When age makes all the difference : Methane production in sediment of contrasting Swedish lakes

Lakes are a significant source of the powerful greenhouse gas methane (CH4) globally. Methaneis produced through microbial processes in anoxic sediments. Methane emission from lakes ishighly variable in space and time. Consequently, is it difficult to predict the methane production rate and at present time it cannot be predicted from sediment characteristics. Therefore, methane production in the sediment of contrasting Swedish lakes was investigated, in order to find out if methane production rate can be related to sediment characteristics, and if a predictive model that recently was developed for Brazilian reservoirs is applicable to Swedish lakes. For this, sediment cores were collected from six lakes, differing in their sediment characteristics and geographical position as well as one river. The sediment cores were sliced into one centimetre thick layers. The different layers were incubated and methane production rate was measured. The sediment layers were also analysed for water content, median grain size, total nitrogen and carbon content as well as age. The influence of sediment age and C:N ratio as predictors for methane production were tested with a mixed linear model and a non-linear model. Both models showed that age had a significant effect on methane production rate (p < 0.001). The C:N ratio also had a statistically significant effect on methane production rate only shown with the non-linear model, however this effect was weak. Applying the recently published predictive model for methane production rate in Brazilian reservoir sediments to this data from the Swedish lakes, provided a good prediction of methane production rate in the nutrient-rich Swedish lakes, however it overestimated the methane production rate of the humic-rich boreal lakes and sediment older than 50 years. In summary, a model using age as predicting factor was developed fitting all the studied Swedish lakes. In addition, the predictive model developed in Brazilian reservoirs for the methane production rate was valid only for the studied nutrient-rich Swedish lakes and the studied oligotrophic Swedish lakes.