Spatial variation in groundwater and stream water carbon chemistry in a Neotropical rainforest

Background/Question/Methods

            Understanding global carbon cycling is increasingly important in a world facing CO₂-induced warming.  The significance of riverine carbon transport and storage is now widely acknowledged; however, most research on carbon cycling in freshwater systems has been conducted either in temperate zones or in large tropical rivers.  This study addresses carbon cycling in small tropical streams.  Groundwater seeps and surface water from low-order streams draining La Selva Biological Station in lowland Costa Rica were sampled in the dry season for CO₂, pH, DOC, conductivity, and reduced iron (since oxidized iron is notably present at some seeps when groundwater is exposed to air).  CO₂ concentrations were determined via headspace equilibrium analyses, and Fe²⁺ concentrations were calculated using the bipyridine/acetate spectrophotometric method.

Results/Conclusions

CO₂ concentrations ranged from 0.2% to 11% in seeps, with no clear patterns of association with pH, conductivity, or concentrations of Fe²⁺ or other solutes.  CO₂ concentrations ranged from 0.1% to 9% in surface water, but mean seep CO₂ concentration was nearly double that of surface water, indicating that CO₂ rapidly evades (outgasses) when CO₂-supersaturated groundwater is exposed to the atmosphere.  These results suggest that riverine evasion of inorganic carbon derived from soil respiration may be an important pathway in the carbon cycle of Central American rainforests.