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fluorescence peak (peak C), or the relative intensity of the humic-like peak versus a micro-
bially derived (or protein-like) peak (peak T and/or M). These indices have been used for a
variety of purposes from studying land use patterns to estuary dynamics and can character-
ize organic matter in filtered water samples, fulvic or humic extracts, or SOM leachates.
Owing to its high sensitivity, fluorescence is an effective technique for identifying subtle
changes in DOM character and composition. However, EEMs can be difficult to analyze
because of the large amount of information found in them. Fluorescence indices provide
a useful tool to isolate specific characteristics of the fluorescence signature that have been
correlated to aspects of DOM chemical composition. Several indices capture shifts in the
emission wavelengths of the humic-like (peak C) fluorescence which are correlated with
the aromaticity, hydrophobicity, extent of humification, source of precursor materials, and
electron shuttling ability of humics. These indices include the humification index proposed
by Kalbitz et al. (
1999
), the FI proposed by McKnight et al. (
2001
), the redox index sug-
gested by Miller et al. (
2006
), and the indices used to study humic material in stalagmites
by Proctor et al. (
2000
) and Perrette et al. (
2005
). Other indices focus on the relation-
ship between the humic-like and protein-like peaks in an attempt to compare the relative
importance of each component of organic matter. These indices include the freshness index
developed by Parlanti et al. (
2000
) and the peak T/peak C ratio introduced by Baker et al.
(2001). The humification index by Zsolnay et al. (
1999
) utilizes portions from each of these
regions. Although each of these indices was developed with a specific goal in mind, many
of them have converged to identifying similar chemical changes in DOM that cause shifts
in the EEM signature and have proven useful in studies beyond the question they were
originally developed to address.
Fluorescence indices are powerful tools for analyzing DOM and tracking variations
between samples, but doing so properly requires careful consideration of spectroscopic
techniques. It is also important to realize that each index was originally developed for a
specific set of samples, and may be useful only in certain environments, such as marine or
soil ecosystems. Thus, when choosing to apply a specific index, it is important to ensure
the index is relevant to the samples and the question at hand. The use of an established
index can help promote synthesis, comparison, and interpretation across studies and eco-
systems (Jaffé et al.,
2008
). Certainly, there may be cases where independent development
of a new index may be appropriate for a specific sample set or for use with an
in situ
probe,
and this process could be guided by our understanding as to how previous indices were
developed.
Acknowledgments
We thank Michael SanClements, Natalie Mladenov, and Paula Coble, as well as three
anonymous reviewers for their help with figures and comments on drafts of this chapter. We
also thank Eric Parrish for graphic design work on the figures. While writing this chapter,
authors were funded by NSF-0724960, NSF EAR-0738910, and ANT-0839027.
