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

Abstract

The partial pressures of CO₂ (pCO₂) andCH₄ (pCH₄) in streams are not only governed byinstream processes, but also by transformations occurring in soil andgroundwater ecosystems. As such, stream water pCO₂ andpCH₄ can provide a tool to assess ecosystem respiration andanaerobic metabolism throughout drainage basins. We conducted three surveyssampling the gas content of streams in eastern Tennessee and western NorthCarolina to assess factors regulating ecosystem metabolism in catchmentswith contrasting geomorphologies, elevations and soil organic matterstorage. In our first survey, the influence of drainage basin geomorphologyon ecosystem respiration was examined by sampling streams drainingcatchments underlain by either shale or dolomite. Geomorphology isinfluenced by geology with shale catchments having shallower soils, broader,unconstrained valley floors compared with dolomite catchments.pCO₂ varied little between catchment types but increased froman average of 3340 ppmv in spring to 9927 ppmv in summer or 9.3 and 28 timesatmospheric equilibrium (pCO_2(equilib)), respectively. Incontrast, pCH₄ was over twice as high in streams drainingshale catchments (306 ppmv; pCH_4(equilib) = 116) compared withmore steeply incised dolomite basins (130 ppmv; pCH_4(equilib)= 51). Using the ratio of pCH₄:pCO₂ as an indexof anaerobic metabolism, shale catchments had nearly twice as muchanaerobiosis (pCH₄:pCO₂ = 0.046) than dolomitedrainages (pCH₄:pCO₂ = 0.024). In our secondsurvey, streams were sampled along an elevational gradient (525 to 1700 m)in the Great Smoky Mountains National Park, USA where soil organic matterstorage increases with elevation. pCO₂ did not vary betweenstreams but increased from 5340 ppmv (pCO_2(equilib) = 15) to8565 ppmv (pCO_2(equilib) = 24) from spring to summer,respectively. During spring pCH₄ was low and constant acrossstreams, but during summer increased with elevation ranging from 17 to 2068ppmv (pCH_4(equilib) = 10 to 1216). The contribution ofanaerobiosis to total respiration was constant during spring(pCH₄:pCO₂ = 0.017) but during summer increasedwith elevation from 0.002 at 524 m to 0.289 at 1286 m. In our last survey,we examined how pCO₂ and pCH₄ changed withcatchment size along two rivers (ca. 60 km stretches in both riverscorresponding to increases in basin size from 1.7–477km² and 2.5–275 km²). pCO₂and pCH₄ showed opposite trends, with pCO₂decreasing ca. 50% along the rivers, whereas pCH₄roughly doubled in concentration downstream. These opposing shifts resultedin a nearly five-fold increase of pCH₄:pCO₂along the rivers from a low of 0.012 in headwaters to a high of 0.266 65-kmdownstream. pCO₂ likely declines moving downstream asgroundwater influences on stream chemistry decreases, whereaspCH₄ may increase as the prevalence of anoxia in riversexpands due to finer-grained sediments and reduced hydrologic exchange withoxygenated surface water.