Methanogenesis, redox and carbon isotope biogeochemistry: Georgetown Lake, Montana.

摘要

This study at Georgetown Lake, MT was conducted to: 1) to develop the methodology at Montana Tech for identifying and quantifying dissolved methane concentrations in the water column; 2) determine the DOC concentration and C-stable isotope composition in the water column and shallow sediments; and 3) verify previous findings and extend our knowledge and understanding of the "under-ice" water column redox biogeochemistry.;The biogeochemistry of the lake has been changing over the last several decades due to the lake's shallow nature and increasing nutrient loads resulting from development, deforestation (pine bark beetle), and heavy recreational use. A seasonal anoxic layer near the lake bottom under ice cover has been previously identified and studied and this study builds on those results. Lake anoxia has been cited as being responsible for winter fish kills and threatens the long-term lake ecology. Additionally, this anoxic zone provides redox conditions that support the formation of a number of reduced chemical species such as H2S, NH4+ and CH 4.;Previous studies conducted at Georgetown Lake have identified increasing concentrations of dissolved inorganic carbon (DIC) near the bottom of the lake and this DIC was also isotopically heavier (delta13C- DIC) than in the upper reaches of the water column. Based on this observation methane production was suspected to be occurring, since acetoclastic methanogenesis produces isotopically depleted CH4 and enriched CO2.;To determine if methanogenesis is an active biogeochemical process in Georgetown Lake, water samples were gathered during periods of ice covered and ice-free conditions. C-stable isotopes were also used to examine the composition of dissolved, particulate, and sediment organic/inorganic carbon to characterize the extent of methanogenesis. CH4 concentrations in the anoxic zone were measured up to about 1.2 mM during ice cover, with summer concentrations of dissolved CH4 at 65 &mgr;M. Trapped gas from sediments was also collected and determined to have about 47 ppmv CH4. DIC was about 10-fold higher in concentration in the water column than DOC and the delta 13C- DIC was substantially enriched (-3 to -5‰) relative to delta 13C- DOC (-24 to -32‰). Additionally, the delta 13C-DOC was similar to that of the surrounding terrestrial vegetation measured (-26 to -29‰). Shallow sediment cores had a concentration of organic carbon (OC) about 10-fold higher than that of inorganic carbon (IC). OC, IC, delta 13C- DIC and delta 13C- DOC all changed as a function of depth in the sediment as a result of microbial activity modifying the buried material.;Methane build up in the water of Georgetown Lake during the winter could result in the release of methane to the atmosphere after ice breakup. As a greenhouse gas, methane is more potent than carbon dioxide. A better understanding of the lake carbon cycling will help to determine if Georgetown Lake is a net source or sink of carbon on an annual basis, and if there is a significant contribution of methane into the atmosphere.