Spatial Heterogeneity of Methane Ebullition in a Large Tropical Reservoir

Abstract

Tropical reservoirs have been identified as important methane (CH(4)) sources to the atmosphere, primarily through turbine and downstream degassing. However, the importance of ebullition (gas bubbling) remains unclear. We hypothesized that ebullition is a disproportionately large CH(4) source from reservoirs with dendritic littoral zones because of ebullition hot spots occurring where rivers supply allochthonous organic material. We explored this hypothesis in Lake Kariba (Zambia/Zimbabwe; surface area >5000 km(2)) by surveying ebullition in bays with and without river inputs using an echosounder and traditional surface chambers. The two techniques yielded similar results, and revealed substantially higher fluxes in river deltas (∼10(3) mg CH(4) m(-2) d(-1)) compared to nonriver bays (<100 mg CH(4) m(-2) d(-1)). Hydroacoustic measurements resolved at 5 m intervals showed that flux events varied over several orders of magnitude (up to 10(5) mg CH(4) m(-2) d(-1)), and also identified strong differences in ebullition frequency. Both factors contributed to emission differences between all sites. A CH(4) mass balance for the deepest basin of Lake Kariba indicated that hot spot ebullition was the largest atmospheric emission pathway, suggesting that future greenhouse gas budgets for tropical reservoirs should include a spatially well-resolved analysis of ebullition hot spots.

Pumping methane out of aquatic sediments – ebullition forcing mechanisms in an impounded river

“…The spatial variability of ebullition in impounded rivers was recently shown to correlate strongly with spatial patterns of sedimentation . In a large reservoir, DelSontro et al (2011) found higher ebullitive fluxes in river delta bays compared to non-river bays, which may also point towards sedimentation as the main cause of the spatial distribution of ebullition. To build on this work, we focus the current study on the temporal variability of ebullition in greater detail at a site known for spatially variable ebullition in order to investigate its underlying processes better.…”

Intense methane ebullition from open water area of a shallow peatland lake on the eastern Tibetan Plateau

“…The results showed that the rate of methane ebullition in Lake Medo was one to two orders of magnitude higher than that from most lakes in northern region (Mattson and Likens, 1990;Huttunen et al, 2003;Bastviken et al, 2004), including that from the Lake Villasjön in Sweden which is most similar functionally, climatically, and morphologically to Lake Medo (Wik et al, 2013), but lower than that from subtropical and tropical lakes (Keller and Stallard, 1994;Martinez and Anderson, 2013). When compared with wetland ecosystems other than lake, the methane ebullition rate from Lake Medo was much higher than that from tropical floodplain (Marani and Alvala, 2007) and that from rice paddies in temperate or even tropical regions (Ziska et al, 1998;Meijide et al, 2011); Lake Medo on the Tibetan Plateau presented a methane ebullition rate much lower than tropical reservoirs (Sturm et al, 2014;Delsontro et al, 2011), but much higher than even the highest methane emission rate from temperate reservoirs reported so far (Delsontro et al, 2010). Therefore, it could be concluded that the methane ebullition from Lake Medo is much higher than that from wetlands ecosystems in temperate and northern regions (Table 3).…”

Spatial and Temporal Correlates of Greenhouse Gas Diffusion from a Hydropower Reservoir in the Southern United States

“…Sites were distributed among six lentic main lake locations within Douglas Lake placed at increasing distances from Douglas Dam (M1-M6); four lentic cove locations within Douglas Lake placed at increasing distances from Douglas Dam (C1-C4), and three lotic locations on the Nolichucky River (U1), French Broad River (U2), and Pigeon River (U3) upstream of Douglas Lake. This distribution of sites captured spatial variation in environmental characteristics known to affect GHG production and surface diffusion [3,9,12].…”

“…First, accumulation of dissolved GHGs in the water column, primarily originating from microbial metabolism, leads to passive diffusion from the reservoir surface to the atmosphere [2,5]. Second, gas bubble ebullition from the sediments of the reservoir also contribute to GHG emissions, particularly in the form CH4 from shallow areas of reservoirs [6][7][8][9]. More recent investigations have documented the ebullition of microbubbles generated from oxic production of CH4 in the water column is also a source of GHG emission from reservoirs [10].…”