Greenhouse gas emissions from small streams in a lake-rich landscape: the importance of incorporating methane

Small streams may be important components of regional and global carbon budgets. Although they have small total surface area relative to terrestrial and lake ecosystems, emission rates of CO₂ and CH₄ are often significantly higher due to factors such as soil/groundwater connectivity and rapid gas exchange across high turbulence air-water interfaces. The goals of this study were to constrain regional stream gas flux estimates in the low relief, lake-rich landscape of the North Temperate Lakes (NTL) LTER and to evaluate dynamics associated with storms and diurnal cycles. We determined growing season CO₂ and CH₄ emissions from 36 sites across 11 small streams that varied in their connectivity with lakes and wetlands. Additionally, stream pCO₂ was monitored hourly with an in-situ pCO₂ sensor at one headwater stream. These data were complemented by a 25-hour continuous sampling event for pCO₂, pCH₄, and concurrent CO₂ flux measurements using the chamber technique.

We found that streams were always supersaturated in CH₄ with respect to the atmosphere (mean > 75 µatm, n=56). Stream CH₄ emissions during the study period were 0.20 g C m² d^-1. Assuming 200 days of open water, streams emit 40 g C m² yr^-1 as CH₄, which is 10x and 4x greater than CH₄ fluxes from lakes and wetlands in this region, respectively. As expected, streams were almost always supersaturated in CO₂ (mean >3500 µatm, n=66), resulting in a mean flux of 1400 g C m² yr^-1.  This flux is at least 4x higher than previous estimates. Diurnal patterns observed with the in-situ sensor and 25-hour sampling together suggest that daytime sampling (especially during baseflow conditions) may lead to an underestimate of fluxes for both CO₂ and CH₄. For example, our continuous pCO₂ record revealed high diurnal variability (Δ~1000ppm) and illustrates how storm events can increase stream pCO₂. Stream pCH₄ also responds to changes in discharge following storms, but unlike pCO₂, pCH₄ increased on the receding limb of a storm. High stream emissions were supported by both high gas concentrations as well as high gas transfer velocities (GTV) (mean= 4.8 m d^-1), which was highly correlated with stream discharge (r²= 0.81, p<0.05). Overall, these high concentrations and fluxes indicate that streams contribute significantly to the carbon budget of the region, especially due to high methane emissions.