QUANTIFYING THE EFFECTS OF LITHOLOGY AND URBANIZATION ON CONCENTRATIONS OF ORGANIC AND INORGANIC CARBON IN COASTAL MONTANE TROPICAL RIVERS

Coastal montane tropical rivers are thought to play a disproportionately large role in the global delivery of carbon from terrestrial watersheds to the world ocean. As part of the Luquillo Critical Zone Observatory, we are sampling DIC, pCO2, POC, DOC, and the optical characteristics of DOC weekly in three coastal rivers in Puerto Rico, building on a longer-term (decadal) record of weekly solute sampling. The rivers drain volcaniclastic terrain with forest cover (Rio Mameyes), volcaniclastic terrain with urban cover (Rio Piedras), and quartz diorite with forest cover (Rio Icacos). Weathering rates are rapid in each landscape, resulting in substantial concentrations of DIC. Results show a strong dependence of concentrations on stream discharge in each river, and a larger range of DOC concentrations in the quartz diorite than the two volcaniclastic terrains. Initial estimates suggest that fluxes of both organic and inorganic carbon are approximately equal in each watershed, but the balance between organic and inorganic carbon shifts toward dominance by organic fractions at high flow.