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.