Spatial resolution of methane production and ebullition in Lake Alsta

Sammanfattning: Freshwater ecosystems cover about 4 % of the Earth’s surface. Yet they are important components in the global C cycle, as they in addition to their within-lake primary production, receive and process organic matter loads originating from terrestrial environments. The boreal forest biome has the highest density of lakes globally. In general, lakes in the boreal forest biome are shallow with high proportions of littoral sediments and are considered methane hotspots in the landscape. The major pathway of methane (CH4) from lakes (depth ≤ 10 m) to the atmosphere is via CH4 ebullition (i.e., gas bubbles). Moreover, CH4 ebullition is highly irregular in space and time. While enhanced CH4 ebullition rates have been reported to coincide with temporal forces (e.g., pressure drop) and showed a spatial variability with higher CH4 ebullition rates at lake inlets, none of the present models can currently represent the variability of CH4 ebullition over space and time. To improve the understanding behind spatial drivers of CH4 ebullition, sediment characteristics in relation to CH4 ebullition were investigated in Lake Alsta, a shallow and eutrophic lake in Sweden. In-situ CH4 ebullition rates were analysed along with sediment TN, TOC, C:N ratio and potential CH4 production rates. Sediment TN could explain CH4 production (R20.39, p-value<0.001), while the degree of explanation of CH4 ebullition rate was low yet significant (R2=0.14, p-value=0.03). However, the combination of fine sediments, together with high loads of aquatic- and terrestrial organic matter and nitrogen are likely spatial factors driving the high CH4 ebullition rates at both inlets in Lake Alsta, Sweden. The CH4 ebullition at its deepest point cannot be distinguished from the ebullition rates at the inlets, and the outlet has a significantly lower CH4 ebullition rate than all other sites. This highlights that the distribution of sediment and the quality of organic matter (i.e., C:N ratio) within the lake affects CH4 ebullition. In addition, including littoral vegetation into CH4 ebullition models and analysing sediment redox potentials might further give explanation to spatial differences.