The effect of regional groundwater on carbon dioxide and methane emissions from a lowland rainforest stream in Costa Rica

Abstract

In the tropical rainforest at La Selva Biological Station in Costa Rica, regional bedrock groundwater high in dissolved carbon discharges into some streams and wetlands, with the potential for multiple cascading effects on ecosystem carbon pools and fluxes. We investigated carbon dioxide (CO 2 ) and methane (CH 4 ) degassing from two streams at La Selva: the Arboleda, where approximately one third of the streamflow is from regional groundwater, and the Taconazo, fed exclusively by local groundwater recharged within the catchment. The regional groundwater inflow to the Arboleda had no measurable effect on stream gas exchange velocity, dissolved CH 4 concentration, or CH 4 emissions but significantly increased stream CO 2 concentration and degassing. CO 2 evasion from the reach of the Arboleda receiving regional groundwater (lower Arboleda) averaged 5.5 mol C m À2 d À1 ,~7.5 times higher than the average (0.7 mol C m À2 d À1 ) from the stream reaches with no regional groundwater inflow (the Taconazo and upper Arboleda). Carbon emissions from both streams were dominated by CO 2 ; CH 4 accounted for only 0.06-1.70% of the total (average of both streams: 5 × 10 À3 mol C m À2 d À1 ). Annual stream degassing fluxes normalized by watershed area were 48 and 299 g C m À2 for the Taconazo and Arboleda, respectively. CO 2 degassing from the Arboleda is a significant carbon flux, similar in magnitude to the average net ecosystem exchange estimated by eddy covariance. Examining the effects of catchment connections to underlying hydrogeological systems can help avoid overestimation of ecosystem respiration and advance our understanding of carbon source/sink status and overall terrestrial ecosystem carbon budgets.Regional groundwater flow is a natural hydrogeological process by which groundwater moves long distances beneath surface topographic divides, possibly recharging in one watershed and discharging in another many kilometers away [Tóth, 2009;Schaller and Fan, 2009;Smerdon et al., 2012;Pacheco, 2015], thus creating the potential for relatively long-distance subsurface transport of C between watersheds and ecosystems. In addition to biogenic contributions from the surface, dissolved CO 2 and CH 4 in regional groundwater can originate from volcanic degassing, nonvolcanic escape of gases from the upper mantle, intrusive magma chambers, carbonate-bearing rocks in the crust, hydrocarbon accumulations [Mörner and Etiope, 2002], and remineralization of ancient sedimentary organic matter [Lovley and Anderson, 2000;Park et al., 2009;Liu et al., 2014]. Aquifers transporting high concentrations of dissolved C have been identified in numerous places around the world, for example, Portugal [Cruz and Amaral, 2004], the Canary Islands [Marrero et al., 2008], OVIEDO-VARGAS ET AL.

Seasonal Shift From Biogenic to Geogenic Fluvial Carbon Caused by Changing Water Sources in the Wet‐Dry Tropics

“…First, the effect of CO 2 evasion upon groundwater emergence likely resulted in enriched δ 13 C DIC values in the river relative to shallow groundwater, thereby causing a potential underestimation of the shallow groundwater contribution to the DIC flux in the mixing analysis. We would indeed expect large proportions of DIC being delivered to the Howard River via shallow flow paths, as p CO 2 within the shallow aquifer was >40,000 ppm on average (Rudge, ), and generally, the role of subsurface waters as carrier of biogenic C is increasingly recognised in the tropics (Duvert et al, ; Johnson et al, ; Oviedo‐Vargas et al, ).…”

“…In contrast, where DIC originates from deeper groundwater, baseflow conditions can result in higher riverine DIC concentrations (Mann et al, ; Shih et al, ). Because different water flow paths can carry DIC of different origins (Oviedo‐Vargas et al, ), understanding the DIC export dynamics in rivers of the wet‐dry tropics requires an assessment of both water and DIC sources and pathways.…”

Seasonal Drivers of Carbon Dioxide Dynamics in a Hydrologically Modified Subtropical Tidal River and Estuary (Caboolture River, Australia)

“…Groundwater is known to be a source of CO 2 to surface waters, and other studies using natural tracers have shown that groundwater can be a major driver of surface water p CO 2 (Atkins et al, ; Maher et al, ; Sadat‐Noori et al, ; Oviedo‐Vargas et al, ). Over the annual cycle, positive significant linear relationships ( p < 0.001) were found between p CO 2 and 222 Rn (Figure ).…”

Effect of hydroperiod on CO2fluxes at the air-water interface in the Mediterranean coastal wetlands of Doñana

“…Aquifers transporting high concentrations of dissolved C have been identified over the world [ Cruz and Amaral , ; Marrero et al , ; Evans et al , ; Kampman et al , ; Yamada et al , ; Genereux et al , ; Rivé et al , ; Kucharič et al , ]. It has been also shown that groundwater discharge from aquifers represents a significant source of atmospheric CO 2 to stream waters [ Crawford et al , ; Dinsmore et al , ; Oviedo‐Vargas et al , ] and coastal wetlands [ Cai et al , ; Atkins et al , ]. In this case, water composition of the Doñana aquifer has been found to be similar to that of precipitation water although more concentrated due to the effect of evapotranspiration in the soil, being also characterized by a slightly acid pH and with high concentrations of nitrates of agricultural origin [ Olías et al , ].…”