Abstract
AbstractThe potent greenhouse gas methane (CH4) is readily emitted from tropical reservoirs, often via ebullition (bubbles). This highly stochastic emission pathway varies in space and time, however, hampering efforts to accurately assess total CH4 emissions from water bodies. We systematically studied both the spatial and temporal scales of ebullition variability in a river inflow bay of a tropical Brazilian reservoir. We conducted multiple highly resolved spatial surveys of CH4 ebullition using a hydroacoustic approach supplemented with bubble traps over a 12‐month and a 2‐week timescale to evaluate which scale of variation was more important. To quantify the spatial and temporal variability of CH4 ebullition, we used the quartile coefficients of dispersion at each point in space and time and compared their frequency distributions across the various temporal and spatial scales. We found that CH4 ebullition varied more temporally than spatially and that the intra‐annual variability was stronger than daily variability within 2 weeks. We also found that CH4 ebullition was positively related to water temperature increase and pressure decrease, but no consistent relationship with water column depth or sediment characteristics was found, further highlighting that temporal drivers of emissions were stronger than spatial drivers. Annual estimates of CH4 ebullition from our study area may vary by 75–174% if ebullition is not resolved in time and space, but at a minimum we recommend conducting spatially resolved measurements at least once during each major hydrologic season in tropical regions (i.e., in dry and rainy season when water levels are falling and rising, respectively).
Highlights
Spatial variability of CH4 ebullition We found that high CH4 ebullition can occur at multiple depths and locations along the bay, from the shallow northern part of the bay close to the river inflow (0–5 m) to the deeper part further south (10–15 m; Fig. 1a)
CH4 ebullition estimated via the hydroacoustic approach varied over four to five orders of magnitude within each depth category, presumably due to the much higher spatial resolution achieved at all depths with the echosounder surveys (Fig. 2a) compared to that achieved with bubble trap deployments (Fig. 2b)
Variability of CH4 ebullition at temporal and spatial scales While many previous studies have pointed out the importance of spatial variability in low-latitude reservoir CH4 ebullition (Grinham et al 2011; DelSontro et al 2015; Hilgert et al 2019), and some others have found seasonal patterns (Deshmukh et al 2014; Yang et al 2018; Marcon et al 2019), this is, to our knowledge, the first study to systematically compare the variability of CH4 ebullition both spatially and over different scales of time
Summary
While there are a number of studies that measured CH4 ebullition in reservoirs at low latitudes, both in the tropics (Keller and Stallard 1994; Abril et al 2005; Ramos et al 2006; DelSontro et al 2011; de Mello et al 2018) and subtropics (Grinham et al 2011; Deshmukh et al 2014; Beaulieu et al 2016; Guérin et al 2016; Grinham et al 2018; Yang et al 2018; Hilgert et al 2019; Marcon et al 2019), only a few of them (Keller and Stallard 1994; Deshmukh et al 2014; Yang et al 2018; Marcon et al 2019) have studied temporal trends of ebullition in enough detail to identify seasonal trends None of these studies have systematically compared the magnitude of the variability of CH4 ebullition over scales of both space and time. The overall aim was to determine the best way to representatively measure CH4 ebullition with minimum sampling effort
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