Quantifying bubble-mediated transport by ebullition from aquatic sediments

Abstract

The widespread release of gas bubbles from aquatic sediments (ebullition) has been receiving growing scientific interest because of its globally relevant contribution to methane emissions. Besides being an efficient transport pathway for methane and other gases to the atmosphere, these bubbles have the potential to mobilize resources and pollutants previously buried in the sediment by carrying solutes and particles on their surface. The phenomenon of bubbles transporting substances other than gases is well studied in open water and widely used in technical applications, such as froth flotation or dissolved air floatation. Research on the transport capabilities of natural bubbles forming in, and being released from, aquatic sediments is exceedingly rare. Ebullition resulting from biogenic gas production in sediments is characterized by large spatial and temporal variability and bubble sizes exceed those typically used in technical applications. Here we summarize the current state of research concerning bubble mediated transport (BMT) from aquatic sediments and develop a perspective based on these findings and own experimental results. We present measurements from a shallow reservoir to explore methods to monitor BMT and gather data on ebullition over 1 year. We found consistent bubble size spectra, despite large temporal variations of ebullition fluxes. We highlight some of the inherent difficulties of research in this area and argue that more experiments are needed for improving empirical and mechanistic understanding of BMT.

Reservoir Drawdown Highlights the Emergent Effects of Water Level Change on Reservoir Physics, Chemistry, and Biology

“…Our results suggest that alternative mechanisms may be responsible for increased nutrient concentrations during and after drawdown. These mechanisms could include shallowing of the epilimnion, which may concentrate nutrients in a smaller epilimnetic layer (Chapra & Reckhow, 1983; Snodgrass, 1977), increased ebullition, which may allow for solute transport out of sediment (Deemer & Harrison, 2019; Delwiche et al., 2020; Schwarz et al., 2023), and enhanced wave action on newly exposed sediments during drawdown, which may contribute to inputs of nutrients and metals from littoral areas (Furey et al., 2004). Because soluble Fe and Mn concentrations peaked earlier than total Fe and Mn concentrations (Figure 7), it seems likely that these metals were entrained from the anoxic hypolimnion in their reduced (soluble) state, before being oxidized in the epilimnion (Davison, 1993; Krueger et al., 2020; Lewis et al., 2023; Munger et al., 2019).…”

“…Littoral erosion and sediment drying due to water level fluctuations may lead to inputs of nitrogen (N), phosphorus (P), and carbon (C) from littoral sediments, further increasing surface concentrations during drawdown (Klotz & Linn, 2001). Likewise, methane ebullition may increase during drawdown due to decreased hydrostatic pressure (Beaulieu et al., 2018; Deemer & Harrison, 2019; Harrison et al., 2017), potentially increasing the suspension of bottom sediments and/or increasing cyanobacterial recruitment to the water column (Deemer & Harrison, 2019; Delwiche et al., 2020; Schwarz et al., 2023). Increased nutrient concentrations can subsequently also lead to increased phytoplankton biomass (e.g., Baldwin et al., 2008; Naselli‐Flores & Barone, 2005).…”

Reservoir drawdown highlights the emergent effects of water level change on reservoir physics, chemistry, and biology

“…Our results suggest that alternative mechanisms may be responsible for increased nutrient concentrations during and after drawdown. These mechanisms could include shallowing of the epilimnion, which may concentrate nutrients in a smaller epilimnetic layer (Chapra & Reckhow, 1983;Snodgrass, 1977), increased ebullition, which may allow for solute transport out of sediment (Delwiche et al 2020;Schwarz et al 2023;Deemer and Harrison, 2019), and enhanced wave action on newly exposed sediments during drawdown, which may contribute to inputs of nutrients and metals from littoral areas (Furey et al, 2004). Because soluble Fe and Mn concentrations peaked earlier than total Fe and Mn concentrations (Figure 7), it seems likely that these metals were entrained from the anoxic hypolimnion in their reduced (soluble) state, before being oxidized in the epilimnion (Davison, 1993;Krueger et al, 2020;Munger et al, 2019;Lewis et al, 2023).…”

“…Littoral erosion and sediment drying due to water level fluctuations may lead to inputs of nitrogen (N), phosphorus (P), and carbon (C) from littoral sediments, further increasing surface concentrations during drawdown (Klotz & Linn, 2001). Likewise, methane ebullition may increase during drawdown due to decreased hydrostatic pressure (Beaulieu et al, 2018;Harrison et al, 2017; manuscript submitted to JGR Biogeosciences 4 Deemer and Harrison, 2019), potentially increasing the suspension of bottom sediments and/or increasing cyanobacterial recruitment to the water column (Delwiche et al 2020;Schwarz et al 2023;Deemer and Harrison, 2019). Increased nutrient concentrations can subsequently also lead to increased phytoplankton biomass (e.g., Baldwin et al, 2008;Naselli-Flores & Barone, 2005).…”