Methane Emission From the Surface of the Mozhaisk Valley-Type Reservoir

This article deals with spatio-temporal changes in methane emission from the surface of the Mozhaisk reservoir. Seasonal changes in methane content and flux were revealed for different morphological areas of the reservoir, based on field observation data obtained in 2015–2018. In the low-flow Mozhaisk reservoir, the methane content in the boundary and bottom layers of the deep-water areas at the end of the summer stratification period may differ by three orders of magnitude. According to results from measuring with floating chambers in the central area of the reservoir from early June to the end of the period of direct stratification (August–September) the total methane flux increased from less than 1 to 16 mgC-CH 4 /(m 2 /hr). Time-coincident measurements with floating chambers of two types revealed characteristic values of the methane flux components and their change over the sampling period. It was found that at the period of stratification the diffusive flux predominates with the mean values 0.2 mgC-CH 4 /(m 2 /hr). A further increase in the total methane flux is associated with an increase of its bubble component. According to calculations, the diffusive flux reaches its maximum values in late summer in the shallow zone of the reservoir. It is established that a significant increase of the values of the total methane flux is observed when the upper boundary of the oxygen-free zone reaches the lower boundary of the epilimnion. The methane flux density reaches its largest values prior to destruction of the direct stratification. Comparison of field measurements with literature data showed that the magnitude of emission from reservoirs with a slow water exchange in the temperate zone can be underestimated in the evaluation of global methane emission.