The importance of biogenic methane for fishery productivity in Tonle Sap Lake, Cambodia

Background/Question/Methods

The magnitude and pathways by which carbon and energy enter food webs is of fundamental ecological importance and relevant to ecological theory describing community interaction, spatial subsidies, and ecosystem function. There has been significant recent attention into quantifying the extent to which allochthonous versus autochthonous carbon supports lake food webs. In the case of fisheries, such questions take on additional relevance when human societies are heavily dependent on productive ecosystems to supply food, employment, and cultural identity. The Lower Mekong River basin, including Tonle Sap Lake (TSL), is the largest inland fishery in the world and a dominant source of protein and income for much of Southeast Asia. Maintaining ecosystem productivity in the face of large-scale environmental change from hydroelectric dams is critical for economic and social well-being in the region. We used stable isotope and Fatty Acid Methyl Ester (FAME) biomarkers to investigate basal carbon sources to the fishery and infer how these sources may change with dam construction. Here we focus specifically on the role of biogenic methane oxidation in supporting fisheries productivity.

Results/Conclusions

The TSL experiences frequent anoxic conditions and high terrigenous organic matter loading which results in significant sediment methane production. Multiple fish species important for commercial and subsistence harvest showed evidence of methane derived basal resource subsidies. Individuals from a class of hypoxia-tolerant fishes (blackfish) had tissue carbon isotope values (δ¹³C) ranging from -36 to -57 per mil. These extremely depleted values are best explained by utilization of biogenic methane by methane oxidizing bacteria (MOB) and subsequent grazing of these bacterial by benthic insects and ultimately fishes. The presence of MOB in the food web was confirmed by identifying 16:1ω8 and 18:1ω8 FAME biomarkers specific to these bacteria in fish tissues. A second class of migratory, hypoxia-intolerant fishes (whitefish) generally had higher tissue δ¹³C indicative of an algal or terrestrial carbon source and lacked MOB specific FAMEs. Blackfish compromise approximately 40% of the total catch from the TSL with an annual production of ~250,000 tonnes. Alterations to the sediment and oxygen dynamics of the TSL, as a result of hydroelectric dams, has the potential to significantly alter fish population dynamics and influence the role methane plays as a carbon source to this economically and socially important fishery.