How Important is Sampling Frequency? Exploring Temporal Dynamics of GHG Emissions Using Low-Cost Sensors in Lake Mesocosms

Greenhouse gas (GHG) emissions from freshwater ecosystems contribute significantly to global carbon budgets. However, these emissions remain poorly constrained due to limited high-frequency measurements. In this study, we tested a low-cost, high-frequency GHG measurement system in a long-running mesocosm experiment in Lemming, Denmark, over a seven-month period, focusing on CO₂ and CH₄ fluxes. We propose a methodology for calculating diffusive CH₄ fluxes using high-frequency sensor data and tested the effects of sampling interval upscaling of emissions by conducting theoretical experiments. 

Our findings reveal substantial temporal variability in GHG emissions, particularly for CH₄, with ebullitive fluxes dominating with high daily variation. Pronounced diurnal fluctuations were observed for CO₂ and diffusive CH₄ fluxes, while ebullitive CH₄ emissions showed no clear diurnal pattern. Our results show that relying solely on daytime measurements leads to a significant underestimation of overall CO₂ fluxes, whereas daytime measurements of diffusive CH₄ fluxes significantly overestimate the total flux. 

The results of the theoretical sampling interval experiments emphasize that infrequent sampling can introduce substantial uncertainty and lead to an underestimation of total emissions, particularly for ebullitive CH₄ fluxes. Simulated experiments demonstrated that increasing the sampling interval from daily to monthly markedly increased uncertainty, whereas weekly sampling intervals better captured overall GHG flux patterns and reduced the uncertainty compared to less frequent sampling. 

Our results highlight the importance of high-frequency GHG measurements in capturing both diurnal and seasonal variations, improving the accuracy of flux estimates, and reducing uncertainties in upscaling emissions to broader scale. Therefore, we emphasize the critical importance of optimizing sampling intervals and incorporating diurnal cycle measurements to enhance the accuracy and reliability of upscaled GHG measurements. Further development and application of low-cost, high-frequency sensor systems are critical to enhance the temporal and spatial coverage of freshwater GHG emissions and support future research in mitigating climate impacts from freshwater ecosystems.