The reactivity of plant-derived organic matter in the Amazon River and implications on aquatic carbon fluxes to the atmosphere and ocean

Abstract:

The remineralization of terrestrially-derived organic carbon (OC), along with direct CO₂ inputs from autochthonous plant respiration in floodplains, results in an evasive CO₂ gas flux from inland waters that is an order of magnitude greater than the flux of OC to the ocean. This phenomenon is enhanced in tropical systems as a result of elevated temperatures and productivity relative to temperate and high-latitude counterparts. Likewise, this balance is suspected to be influenced by increasing global temperatures and alterations to hydrologic and land use regimes.

Here, we assess the reactivity of terrestrial and aquatic plant-derived OM near the mouth of the Amazon River. The stable isotopic signature of CO₂ (δ¹³CO₂) was monitored in real-time during incubation experiments performed in a closed system gas phase equilibration chamber connected to a Picarro Cavity Ring-Down Spectrometer. Incubations were performed under natural conditions and with the injection of isotopically labeled terrestrial macromolecules (e.g. lignin) and algal fatty acids. Under natural conditions, δ¹³CO₂ became more depleted, shifting from roughly -23‰ to -27‰ on average, suggesting that C3 terrestrial vegetation was the primary fuel for CO₂ production. Upon separate injections of ¹³C-labeled lignin and algal fatty acids, δ¹³CO₂ increased near instantaneously and peaked in under 12 hours. Roughly 75% of the labeled lignin was converted to CO₂ at the peak in δ¹³CO₂, whereas less than 20% of the algal fatty acids were converted to CO₂ (preliminary data subject to change). The rate of labeled-OC remineralization was enhanced by the addition of a highly labile substrate (e.g. ethyl acetate). Likewise, constant measurements of O₂/pCO₂ along the lower river revealed anomalously high CO₂ and low O₂ levels near the confluence of the mainstem and large tributaries with high algal productivity. These collective results suggest that the remineralization of complex terrestrial macromolecules is a significant source of CO₂ to tropical rivers, whereas algal-derived OC is primarily incorporated into the microbial loop/higher trophic levels and enhances the breakdown of more complex terrestrially-derived molecules (e.g. the “priming effect”).