Influence of drainage basin topography and elevation on carbon dioxide and methane supersaturation of stream water

The partial pressures of CO2 (pCO(2)) and CH4 (pCH(4)) in streams rue not only governed by instream processes, but also by transformations occurring in soil and groundwater ecosystems. As such, stream water pCO(2) and pCH(4) can provide a tool to assess ecosystem respiration and anaerobic metabolism throughout drainage basins. We conducted three surveys sampling the gas content of streams in eastern Tennessee and western North Carolina to assess factors regulating ecosystem metabolism in catchments with contrasting geomorphologies, elevations and soil organic matter storage. In our first survey, the influence of drainage basin geomorphology on ecosystem respiration was examined by sampling streams draining catchments underlain by either shale or dolomite. Geomorphology is influenced by geology with shale catchments having shallower soils, broader, unconstrained valley floors compared with dolomite catchments. pCO(2) varied little between catchment types but increased from an average of 3340 ppmv in spring to 9927 ppmv in summer or 9.3 and 28 times atmospheric equilibrium (pCO(2(equilib))), respectively. In contrast, pCH(4) was over twice as high in streams draining shale catchments (306 ppmv; pCH(4(equilib)) = 116) compared with more steeply incised dolomite basins (130 ppmv; pCH(4(equilib)) = 51). Using the ratio of pCH(4):pCO(2) as an index of anaerobic metabolism, shale catchments had nearly twice as much anaerobiosis (pCH(4):pCO(2) = 0.046) than dolomite drainages (pCH(4):pCO(2) = 0.024). In our second survey, streams were sampled along an elevational gradient (525 to 1700 m) in the Great Smoky Mountains National Park, USA where soil organic matter storage increases with elevation. pCO(2) did not vary between streams but increased from 5340 ppmv (pCO(2(equilib)) = 15) to 8565 ppmv (pCO(2(equilib)) = 24) from spring to summer, respectively. During spring pCH(4) was low and constant across streams, but during summer increased with elevation ranging from 17 to 2068 ppmv (pCH(4(equilib)) = 10 to 1216). The contribution of anaerobiosis to total respiration was constant during spring (pCH(4):pCO(2) = 0.017) but during summer increased with elevation from 0.002 at 524 m to 0.289 at 1286 m. In our last survey, we examined how pCO(2) and pCH(4) changed with catchment size along two rivers (ca. 60 km stretches in both rivers corresponding to increases in basin size from 1.7-477 km(2) and 2.5-275 km(2)). pCO(2) and pCH(4) showed opposite trends, with pCO(2) decreasing ca. 50% along the rivers, whereas pCH(4) roughly doubled in concentration downstream. These opposing shifts resulted in a nearly five-fold increase of pCH(4):pCO(2) along the rivers from a low of 0.012 in headwaters to a high of 0.266 65-km downstream. pCO(2) likely declines moving downstream as groundwater influences on stream chemistry decreases, whereas pCH(4) may increase as the prevalence of anoxia in rivers expands due to finer-grained sediments and reduced hydrologic exchange with oxygenated surface water.