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Shining light on the storm: in-stream optics reveal hysteresis of dissolved organic matter character

Abstract

The quantity and character of dissolved organic matter (DOM) can change rapidly during storm events, affecting key biogeochemical processes, carbon bioavailability, metal pollutant transport, and disinfection byproduct formation during drinking water treatment. We used in situ ultraviolet–visible spectrophotometers to concurrently measure dissolved organic carbon (DOC) concentration and spectral slope ratio, a proxy for DOM molecular weight. Measurements were made at 15-minute intervals over three years in three streams draining primarily agricultural, urban, and forested watersheds. We describe storm event dynamics by calculating hysteresis indices for DOC concentration and spectral slope ratio for 220 storms and present a novel analytical framework that can be used to interpret these metrics together. DOC concentration and spectral slope ratio differed significantly among sites, and individual storm DOM dynamics were remarkably variable at each site and among the three sites. Distinct patterns emerged for storm DOM dynamics depending on land use/land cover (LULC) of each watershed. In agricultural and forested streams, DOC concentration increased after the time of peak discharge, and spectral slope ratio dynamics indicate that this delayed flux was of relatively higher molecular weight material compared to the beginning of each storm. In contrast, DOM character during storms at the urban stream generally shifted to lower molecular weight while DOC concentration increased on the falling limb, indicating either the introduction of lower molecular weight DOM, the exhaustion of a higher molecular weight DOM sources, or a combination of these factors. We show that the combination of high-frequency DOM character and quantity metrics have the potential to provide new insight into short-timescale DOM dynamics and can reveal previously unknown effects of LULC on the chemical nature, source, and timing of DOM export during storms.

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Acknowledgements

This material is based upon work supported by the National Science Foundation under VTEPSCoR Grant No. EPS-1101317, EPS-IIA1330446, and OIA 1556770, and NSF EAR Grant No. 1561014 to AWS. We thank Ryan Sleeper, Saul Blocher, Joshua Benes, and two anonymous reviewers for their helpful contributions to this work. Any opinions, findings, and conclusions, or recommendations expressed in this material are those of the authors and have been approved by the U.S. Geological Survey, but do not necessarily reflect the views of the National Science Foundation, Vermont EPSCoR, or any other supporting organization. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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Correspondence to M. C. H. Vaughan.

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Vaughan, M.C.H., Bowden, W.B., Shanley, J.B. et al. Shining light on the storm: in-stream optics reveal hysteresis of dissolved organic matter character. Biogeochemistry 143, 275–291 (2019). https://doi.org/10.1007/s10533-019-00561-w

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  • DOI: https://doi.org/10.1007/s10533-019-00561-w

Keywords

  • High-frequency sensors
  • Dissolved organic matter
  • Spectral slope
  • Hysteresis
  • Storms