Hydrologic and Land Cover Effects on Sources and Fate of In-Stream Fluorescent Dissolved Organic Matter

Dissolved organic matter (DOM) is a critical component of the carbon cycle linking
terrestrial and aquatic ecosystems. Although many factors influence DOM fluxes and
quality in rivers, controls on DOM compositions in catchments of the western U.S. are
poorly understood. UV and fluorescent spectroscopy is a simpler, faster, and less
expensive DOM fingerprinting technique compared to techniques, such as nuclear
magnetic resonance (NMR) spectroscopy or wet chemical fractionation, and could be
useful for characterizing complex DOM chemistry. However, only 1 % of DOM is
estimated to be fluorescent, and the utility of UV and fluorescent spectroscopy for DOM
characterization needs to be further investigated. This dissertation applied UV and
fluorescent spectroscopy to examine hydrologic and land cover controls on DOM
chemistry in streams of 45 catchments in the forested headwaters and a mixed landscape
of the Willamette River Basin, Oregon, based on two years of monitoring. This
dissertation contributes three major findings. First, freezing of water alters DOM
chemistry by preferentially precipitating aromatic DOM. Second, UV and fluorescent
spectroscopy was able to discriminate DOM delivered from highly processed, proteinrich
deep subsurface sources during dry seasons especially in forested headwater streams of the H. J. Andrews Experimental Forest (HJA), where nitrogen inputs are very low. In
addition, fluorescent DOM chemistry differed among watersheds with varying forest
management history. Third, although fluorescent DOM in a headwater forested system
differed among land use history, fluorescent DOM composition did not vary among
streams draining a well-mixed landscape of urban, pasture/hay, forest, and agricultural
land cover types in the central Willamette River Basin, where nitrogen inputs to streams
are relatively high. Dissolved organic carbon (DOC) concentration decreased and the
fluorescent index indicated an increase in terrestrial sources of DOM from small (1st and
2nd order) to large (4th to 6th order) streams. A protein-like DOM component that was
detected in the headwater forested study site (HJA) was not detected in stream samples
from the middle basin study site. These findings indicate that in-stream respiration,
fueled by nutrient additions from agriculture runoff, consumes bioavailable, labile DOM
(proteins) preferentially relative to more recalcitrant, terrestrial sources of DOM along
water flow paths from headwater streams to major rivers. This study shows the clear
applicability of the fluorescent characterization of DOM in identifying hydrologic and
landscape controls as well as varying DOM chemistry and functions throughout
watershed ecosystems.