Comparing methane ebullition variability across space and time in a Brazilian reservoir

Abstract

The potent greenhouse gas methane (CH 4 ) 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 CH 4 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 CH 4 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 CH 4 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 CH 4 ebullition varied more temporally than spatially and that the intra-annual variability was stronger than daily variability within 2 weeks. We also found that CH 4 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 CH 4 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).

Spatially Resolved Measurements in Tropical Reservoirs Reveal Elevated Methane Ebullition at River Inflows and at High Productivity

“…From a well‐studied inflow‐bay in CDU and from the main inflow in FUN, we calculated the water volumes of the river‐inflow areas (i.e., the part of the bay which we assumed to be under the influence of the incoming river of this bay) using the surface volume tool in ArcGIS. In CDU, we delimited the river‐inflow area from the non‐inflow (i.e., lake‐like) reservoir area based on field observations of water flow during 15 field campaigns over one year in a northern bay with river inflow (Linkhorst et al., 2020) (Figure S7: river inflow 6); downstream of the placed delimitation, we could not observe a visible unidirectional flow of surface water, and assumed that this corresponds to the end of the river inflow area. In FUN, we delimited the river inflow area behind a large lateral land dam, that is, a levee, at a shallow sill in the main stem in the northernmost large bend of the reservoir (Figure S8: river inflow 1).…”

“…Our previous comparison of spatial and temporal variability in CDU (Linkhorst et al, 2020) suggested to sample each hydrological season (rising and falling water level) for annual upscaling of CH 4 ebullition. In LINKHORST ET AL.…”

“…Unfortunately, CH 4 ebullition is difficult to accurately quantify because it is highly variable across space and time, even within a single system (DelSontro et al., 2011; Linkhorst et al., 2020; Maeck & Lorke, 2014; Maeck et al., 2013). Obtaining measurements representative for an entire water body is not trivial, especially when even massive sampling efforts usually only cover a fraction of the reservoir during a limited time period.…”

Spatial and Temporal Variability of Diffusive CO2 and CH4 Fluxes From the Amazonian Reservoir Petit‐Saut (French Guiana) Reveals the Importance of Allochthonous Inputs for Long‐Term C Emissions

“…This study examined gas diffusive fluxes without regard to ebullition since a previous study reported it is the main emission pathway within the reservoir for CO 2 (89%) and for CH 4 (76%) (Abril et al, 2005). Yet the spatiotemporal variability of the ebullitive CH 4 flux has not been accurately addressed at Petit‐Saut while the contribution of such a pathway to the total gas emissions by the reservoir likely exhibits high spatial and temporal variability (e.g., DelSontro et al, 2011; Linkhorst et al, 2020). In particular, ebullition may be an important emissions pathway notably in littoral and transition areas and therefore may exhibit a similar spatial variation to the diffusive flux that we reported here.…”

Hotspots of Diffusive CO2 and CH4 Emission From Tropical Reservoirs Shift Through Time

“…gain knowledge on how to best distribute measurement effort between spatial and seasonal coverage (Linkhorst et al, 2020;Wik et al, 2016).…”