Seasonal and longitudinal dynamics of DOM within headwater streams of a tropical evergreen forest

Dissolved organic matter (DOM) contributes to the energy flow in tropical rivers that dominate the global budget of riverine runoff to the ocean. Yet, the processes that control downstream DOM concentration and composition in the tropics are much less understood compared to higher latitudes. Here, we investigate seasonal and downstream changes in DOM concentration and composition in headwater streams draining the seasonally dry evergreen forest of the Río Tempisquito watershed in Costa Rica. We used dissolved organic carbon (DOC) analysis as a measure of DOM concentrations and UV-Vis, fluorescence spectroscopy and FT-ICR-MS to measure DOM composition. Seasonal differences in litterfall and day-to-day changes in rainfall controlled the DOC concentration. Elevated DOC concentrations were associated with inputs of terrestrial, humic-like DOM into the stream, and light rain falling on fresh leaf litter in the dry season led to nearly equivalent DOC concentrations as heavier rain falling on degraded litter in the wet season. The average nominal oxidation state of carbon (NOSC) in DOM molecular formulae differed between wet and dry seasons (seasonally) and with distance downstream (longitudinally). A higher NOSC of stream DOM in the dry versus wet season may be due to a peak in fresh litterfall in the dry season followed by the leaching of degraded litter during precipitation in the wet season. The shift in DOM toward a higher NOSC downstream in both seasons may be due to the selective metabolism of a more reduced, labile pool within DOM during downstream transport. We propose that the downstream changes in DOM composition in the dry tropical headwater streams of this study are driven by the preferential microbial metabolism of labile, reduced DOM compounds. These findings extend the River Continuum Concept by showing that DOM oxidation state can be a sensitive indicator of microbial metabolism in the absence of measurable decreases in downstream DOM concentrations.