UC Riverside

The volume of methane (CH₄) and other gases in sediments and the rate of CH4 ebullition were determined for a shallow eutrophic freshwater lake in Southern California. Gas volume, principally as CH₄, was measured at 28 sites in July 2010, followed by monthly sampling at 7 sites through December 2011. Gas volumes measured in July 2010 at the 28 sites exhibited a complex dependence on sediment properties; the volume of CH₄ and other gases were negligible in very coarse-textured sediment with low water and organic carbon contents. Gas volumes increased strongly with increased silt content, and were highest in sediments with intermediate water contents (60-70%), organic carbon contents (2-3%) and depths (approximately 4 m). Methane was the dominant gas collected from sediment (80 - 90%), while carbon dioxide comprised roughly 2 - 3% of sediment gas in the lake. Gas sampling during cool winter months found very low or undetectable volumes of gas present, while sediment gas volumes increased markedly during the spring and early summer months, and then declined in late summer and fall. The rate of CH₄ ebullition, quantified with an echosounder, also varied markedly across the lake and seasonally. High rates of ebullition were measured at all 7 sites in July 2011 (up to 96 mmol CH₄ m^-2 d^-1), while the rates were >50% lower in September and negligible in December 2010. Ebullition rates were inversely correlated with depth and most other sediment properties, but strongly positively correlated with sand content. No simple relationship between ebullition rate and sediment gas volume across the set of sites was found, although ebullition rates at individual sites were positively related to gas volume. Measurements of acoustic target strength of individual bubbles across the sediments and over time were also made. Bubble size and volume varied weakly across the different sediments and depths, with no strong correlation with sediment properties. A statistical characterization of the population of bubbles found broadly normally-distributed bubble size, with kurtosis values of -0.84 to +1.11 and skewness values of -0.23 to 0.62. The ensemble-average weighted mean bubble volume was 0.0940.023 cm³, although individual bubbles with target strengths as low as -68dB (0.0013 cm³) and as high as -43dB (4.689 cm³) were found.