Dissolved CH₄ coupled to photosynthetic picoeukaryotes in oxic waters and to cumulative chlorophyll <i>a</i> in anoxic waters of reservoirs

Methane (CH4) 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, CH4 is produced during the anaerobic mineralization of organic carbon in anoxic sediments by methanogenic archaea. However, the origin of the recurrent and ubiquitous CH4 supersaturation in oxic waters (i.e., the methane paradox) is still controversial. Here, we determined the dissolved CH4 concentration in the water column of 12 reservoirs during summer stratification and winter mixing to explore CH4 sources in oxic waters. Reservoir sizes ranged from 1.18 to 26.13 km2. We found that dissolved CH4 in the water column varied by up to 4 orders of magnitude (0.02–213.64 µmol L−1), and all oxic depths were consistently supersaturated in both periods. Phytoplanktonic sources appear to determine the concentration of CH4 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 CH4 concentration. In oxic waters, the photosynthetic picoeukaryotes' abundance was significantly correlated to the dissolved CH4 concentration during both the stratification and the mixing. The mean depth of the reservoirs, as a surrogate of the vertical CH4 transport from sediment to the oxic waters, also contributed notably to the CH4 concentration in oxic waters. Our findings suggest that photosynthetic picoeukaryotes can play a significant role in determining CH4 concentration in oxic waters, although their role as CH4 sources to explain the methane paradox has been poorly explored.

Dissolved CH₄ coupled to Photosynthetic Picoeukaryotes in Oxic Waters and Cumulative Chlorophyll-a in Anoxia

CH4 emissions from reservoirs are responsible for the majority of the atmospheric climatic forcing of these aquatic ecosystems, comparable to emissions from paddies or biomass burning. Primarily, CH4 is produced during the anaerobic mineralization of organic carbon in the anoxic sediments by methanogenic archaea. However, the origin of the recurrent and ubiquitous CH4 supersaturation in oxic waters (i.e., methane paradox) is still controversial. Here, we determined the dissolved CH4 concentration in the water column of twelve reservoirs during the summer stratification and the winter mixing. We obtained that the dissolved CH4 concentration varied up to four orders of magnitude (0.02–213.64 μM), and all depths were consistently supersaturated (710–7082234 %) in both periods. Phytoplanktonic sources of carbon appear to determine the concentration of CH4 in the reservoirs. In the anoxic waters, the depth-cumulative chlorophyll-a concentration, a proxy for the total phytoplanktonic biomass exported to sediments, determined the CH4 concentration. In the oxic waters, the photosynthetic picoeukaryotes abundance significantly determined the dissolved CH4 concentration both during the stratification and the mixing. The mean depth of the reservoirs, as a surrogate of the CH4 transport from sediment to the oxic waters, also contributed in shallow systems. Our findings suggest that photosynthetic picoeukaryotes can have a significant role in determining the CH4 concentration in oxic waters and, in comparison to cyanobacteria, have been poorly explored as CH4 sources.

LAKE 2.0: a model for temperature, methane, carbon dioxide and oxygen dynamics in lakes

A one-dimensional (1-D) model for an enclosed basin (lake) is presented, which reproduces temperature, horizontal velocities, oxygen, carbon dioxide and methane in the basin. All prognostic variables are treated in a unified manner via a generic 1-D transport equation for horizontally averaged property. A water body interacts with underlying sediments. These sediments are represented by a set of vertical columns with heat, moisture and CH4 transport inside. The model is validated vs. a comprehensive observational data set gathered at Kuivajärvi Lake (southern Finland), demonstrating a fair agreement. The value of a key calibration constant, regulating the magnitude of methane production in sediments, corresponded well to that obtained from another two lakes. We demonstrated via surface seiche parameterization that the near-bottom turbulence induced by surface seiches is likely to significantly affect CH4 accumulation there. Furthermore, our results suggest that a gas transfer through thermocline under intense internal seiche motions is a bottleneck in quantifying greenhouse gas dynamics in dimictic lakes, which calls for further research.

Plant-mediated CH₄ transport and contribution of photosynthates to methanogenesis at a boreal mire: a ¹⁴C pulse-labeling study

Plant-mediated methane (CH4) transport and the contribution of recent photosynthates to methanogenesis were studied on two dominating vascular plant species – Eriophorum vaginatum and Scheuchzeria palustris – at three types of microrelief forms (hummocks – E. hummocks, lawns – E. lawns and hollows – S. hollows) of a boreal natural minerogenic, oligotrophic fen in Eastern Finland. 14C-pulse labeling of mesocosms with shoots isolated from entire belowground peat under controlled conditions allowed estimation of plant-mediated CH4 flux and contribution of recent (14C) photosynthates to total CH4. The results showed (i) CH4 flux increased in the order E. hummocks ≤ E. lawns < S. hollows corresponding to the increasing water table level at the relief microforms as adjusted to field conditions. (ii) Plant-mediated CH4 flux accounted for 38, 31 and 51 % of total CH4 at E. hummocks, E. lawns and S. hollows, respectively. (iii) Contribution of recent photosynthates to methanogenesis accounted for 0.03 % for E. hummocks, 0.06 % for E. lawns and 0.13 % for S.hollows of assimilated 14C. Thus, microsites with S. palustris were characterized by higher rates of transported CH4 from the peat column to the atmosphere when compared to E. vaginatum of drier lawns and hummocks. Contribution of recent photosynthates to methanogenesis was dependent on the plant biomass within-species level (E. vaginatum at hummocks and lawns) but was not observed between species: smaller S. palustris had higher flux of 14CH4 as compared to larger E. vaginatum. Therefore, for the assessment of CH4 dynamics over meso- and macroscale as well as for the implication and development of the modeling of CH4 fluxes, it is necessary to account for plant species-specific differences in CH4 production, consumption and transport and the attribution of those species to topographic forms of microrelief.

Contribution of recent plant photosynthates of <i>Eriophorum vaginatum</i> and <i>Scheuchzeria palustris</i> to methanogenesis and CH₄ transport at a boreal mire: a ¹⁴C pulse-labeling study

Contribution of recent photosynthates to methanogenesis and plant-mediated methane (CH4) transport were studied on two dominating vascular plant species – Eriophorum vaginatum and Scheuchzeria palustris – at three microform types (hummocks, lawns and hollows) of a boreal natural minerogenic, oligotrophic fen in Eastern Finland. Measurements of total CH4 flux, isolation of shoots from entire peat and 14C-pulse labeling of mesocosms under controlled conditions allowed estimation of plant-mediated CH4 flux and contribution of recent (14C) photosynthates to total CH4. The obtained results showed (i) CH4 flux increases in the order E. hummocks ≤ E. lawns < S. hollows corresponding to the increasing water table level of the microforms as derived from in situ measurements. (ii) Plant-mediated CH4 flux accounted for 38, 31 and 51 % of total CH4 at E. hummocks, E. lawns and S. hollows, respectively. (iii) Contribution of recent photosynthates to methanogenesis accounted for 0.03 % for E. hummocks, 0.06 % for E. lawns and 0.13 % for S. hollows of assimilated 14C. Thus, S. palustris microsites are characterized by a higher efficiency for transporting CH4 from the peat column to the atmosphere when compared to E. vaginatum of drier lawns and hummocks. Contribution of recent plant photosynthates to methanogenesis was not depended on the amount of plant biomass: smaller S. palustris had higher 14CH4 as compared to larger E. vaginatum. Therefore, for the assessment of CH4 production and emission over meso- and macroscales as well as for the implication and development of C modeling of CH4 fluxes, it is necessary to account for plant species-specific processes including CH4 production, consumption and transportation and the attribution of those species to topographic microforms.