A process-based stable isotope approach to carbon cycling in recently flooded upland boreal forest reservoirs
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Reservoirs impound and store large volumes of water and flood land. The water is used for electricity generation, irrigation, industrial and municipal consumption, flood control and to improve navigation. The decomposition of flooded soil and vegetation creates greenhouse gases and thus reservoirs are a source of greenhouse gases. Reservoirs are not well studied for greenhouse gas flux from the water to the atmosphere. The FLooded Upland Dynamics EXperiment (FLUDEX) involves the creation of three experimental reservoirs in the upland boreal forest to study greenhouse gas and mercury dynamics. The balance of biological processes, decomposition, primary production, CH<sub>4</sub> oxidation and the nitrogen cycle in the reservoirs controls the greenhouse gas flux from the reservoir to the atmosphere. Understanding the importance and controlling factors of these processes is vital to understanding the sources and sinks of greenhouse gases within reservoirs. The carbon and oxygen dynamics near the sediment-water interface are very important to the entire reservoir because many processes occur in this area. Light and dark benthic chambers were deployed, side-by-side, to determine the benthic flux of DIC and CH<sub>4</sub> across the sediment-water interface and to determine the role of benthic photoautotrophs in benthic DIC, CH<sub>4</sub> and O<sub>2</sub> cycling. Benthic chambers have shown photoautotrophs use the decomposing soil, rocks and exposed bedrock as a physical substrate to colonize and the CO<sub>2</sub> produced by the decomposing soil as a carbon source since the delta<sup>13</sup>C-DIC value of the DIC added to light chambers is enriched relative to dark chambers and net photosynthesis rates are linked to community respiration. Benthic photoautotrophs consume 15-33% of the potential DIC flux into the water column. CH<sub>4</sub> produced by the decomposition of soils is partially oxidized by methanotrophs that use the photosynthetically produced oxygen. The delta<sup>13</sup>C-CH<sub>4</sub> values of the CH<sub>4</sub> added to light chambers is enriched relative to dark chambers and 15-88% of the potential CH<sub>4</sub> flux into the water column is oxidized. An isotope-mass budget for DIC and CH<sub>4</sub> is presented for each reservoir to identify the importance of processes on areservoir scale. Input of DIC to the reservoirs from overland flow can be important because concentration is greater and delta<sup>13</sup>C-DIC values are depleted relative to inflow from Roddy Lake. Estimates of total reservoir primary production indicate that 3-19% of the total DIC production from decomposition is removed by photoautotrophs. The carbon cycling in biofilm and the importance of periphytic primary production needs to be better understood. Dissolved delta<sup>13</sup>C-CH<sub>4</sub> values of CH<sub>4</sub> in reservoir outflow enriched 45-60permil, indicating that CH<sub>4</sub> oxidation was an important CH<sub>4</sub> sink within the reservoirs. Stable carbon isotope data indicates that the CH<sub>4</sub> in the bubbles is partially oxidized so the site of bubble formation is the upper portion of the flooded soil. The fraction of CH<sub>4</sub> converted to CO<sub>2</sub> in the FLUDEX reservoirs is similar to that of the wetland flooded for the Experimental Lakes Area Reservoir Project (ELARP). Approximately half of the dissolved CH<sub>4</sub> in the FLUDEX reservoirs was removedby CH<sub>4</sub> oxidation. The ebullitive flux of CH<sub>4</sub> from FLUDEX reservoirs is reduced 25-75% by CH<sub>4</sub> oxidation. The CH<sub>4</sub> flux to the atmosphere from peat surface of the ELARP reservoir became less oxidized after flooding: 91% to 85% oxidized. The floating peat islands of the ELARP reservoir were less oxidized than the peat surface. Similar to the CH<sub>4</sub> in the FLUDEX reservoirs, CH<sub>4</sub> in the ELARP peat islands was oxidized 56%. CH<sub>4</sub> oxidation is an important process because it reduces the global warming potential of the greenhouse gas flux since CO<sub>2</sub> is less radiatively active than CH<sub>4</sub>.