Carbon dioxide efflux and ecosystem metabolism of small forest lakes

Abstract

Small lakes are numerous in the landscape and closely connected to the terrestrial environment, which strongly influences the system scale carbon cycling. However, despite their importance in large-scale carbon budgets, small lakes remain understudied compared to large lakes. We investigated oxygen and carbon dynamics in four small Danish forest lakes over a year. Continuous pH measurements were used to establish a high-frequency time series of CO₂ partial pressure and 1169 direct measurements of air–water CO₂ flux were performed using floating chambers. Net ecosystem oxygen production (NEP) was derived from free-water oxygen measurements in order to quantify the contribution of in-lake aerobic metabolism to the air–water CO₂ flux. We found that the forest lakes, on average, were tenfold CO₂ supersaturated. The two most intensively studied lakes had mean CO₂ effluxes of 36.3 mol m⁻² year⁻¹. The CO₂ effluxes exceeded NEP during all months implying that the CO₂ generated by aerobic respiration alone could not account for the observed CO₂ efflux. The observed discrepancy is likely promoted by a hydrologic CO₂ input and/or anaerobic sediment processes generating CO₂ without a concurrent consumption of O₂. A broad-scale national analysis showed that the CO₂ efflux increased as lake size decreased despite lower gas exchange velocity. Small lakes also showed higher excess CO₂ efflux compared to O₂ influx. Overall, small lakes and forest lakes in particular have high CO₂ effluxes and high CO₂ supersaturation. However, the contribution of hydrological inputs and anaerobic sediment processes to the net CO₂ efflux remains elusive.