Late summer peak in pCO₂ corresponds with catchment export of DOC in a temperate, humic lake

Humic lakes play a key role in the processing of organic carbon (OC) mobilised from their catchments, but knowledge of OC dynamics in lakes within maritime temperate climates is limited. Climate exerts a significant influence on mechanisms of OC capture, storage, and processing on the wet and cloudy west coast of Ireland. We examined a high-frequency dataset of partial pressure of CO₂ (pCO₂) in the surface waters of Lough Feeagh collected over 1 year. The annual pattern in pCO₂ ranged between 491 and 1169 µatm and was strongly related to allochthonous riverine OC inputs. In contrast to observations in colder climates, a single peak in pCO₂ occurred in Lough Feeagh in early September. Generalised additive mixed modelling revealed that 2 variables, inflow water colour concentration (a reliable proxy for DOC concentrations) and lake Schmidt stability, together explained 68% of pCO₂ variability. Both the statistical analysis and timing of the peaks in inflow DOC and pCO₂ strongly suggested that catchment carbon export drove pCO₂ supersaturation in the lake, and hence CO₂ emissions. We estimated that between 217 and 370 t CO₂-C (0.55–0.94 t/ha) was emitted during the study period. These results highlight the interplay between catchment OC fluxes and climate in determining pCO₂ dynamics in maritime temperate lakes.