Abstract
AbstractLakes are generally supersaturated in carbon dioxide (CO2) and emitters of CO2 to the atmosphere. However, estimates of CO2 flux () from lakes are seldom based on direct flux measurements and usually do not account for nighttime emissions, yielding risk of biased assessments. Here, we present direct measurements from automated floating chambers collected every 2–3 hr and spanning 115 24 hr periods in three boreal lakes during summer stratification and before and after autumn mixing in the most eutrophic lake of these. We observed 40%–67% higher mean in daytime during periods of surface water CO2 supersaturation in all lakes. Day‐night differences in wind speed were correlated with the day‐night differences in the two larger lakes, but in the smallest and most wind‐sheltered lake peaks of coincided with low‐winds at night. During stratification in the eutrophic lake, CO2 was near equilibrium and diel variability of insignificant, but after autumn mixing was high with distinct diel variability making this lake a net CO2 source on an annual basis. We found that extrapolating daytime measurements to 24 hr periods overestimated by up to 30%, whereas extrapolating measurements from the stratified period to annual rates in the eutrophic lake underestimated by 86%. This shows the importance of accounting for diel and seasonal variability in lake CO2 emission estimates.
Highlights
Lakes have been estimated to emit 0.32–0.64 Pg C year−1 as CO2 to the atmosphere (Aufdenkampe et al, 2011; Cole et al, 2007; Holgerson & Raymond, 2016; Raymond et al, 2013; Tranvik et al, 2009)
Autumn mixing occurred in VEN from September 11 to 13, resulting in an increaEse in FCO2 (Figure 2a), whereas PRS and LJU were partly stratified for the full sampling period and are regarded as stratified lakes
All lakes were net sources of CO2 to the atmosphere during the sampling campaigns based on the areal cumulative flux (Figure 3a), and mean (±1 SD) E24 hr FCO2 was estimated as 5.3 ± 17.1, 76 ± 26.2, 24.3 ± 11.4, and 9.9 ± 3.2 mmol m−2 d−1 in VENbef, VENaft, PRS, and LJU, respectively
Summary
Lakes have been estimated to emit 0.32–0.64 Pg C year−1 as CO2 to the atmosphere (Aufdenkampe et al, 2011; Cole et al, 2007; Holgerson & Raymond, 2016; Raymond et al, 2013; Tranvik et al, 2009). This is of the same order of magnitude as the CO2 emissions from land use change, or the carbon transport from continents to the ocean (Ciais et al, 2013), making CO2 emissions from lakes important in the global carbon cycle.
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