Dissolved CH 4 coupled to Photosynthetic Picoeukaryotes in Oxic 1 Waters and Cumulative Chlorophyll-a in Anoxia in Reservoirs

Methane (CH 4 ) emissions from reservoirs are responsible for most of the atmospheric climatic forcing of these 10 aquatic ecosystems, comparable to emissions from paddies or biomass burning. Primarily, CH 4 is produced during the 11 anaerobic mineralization of organic carbon in anoxic sediments by methanogenic archaea. However, the origin of the 12 recurrent and ubiquitous CH 4 supersaturation in oxic waters (i.e., the methane paradox) is still controversial. Here, we 13 determined the dissolved CH 4 concentration in the water column of twelve reservoirs during summer stratification and 14 winter mixing to explore CH 4 sources in oxic waters. Reservoirs size ranged from 1.18 to 26.13 km 2 . We obtained that 15 dissolved CH 4 in the water column varied up to four orders of magnitude (0.02-213.64 µ M), and all oxic depths were 16 consistently supersaturated in both periods. Phytoplanktonic sources appear to determine the concentration of CH 4 in these 17 reservoirs primarily. In anoxic waters, the depth-cumulative chlorophyll-a concentration, a proxy for the phytoplanktonic 18 biomass exported to sediments, was correlated to CH 4 concentration. In oxic waters, the photosynthetic picoeukaryotes 19 abundance was significantly correlated to the dissolved CH 4 concentration both during the stratification and the mixing. The 20 mean depth of the reservoirs, as a surrogate of the vertical CH 4 transport from sediment to the oxic waters, also contributed 21 notably to the CH 4 concentration in oxic waters. Our findings suggest that photosynthetic picoeukaryotes can have a 22 significant role in determining CH 4 concentration in oxic waters, although their role as CH 4 sources to explain methane 23 paradox has been poorly explored. organisms. In the anoxic waters, the depth-cumulative chlorophyll-a concentration, a proxy for the phytoplanktonic biomass exported to sediments, determined the CH 4 concentration. In the oxic waters, we considered different potential CH 4 sources, including the vertical and lateral transport of CH 4 from anoxic zones and in situ 556 production. The mean depth of the reservoirs, as a surrogate of the CH 4 transport from sediment to the oxic waters, 557 contributed in shallow systems. We did not detect methanogenic Archaea or methylphosphonates degradation target genes (i.e. mcrA and phnJ genes, respectively ) , which suggests that these pathways are not responsible for the in situ production of CH 4 in the oxic waters of the study reservoirs. We found that dissolved CH 4 was coupled to the abundance of photosynthetic picoeukaryotes (PPEs) during both periods and to chlorophyll-a concentration and the abundance of and cyanobacteria 561 during the stratification period. These PPEs were non-colonial green algae from the order Chlorococcales (class Chlorophyceae, phylum Chlorophyta), and the genus Chrysochromulina spp. , (class Coccolithophyceae, phylum 563 Haptophyta ). Finally, we combined all the explanatory variables with significant effects and determined their relative contribution to the CH 4 concentration using generalized additive models (GAMs). The abundance of PPEs was the variable explaining most of the variance of dissolved CH 4 concentration during the stratification period, with an effect higher than the 566 cyanobacteria abundance. During the mixing period, the reservoir mean depth and the abundance of the PPEs were the only 567 drivers for CH 4 concentration. Our findings show that the abundance of PPEs can be relevant for explaining the dissolved