Does photomineralization of dissolved organics matter in temperate inland waters?
Abstract
Sunlight can oxidize dissolved organic carbon (DOC) to dissolved inorganic carbon (DIC) and carbon dioxide (CO2) in freshwaters. The importance of complete photooxidation, or photomineralization, as a sink for DOC and source of CO2 remains unclear at the scale of entire watersheds, as most studies are restricted to single water bodies. In this study, we construct a routed mass balance model representing over 70,000 river and stream reaches in the Connecticut River Watershed (CRW), USA, to calculate spectrally resolved photomineralization. We test the hypothesis that photomineralization is a negligible DOC sink and CO2 source across all reaches and flow conditions relative to DOC fluxes and biomineralization (i.e., respiration) rates. Our model is unique in that it quantifies the inter-dependence of reaction rates and transport drivers within the watershed reaches for the full range of historically observed flow conditions, incoming solar irradiance, and canopy cover shading observed throughout the year. Our model predicts average daily areal photomineralization rates in line with other measured photomineralization rates for freshwater bodies, but we find that even for high photomineralization fluxes, corresponding photomineralization uptake velocities are typically at least an order of magnitude smaller than those reported for other instream processes. To directly compare photomineralization fluxes with respiration, as well as apply the model to lentic water bodies, we extract regressions predicting first order rate constants for photomineralization and use them in a larger DOC mass balance model that also predicts respiration, photosynthesis, and terrestrial loading to all lakes and river reaches in the CRW. In median flows and mean light intensities, for an average watershed travel distance, 3-5% of the DOC fluxes are eliminated, indicating that photomineralization is a minor DOC sink in temperate rivers.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFM.B22A..02M