Accounting for temporal changes in carbon dioxide (CO2) effluxes from freshwaters remains a challenge for global and regional carbon budgets. Here, we synthesize 171 site-months of flux measurements of CO2based on the eddy covariance method from 13 lakes and reservoirs in the Northern Hemisphere, and quantify dynamics at multiple temporal scales. We found pronounced sub-annual variability in CO2flux at all sites. By accounting for diel variation, only 11% of site-months were net daily sinks of CO2. Annual CO2emissions had an average of 25% (range 3%–58%) interannual variation. Similar to studies on streams, nighttime emissions regularly exceeded daytime emissions. Biophysical regulations of CO2flux variability were delineated through mutual information analysis. Sample analysis of CO2fluxes indicate the importance of continuous measurements. Better characterization of short- and long-term variability is necessary to understand and improve detection of temporal changes of CO2fluxes in response to natural and anthropogenic drivers. Our results indicate that existing global lake carbon budgets relying primarily on daytime measurements yield underestimates of net emissions.
Emission of CO2from tropical peatlands is an important component of the global carbon budget. Over days to months, these fluxes are largely controlled by water table depth. However, the diurnal cycle is less well understood, in part, because most measurements have been collected daily at midday. We used an automated chamber system to make hourly measurements of peat surface CO2emissions from chambers root‐cut to 30 cm. We then used these data to disentangle the relationship between temperature, water table and heterotrophic respiration (Rhet). We made two central observations. First, we found strong diurnal cycles in CO2flux and near‐surface peat temperature (<10 cm depth), both peaking at midday. The magnitude of diurnal oscillations was strongly influenced by shading and water table depth, highlighting the limitations of relying on daytime measurements and/or a single correction factor to remove daytime bias in flux measurements. Second, we found mean daily Rhethad a strong linear relationship to the depth of the water table, and under flooded conditions, Rhetwas small and constant. We used this relationship between Rhetand water table depth to estimate carbon export from both Rhetand dissolved organic carbon over the course of a year based on water table records. Rhetdominates annual carbon export, demonstrating the potential for peatland drainage to increase regional CO2emissions. Finally, we discuss an apparent incompatibility between hourly and daily average observations of CO2flux, water table and temperature: water table and daily average flux data suggest that CO2is produced across the entire unsaturated peat profile, whereas temperature and hourly flux data appear to suggest that CO2fluxes are controlled by very near surface peat. We explore how temperature‐, moisture‐ and gas transport‐related mechanisms could cause mean CO2emissions to increase linearly with water table depth and also have a large diurnal cycle.
more » « less- Award ID(s):
- 1923491
- PAR ID:
- 10450063
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Global Change Biology
- Volume:
- 25
- Issue:
- 9
- ISSN:
- 1354-1013
- Page Range / eLocation ID:
- p. 2885-2899
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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