Methane bubbles in Lake Kinneret: Quantification and temporal and spatial heterogeneity

The amount of methane bubbles rising from the bottom of Lake Kinneret was quantified by using a dual-beam echo sounder. Both echo-counting (EC) and echo integration (EI) techniques were implemented. Bubbles can confound the identification of fish targets because their acoustic sizes strongly overlap. Analysis of vertical changes in densities in the anoxic hypolimnion (with no fish) indicated that multiple targets were most abundant near the bottom, which caused an essential bias of bubble density estimates by EC. EI was a reliable tool for assessment of bubble density near the bottom. In the upper part of the water column, both techniques provided similar estimates of density of targets (mainly bubbles) because they were measured between dense fish schools during the daytime. The mean acoustic size of rising bubbles decreased in the hypolimnion but increased in the epilimnion, suggesting change in the relative importance of factors controlling gas volume. In summer and fall 2001, the bubbles became predominant echo-reflecting objects in the epilimnion. Temporal and spatial changes in bubble densities were highly heterogeneous, suggesting strong variability in factors affecting the gas ebullition. This variability should be taken into consideration when attempting to quantify the methane ebullition and assessing fish abundance in aquatic systems. Over the last century, the atmospheric concentration of the greenhouse gas methane has risen ;1% per year (Rowland 1985). The reasons for this trend are not completely understood since the global sources and sinks of methane still need quantification (Conrad and Seiler 1988). Methane is an important product of the anaerobic degradation of organic material in bottom sediments (Kiene 1991). Gas ebullition from the bottom sediments of natural waters could substantially envelop the total methane flux; however, only a few studies have been carried out to assess gas bubble emission from the floor of aquatic ecosystems using different techniques or a combination of methods (e.g., Huttunen et al. 2001; Whiting and Chanton 2001; Leifer and Patro 2002). Gas traps, video/photo, and acoustic techniques are used to measure gas ebullition. The first two methods are good to quantify gas emission in confined areas. Acoustic methods might be helpful to evaluate spatial variability of bubbles, which are strong scatterers of acoustic energy, in deep enough aquatic systems (Vagle and Farmer 1991; Jackson et al. 1998; Hornefius et al. 1999; Quigley et al. 1999). Acoustic remote-sensing methods offer the advantage of being able to quickly scan large volumes of the water column synoptically, but they typically lack the ability to resolve and taxonomically classify individual targets. Gas bubbles rising from the bottom can contaminate the assessment of fish densities with echo sounders (Rudstam and Johnson 1992). 1 Corresponding author ([email protected]).