Freshwater systems are an important component on global carbon and nutrient cycles; having a disproportionally large impact given their small global surface area (Davidson et al., 2015). Generally, inland aquatic systems are considered to be carbon sources on an annual basis, however, their output can vary significantly (Tangen et al., 2016). The size of water bodies can greatly effect greenhouse gas fluxes, as smaller water bodies exhibit more intense changes and higher variability; which in turn alters the potential greenhouse gas emissions. Climate models predict an intensification in the frequency and intensity of extreme weather events, such as heat waves, which will have a burdening impact on the ecosystems. As the rate of summer heat waves is likely to increase, Bartosiewicz et al. look to answer what the effect will be on greenhouse gas fluxes in a lake during heat waves.
This study was conducted on Lake Jacques, Quebec, a shallow lake with an average depth of only 0.75 meters. They analyzed emissions from the lake over two summers in 2011 and 2012, one which had much hotter, drier, heat wave like conditions (2012), while the other was wetter and closer to average climate conditions for the area (2011). Greenhouse gas emissions were measured as a total global warming potential (GWP), the sum of CH4, N2O and CO2 in CO2 equivalents, corrected for the individual global warming potential of each gas (Bartosiewicz et al., 2016). These gases enter the atmosphere from the lake both by diffusion and ebullition, and by measuring both fluxes Bartosiewicz et al. were able to see changes in source as well as magnitude of greenhouse gas produced by the lake. Measurements were made using the headspace technique through equilibration of 2L of water for the diffusion, and using submersible funnels to collect gas bubbles for measuring ebullition (Bartosiewicz et al., 2016). Several other variables were measured to help correlate any changes in emissions, including measures of biological productivity, temperature, rainfall etc.
The findings were that CO2 and CH4 fluxes was significantly higher in 2011 than in 2012, and N2O showing no significant change, with the average totals of GWP being 189 mmol C m-2 day-1 in 2011 and 107 mmol C m-2 day-1 in a 2012. These results show a decrease in greenhouse gas flux during the hotter summer, or increased climate change conditions which is contrary to what was predicted. The likely causes of the lower greenhouse gas flux during the heat wave summer is due to the increased intensity and length of stratification within the lake. During the hotter summer there was a greater temperature difference, causing the hypolimnion to be cooler and slow down microbial activity; which was responsible for much of the CH4 production. The 2012 summer also experience 3 times the phytoplankton biomass, which likely influenced the gas fluxes (Bartosiewicz et al., 2016). The authors note that the research would have benefited from more continuous measurements, given the highly variable conditions of shallow water bodies.
These findings exemplify the variability of shallow freshwater systems, in that greenhouse gas production is greatly effected by a number of factors. Temperature, stratification and biological productivity all had some effect on the flux of greenhouse gases. Therefore, it is important to understand how changes in climate conditions will have an effect on small freshwater bodies to better model climate change and develop beneficial policies and strategies for the future.