LONGITUDINAL DYNAMICS OF DOM WITHIN HEADWATER STREAMS OF A TROPICAL EVERGREEN FOREST

Dissolved organic matter (DOM) in tropical stream ecosystems contributes to the tropical rivers that dominate the global budget of riverine runoff to the ocean. Yet, the processes that control downstream DOM quality and quantity in the tropics are much less understood compared to higher latitudes. Here, we investigate seasonal and downstream changes in DOM in the dry evergreen forest of the Rio Tempisquito watershed in Costa Rica using UV-Vis, fluorescence, and FT-ICR-MS. We found that seasonal differences in litterfall and daily changes in rainfall controlled the input of terrestrial, humic-like DOM into the stream and led to higher DOC concentrations. By analyzing downstream changes in DOM in three streams during the wet and dry seasons, we show that the average DOM molecular composition consistently shifted from reduced to more oxidized downstream. These longitudinal increases in the oxidation state of DOM coincided with an increase in a pool of humic-like DOM that is thought to come from microbial oxidation. Given that the quality of DOM input laterally should be similar across the longitudinal gradient and shading and high flow velocities should limit in-stream primary production and photodegradation, we propose that downstream changes in DOM quality in dry tropical headwater streams are driven by the preferential microbial metabolism of a pool of labile, reduced DOM compounds. These findings suggest that downstream increases in DOM oxidation state may scale with water depth, velocity, and discharge – all factors that increase the uptake length for DOM in streams.  

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Primary Presenter: Samantha Sullivan, Old Dominion University (ssulliva@odu.edu)

Authors:

Samantha Sullivan, Department of Chemistry and Biochemistry, Old Dominion University  (ssulliva@odu.edu)

Jennifer Bowen, Department of Earth System Science, Stanford University  (jcbowen@stanford.edu)

Louis Kaplan, Stroud Water Research Center  (lakaplan@stroudcenter.org)

Rose Cory, Department of Earth and Environmental Sciences, University of Michigan  (rmcory@umich.edu)

Patrick Hatcher, Department of Chemistry and Biochemistry, Old Dominion University  (phatcher@odu.edu)