidification of lakes. Plain Language Summary Over the past decades, terrestrial dissolved organic carbon (DOC) inputs to lakes have been increasing in boreal and northern ecosystems, raising questions about changes in the carbon dynamics of inland waters, especially carbon dioxide (CO 2 ) production. Here we present long‐term trends in pressure of CO 2 (pCO 2 ) concurrent with the continuous increase in DOC across 15 lakes in Ontario (Canada). We show that from 1980 to 1999, there were no or few changes in pCO 2 trends, but that net increases have been observed from 2000 to 2017 in all the studied lakes. As pCO 2 and DOC long‐term trends are weakly coupled, we hypothesized the existence of a DOC threshold needed to stimulate a pCO 2 response. We further propose that the contemporary increase in pCO 2 has the potential to regulate pH, leading to a re‐acidification of these lakes. Key Points Pressure of CO 2 (pCO 2 ) remained rather constant in lake surface water in the 80s, but started to increase in the past two decades Long‐term trends in pCO 2 and dissolved organic carbon (DOC) are weakly coupled, despite a constant increase in DOC Recent changes in pCO 2 and DOC may lead to a re‐acidification of inland waters and changes in C balance between burial and emissions