Dissolved CH 4 coupled to photosynthetic picoeukaryotes in oxic waters and to cumulative chlorophyll a in anoxic waters of reservoirs

[摘要] Methane ( CH 4 ) emissions from reservoirs are responsible for most of the atmospheric climatic forcing of these aquatic ecosystems, comparable to emissions from paddies or biomass burning. Primarily, CH 4 is produced during the anaerobic mineralization of organic carbon in anoxic sediments by methanogenic archaea. However, the origin of the recurrent and ubiquitous CH 4 supersaturation in oxic waters (i.e., the methane paradox) is still controversial. Here, we determined the dissolved CH 4 concentration in the water column of 12 reservoirs during summer stratification and winter mixing to explore CH 4 sources in oxic waters. Reservoir sizes ranged from 1.18 to 26.13 km 2 . We found that dissolved CH 4 in the water column varied by up to 4 orders of magnitude (0.02–213.64  µ mol L −1 ), and all oxic depths were consistently supersaturated in both periods. Phytoplanktonic sources appear to determine the concentration of CH 4 in these reservoirs primarily. In anoxic waters, the depth-cumulative chlorophyll  a concentration, a proxy for the phytoplanktonic biomass exported to sediments, was correlated to CH 4 concentration. In oxic waters, the photosynthetic picoeukaryotes' abundance was significantly correlated to the dissolved CH 4 concentration during both the stratification and the mixing. The mean depth of the reservoirs, as a surrogate of the vertical CH 4 transport from sediment to the oxic waters, also contributed notably to the CH 4 concentration in oxic waters. Our findings suggest that photosynthetic picoeukaryotes can play a significant role in determining CH 4 concentration in oxic waters, although their role as CH 4 sources to explain the methane paradox has been poorly explored.

[学科分类] 大气科学