No long‐term trends in pCO₂ despite increasing organic carbon concentrations in boreal lakes, streams, and rivers

Abstract

Concentrations of dissolved organic carbon (DOC) from terrestrial sources have been increasing in freshwaters across large parts of the boreal region. According to results from large‐scale field and detailed laboratory studies, such a DOC increase could potentially stimulate carbon dioxide (CO2) production, subsequently increasing the partial pressure of CO2 (pCO2) in freshwaters. However, the response of pCO2 to the presently observed long‐term increase in DOC in freshwaters is still unknown. Here we tested whether the commonly found spatial DOC‐pCO2 relationship is also valid on a temporal scale. Analyzing time series of water chemical data from 71 lakes, 30 streams, and 4 river mouths distributed across all of Sweden over a 17 year period, we observed significant DOC concentration increases in 39 lakes, 15 streams, and 4 river mouths. Significant pCO2 increases were, however, only observed in six of these 58 waters, indicating that long‐term DOC increases in Swedish waters are disconnected from temporal pCO2 trends. We suggest that the uncoupling of trends in DOC concentration and pCO2 are a result of increased surface water runoff. When surface water runoff increases, there is likely less CO2 relative to DOC imported from soils into waters due to a changed balance between surface and groundwater flow. Additionally, increased surface water runoff causes faster water flushing through the landscape giving less time for in situ CO2 production in freshwaters. We conclude that pCO2 is presently not following DOC concentration trends, which has important implications for modeling future CO2 emissions from boreal waters.

CO2 evasion from boreal lakes: Revised estimate, drivers of spatial variability, and future projections

“…Perhaps, the most interesting result is the strong negative control of P on p CO 2 . Although, previous large‐scale studies have found a positive relationship between P and open water p CO 2 (Rantakari & Kortelainen, ; Sobek et al., ), a recent temporal study over a 17‐year period found a negative or no relation at all between precipitation and p CO 2 in boreal inland waters (Nydahl, Wallin, & Weyhenmeyer, ). Nydahl et al.…”

“…Nydahl et al. () suggested that increased precipitation results in a dilution of CO 2 concentrations in inland waters due to an altered balance between surface and CO 2 ‐rich groundwater flow. In addition, P induced increased surface water runoff can cause a faster water flushing through the landscape giving less time for in situ CO 2 production in inland waters.…”

“…Perhaps, the most interesting result is the strong negative control of P on pCO 2 . Although, found a negative or no relation at all between precipitation and pCO 2 in boreal inland waters (Nydahl, Wallin, & Weyhenmeyer, 2017). Nydahl et al (2017) suggested that increased precipitation results in a dilution of CO 2 concentrations in inland waters due to an altered balance between surface and CO 2 -rich groundwater flow.…”

Where does the river end? Drivers of spatiotemporal variability in CO2 concentration and flux in the inflow area of a large boreal lake

“…After river discharge had peaked in April and May, both CO 2 and DIC concentrations at the river mouth showed a considerable decrease of ~ 50%, compared to the values obtained in March, which was probably related to a dilution effect during snowmelt and to decreased DIC export from soils (Kokic et al ). DOC concentrations at the river mouth, in contrast, did not differ between seasons, which might be explained by the fact that DIC and DOC can be exported from different soil horizons during varying runoff conditions (Giesler et al ; Nydahl et al ). However, despite the difference in DOC and DIC changes, both their spatial gradients reveal the path of river intrusion up to 15 km past the bay area in early June.…”

Large Uncertainty in Estimating pCO2 From Carbonate Equilibria in Lakes

“…Limited direct p CO 2 observations combined with a growing interest in assessing a role of freshwaters in regional and global scales have resulted in studies relying on routinely measured CO 2 ‐related parameters to estimate p CO 2 and carbon flux from inland waters at global (Aufdenkampe et al, ; Cole et al, ; Cole et al, ; Raymond et al, ; Tranvik et al, ) and regional scales (Buffam et al, ; Butman & Raymond, ; Lapierre et al, ; McDonald et al, ) and evaluating temporal trends of inorganic carbon species (Jones et al, ; Nydahl, Wallin, & Weyhenmeyer, ; Seekell & Gudasz, ). We demonstrate, however, that field measurements of pH, ALK, and DIC might be insufficient to provide robust estimates of mean p CO 2 and question if they are sensitive enough to detect long‐term change in chemically heterogeneous lakes (Figures and and Table ).…”