Regional Groundwater Discharge Drives High Carbon Dioxide Emissions from a Lowland Tropical Rainforest Stream

Abstract:

Field measurements of carbon (C) fluxes are fundamental for understanding global C cycling, and the C source/sink status of ecosystems. In the tropical rainforest at La Selva Biological Station in Costa Rica, old regional bedrock groundwater (gw) high in dissolved inorganic C discharges into some streams and wetlands with possible impacts on ecosystem C pools and fluxes. We investigated carbon dioxide (CO₂) and methane (CH₄) degassing from two streams at La Selva: the Arboleda, where ~1/3 of the streamflow is from regional gw, and the Taconazo, fed exclusively by much younger local gw recharged within the catchment. In two reaches (upper and lower) of the Arboleda and Taconazo streams, emissions were determined from tracer injections. In the lower Arboleda (the only reach receiving regional gw) CO₂ fluxes (f_CO2) averaged 5.5 mol C per m² of stream surface per day, ~7.5x higher than the average (0.7 mol C m^-2 d^-1) from the stream reaches with no regional gw inflow (the Taconazo and upper Arboleda). The regional gw inflow had no measurable effect on CH₄ emissions. To further understand the dynamics of enhanced CO₂ degassing from the lower Arboleda, we examined spatiotemporal patterns in f_CO2 using floating chambers. Both static and drifting chambers revealed high spatial heterogeneity in f_CO2 at the scale of 5 to 30 m reaches. Temporal trends were highly localized; in two of three subreaches surveyed repeatedly, f_CO2 increased with stream discharge and did not differ between wet and dry seasons, but the third subreach showed the opposite behavior. Results from static and drifting chambers deviated 31% and -36%, respectively, from tracer injection results. CO₂ degassing from the Arboleda is a large C flux; when averaged over the watershed area it is similar in magnitude to the net ecosystem exchange measured by eddy covariance. Elevated CO₂ emissions from the Arboleda stream are consistent with measurements of higher CO₂ concentration in the air above the Arboleda stream, and low ¹⁴C in plants growing near the Arboleda weir, a zone of high stream gas exchange where geological CO₂ low in ¹⁴C is degassed from the stream and taken up by riparian plants. The outcomes of this research contribute to the understanding of how catchment connections to underlying hydrogeological systems can affect terrestrial ecosystem C budgets.