Carbon dioxide efflux and ecosystem metabolism of small forest lakes

• Published in: Aquatic sciences 2020, Vol.82 (1), p.1-17, Article 9
• Main Authors: Martinsen, Kenneth Thorø, Kragh, Theis, Sand-Jensen, Kaj
• Format: Article
• Language: English
• Subjects: Aerobic respiration; Anaerobic processes; Biomedical and Life Sciences; Carbon cycle; Carbon dioxide; Ecology; Efflux; Forests; Freshwater & Marine Ecology; Gas exchange; Hydrology; Lakes; Life Sciences; Marine & Freshwater Sciences; Metabolism; Oceanography; Oxygen; Oxygen production; Partial pressure; Research Article; Sediment; Supersaturation; Terrestrial environments
• ISSN: 1015-1621; 1420-9055
• DOI: 10.1007/s00027-019-0682-8

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 CO 2 partial pressure and 1169 direct measurements of air–water CO 2 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 CO 2 flux. We found that the forest lakes, on average, were tenfold CO 2 supersaturated. The two most intensively studied lakes had mean CO 2 effluxes of 36.3 mol m −2  year −1 . The CO 2 effluxes exceeded NEP during all months implying that the CO 2 generated by aerobic respiration alone could not account for the observed CO 2 efflux. The observed discrepancy is likely promoted by a hydrologic CO 2 input and/or anaerobic sediment processes generating CO 2 without a concurrent consumption of O 2 . A broad-scale national analysis showed that the CO 2 efflux increased as lake size decreased despite lower gas exchange velocity. Small lakes also showed higher excess CO 2 efflux compared to O 2 influx. Overall, small lakes and forest lakes in particular have high CO 2 effluxes and high CO 2 supersaturation. However, the contribution of hydrological inputs and anaerobic sediment processes to the net CO 2 efflux remains elusive.