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Spatial variability of methane production and methanogen communities within a eutrophic reservoir: evaluating the importance of organic matter source and quantity
Citation:
Berberich, M., J. Beaulieu, T. Hamilton, AND I. Buffam. Spatial variability of methane production and methanogen communities within a eutrophic reservoir: evaluating the importance of organic matter source and quantity. Summer Meeting of the Association for the Sciences of Limnology and Oceanography, Vancouver, British Columbia, CANADA, June 10 - 15, 2018.
Impact/Purpose:
Freshwater reservoirs are a globally significant source of methane (CH4), a potent greenhouse gas. Methane emission rates can vary by several orders of magnitude across reservoirs, however, and the factors that control CH4 production rates are not well known. In this research we investigated the relationship between CH4 production rates and sediment organic matter in a eutrophic reservoir in Ohio, USA. We found that CH4 production was related to both the quantity and quality of organic matter in the sediment. The research suggests that emissions could be mitigated through reduction of allochthonous and autochthonous organic matter inputs to the reservoir. These findings may be of interest to reservoir managers and greenhouse gas inventory compilers.
Description:
Freshwater reservoirs are an important source of the greenhouse gas methane (CH4) to the atmosphere, but there is a wide range of estimates of global emissions, due in part to variability of methane emissions rates within reservoirs. While morphological characteristics, including water depth, contribute to the variation in emission rates, spatial heterogeneity of biological methane production rates by sediment dwelling methanogenic archaea may be another important source of variation. An important constraint on sediment CH4 production rates is the availability of organic matter (OM). Both the quantity and quality of OM have been shown to influence CH4 production rates in laboratory experiments. In particular, CH4 production rates have been shown to respond strongly to algal-derived OM, a highly labile OM source. It is unclear, however, whether this pattern persists at the field scale where other sources of organic matter, such as sediment loads from the watershed, may play an important role in CH4 generation. We measured methane production rates, sediment OM source, OM quantity, and methanogen community composition at fifteen sites in a temperate, eutrophic reservoir to assess OM drivers of spatial variability in CH4 production rates and methanogen communities. Areal CH4 production rates (g CH4 m^-2) were highest in the riverine portion of the reservoir below the main inlet where OM quantity (g OM cm^-2) was greatest, presumably due to high sedimentation rates. The pattern of high CH4 production rates in the riverine portion of the reservoir persisted even when rates were normalized to OM quantity (g CH4 g-1 OM), suggesting that not only was OM more abundant in the riverine zone, it was more readily utilized by methanogens. Sediment stable isotopes and elemental ratios indicated a greater proportion of allochthonous OM in the riverine zone than other areas of the reservoir, suggesting that watershed derived OM is an important driver of CH4 production in the system. Methanogens were abundant at all sampling sites but the functional diversity of methanogens was highest in the riverine zone. Variation in functional diversity of methanogens likely reflects differences in decomposition processes or OM quality across the reservoir. In contrast to previous reports of water column primary productivity as a key predictor of CH4 emission rates in reservoirs, we found that the highest production rates occurred at sites with a strong contribution of terrestrial OM.