Seasonal variation in CH₄ emission during the growing season

There is a seasonal variation in CH₄ emissions for both P. australis and C. rostrata in both Lake Erssjön and Lake Följesjön. The highest diel mean was recorded in June during the first measurement occasion in Lake Erssjön for both plant species. However, in Lake Följesjön the highest diel mean was measured during the third measurement occasion for C. rostrata and during the fourth measurement for P. australis. In general, CH₄ emissions decrease towards the end of the growing season. Seasonal variations in CH₄ emission per g DW were also apparent during the growing season.

Differences in CH₄ emission between species

CH₄ emissions calculated per m² from E. fluviatile were smaller than from the two other species. However, E. fluviatile had larger CH₄ emissions per g DW.

Diel variation in CH₄ emission

There were large variations in the CH₄ emissions within days, during the growing season. However, recurrent peaks in CH₄ emissions at the same time on the different measurement occasions were not found for any of the species. The variation during the days was higher for P. australis than for C. rostrata in Lake Erssjön. This pattern was less obvious in Lake Följesjön.

Model averaging for CH₄ emission

When modelling CH₄ emissions per square meter using all available data, two explanatory variables shown to be important: lake and air temperature. The results clearly showed that CH₄ emissions from emergent aquatic macrophytes in Lake Följesjön were much higher than from Lake Erssjön. When calculating CH₄ emissions per m², emissions did not differ between P. australis and C. rostrata. The GLZ also detect a change in CH₄ emission for the measurement days during the growing season. Air temperature was the environmental variable that was best correlated with CH₄ emission, where CH₄ emissions increased with increasing temperature. On the other hand, an increase in light and wind had a negative effect on CH₄ emissions (Table 1).

When modelling CH₄ emissions per g DW per m², there was a significant difference between the emergent aquatic macrophytes (P. australis and C. rostrata), while the lake type contributed little to the model. Furthermore, several environmental variables seemed equally important in the model, where CH₄ emissions increased with air temperature and air pressure but decreased with increasing light and wind (Table 2).