Controls on greenhouse gas concentrations in polymictic headwater lakes in Ireland

Freshwater lakes are known to release carbon dioxide (CO2) and methane (CH4) to the atmosphere; however, the importance of lakes in global nitrous oxide (N2O) budgets is not yet known. Further, despite the abundance of small lakes on the landscape, neither emissions of these gases nor their drivers are well described. Dissolved concentrations of CO2, CH4 and N2O greenhouse gases were related to water chemistry, hydrology and catchment characteristics in order to identify factors controlling gas concentrations for 121 small Irish headwater lakes (median area: 2.0 ha) in relatively undisturbed catchments; lake–atmosphere gas fluxes were also calculated. The majority of lakes were supersaturated (relative to the atmosphere) with CO2 and N2O while CH4 was above saturation in all lakes. Dissolved gas concentrations were correlated with land cover (rock, forest and grassland), deuterium excess (an indicator of hydrologic character) and lake organic carbon concentrations, although dissolved CO2 exhibited few significant relationships. Principal components analysis indicated that higher levels of CH4 and N2O supersaturation were exhibited under different conditions. Methane supersaturation was highest in low elevation catchments with an evaporative hydrologic character and high organic carbon concentrations. In contrast, lakes characteristic of N2O supersaturation were low in carbon and located in more rapidly flushed higher elevation catchments. Estimated fluxes of CO2, CH4 and N2O to the atmosphere averaged 14, 0.36 and 1.3 × 10− 3 mmol m− 2 d− 1, respectively.

► Greenhouse gas concentrations were measured in 121 undisturbed headwater lakes.
► CO2 and N2O were supersaturated in most lakes and CH4 supersaturation was universal.
► Low elevation, evaporatively enriched lakes had greatest potential for CH4 emission.
► Highest supersaturation of N2O was observed in rocky, throughflow catchments.