Fluorescence index as an indicator of dissolved organic carbon quality in hydrologic flowpaths of forested tropical watersheds
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Over two hundred samples were collected in tropical headwater forested catchments in the lowland Amazon basin near Juruena, Mato Grosso Brazil. These were analyzed for fluorescence characteristics and DOC concentrations, and represented a range of terrestrial hydrologic flowpaths and first-order streams during baseflow and stormflow conditions. The fluorescence index (FI) of McKnight et al. (2001) was found to have a significant relationship with DOC concentrations for stream water at baseflow conditions, but FI values within individual terrestrial flowpaths and stormflow varied little for the range of DOC concentrations observed. FI values were seen to increase for increasing residence time of water within the terrestrial ecosystem, while DOC concentration decreased for increasing hydrologic residence time. The FI of terrestrial flow paths indicated that DOC became increasingly characterized by microbially derived carbon for flow paths with longer residence times, on the order through fall and overland flow < percolating soil water < groundwater. Base flow samples of stream water had a mean FI value of 1.78, compared with 1.51 and 1.44 for through fall and overland flow, respectively, and 1.65 for percolating soil water. The FI values for stream water at base flow were also seen to vary seasonally, and were inversely proportional to DOC concentrations over time.
KeywordsBaseflow Fluorescence index Emergent groundwater Overland flow Stormflow Throughfall
The study was supported by NASA LBA-ECO grant to project group ND-11. The authors greatly appreciate the collaborations of field site hosts Rohden Indústria Lígnea Ltda. and Apolinário Stuhler. We thank Benedito Silveira de Andrade and Elielton Anterio da Souza for field assistance. Discussions with Robert G.M. Spencer and the comments provided by the anonymous reviewers were very useful in revising the manuscript.