Abstract

Most lakes and reservoirs worldwide are supersaturated with respect to water concentrations of carbon dioxide (CO2 ) and methane (CH 4 ), two potent greenhouse gases (GHG). Although surface water GHG concentrations have been measured for more than three decades in lakes and reservoirs around the globe, the mechanisms leading to GHG supersaturation are still obscure, with the relative contributions of the different GHG producing processes hotly debated. In this study, we evaluated the terrestrial organic matter (OM) exports from land to aquatic systems, followed by its degradation and contribution to the surface freshwaters GHG concentrations. Natural lakes, reservoirs as well as lakes and reservoirs with a wood harvested watershed were sampled in the summer of 2007 and analyzed for a broad variety of bulk water chemical parameters and OM molecular proxies. In order to collect sufficient quantities of OM for the amino acid (AA) and lipid molecular analyses, a tangential flow filtration reverse osmosis systems (TFF-RO) was used following the evaluation of its performance for total OM possible molecular fractionation and recoveries using FTIR, stable carbon isotope signatures (e 13 C) and total organic carbon (TOC) concentrations. No significant sample fractionation or carry over was obtained with recoveries ranging between 94.6 and 106.9 % using the TFF-RO system. All lakes and reservoirs sampled in this study were supersaturated in CO2 and CH4 , with wood harvested water bodies and the reservoir having significantly higher water surface CO2 concentrations and emissions to the atmosphere compared to the natural lakes. The bulk water and OM chemical and isotopic analyses showed that the increase in terrestrial dissolved organic carbon and total nitrogen concentrations positively influenced bacterial OM degradation, which drove CO 2 production. Molecular analyses showed a direct relationship between the increases in bacterial biomarker abundances in the dissolved and sedimentary OM fractions, and higher water CO2 concentrations. This result suggests that OM bacterial oxidation is the most important process leading to GHG production in freshwater aquatic systems