Martyn N. Futter

Lakes process large volumes of organic carbon (OC), are important sources of methane (CH 4 ), and contribute to climatic warming. However, there is a lack of data from large lakes >500 km 2 , which creates uncertainty in global budgets. In this data article, we present dissolved CH 4 , OC bioreactivity measurements, water chemistry, and algal biovolumes at 11 stations across Lake Mälaren, the third largest (1,074 km 2 ) Swedish lake. Total phosphorus concentrations show that during the study period the lake was classed as mesotrophic/eutrophic. Overall mean CH 4 concentration from all stations, sampled five times to cover seasonal variation, was 2.51 μg l −1 (0.98–5.39 μg l −1 ). There was no significant seasonal variation although ranges were greatest during summer. Concentrations of CH 4 were greatest in shallow waters close to anthropogenic nutrient sources, whilst deeper, central basins had lower concentrations. Methane correlated positively with measures of lake productivity (chlorophyll a , total phosphorus), and negatively to water depth and oxygen concentration, with oxygen emerging as the sole significant driver in a linear mixed effects model. We collated data from other lakes >500 km 2 ( n = 21) and found a significant negative relationship between surface area and average CH 4 concentration. Large lakes remain an understudied contributor to the global CH 4 cycle and future research efforts should aim to quantify the spatial and temporal variation in their diffusive and ebullitive emissions, and associated drivers.