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residues on larger proteins (e.g., >1000 nm), or perhaps bound to other DOM (possibly
humics). The so-called “visible humic-like” fluorescence at long wavelengths of excita-
tion and emission (C peak) is due primarily to terrestrial (or at least freshwater) humic
substances. Consequently, these humic substances tend to be smaller, being dominant in
the 1-5 kDa (1-5 nm) fraction (e.g., Stolpe et al.,
2010
). Altogether these results indicate
that colloidal fluorescence properties are strongly dominated by protein and various humic
fluorophores, whose intensity reflects both the size of DOM and its source.
7.4 Effect of Temperature
Increasing a solution temperature increases the chance for molecules to interact and thus
fluorophore collisional quenching is expected to be greater at higher temperatures. However,
not all fluorophores are deactivated in the same manner. Thus thermal quenching of DOM
fluorescence is a physical phenomenon that can be used to characterize DOM in natural
waters (Baker,
2005
; Seredynska-Sobecka et al.,
2007
) as much as it is a consideration for
the environmental conditions DOM encounters (Spencer et al.,
2007b
).
Thermal quenching appears to increase the deactivation of the excited state by internal
conversion (Senesi,
1990
). Indeed this effect was observed from 10°C to 45°C by Baker
(
2005
) on examining several riverine and wastewater DOM samples and humic standards.
Interestingly, protein fluorescence appeared most susceptible to thermal quenching in sev-
eral instances as compared to fulvic material fluorescence. As thermal quenching is sensi-
tive to the fluorophores' exposure to the energy supplied by increasingtemperature, these
results imply that some species may be more easily perturbed than others and can provide
information on DOM sources (Baker,
2005
).
Freezing and thawing may extensively modify DOM, especially if it becomes polym-
erized. DOM concentration increases during the freezing process as ice crystallizes. In
Arctic natural waters, as well as laboratory experiments, a temperature-driven size exclu-
sion DOM fractionation produces a brine that is generally higher in molecular weight and
highly fluorescent. Amon et al. (
2003
) found a subsurface fluorescence maximum in the
Arctic halocline in the Nordic Seas that followed the East Greenland Current. Similarly,
Gueguen et al. (
2007
) found a distinctive humic signal in DOM (C peak) in the Arctic halo-
cline, fluorescence that was nearly twice that of the surface layer. Belzile et al. (
2002
) used
synchronous fluorescence to study freeze up in polar lakes and rivers, finding that DOM
exclusion factors were nearly twice those for inorganic solutes. Belzile et al. (
2002
) also
found that freezing effectively size fractionates DOM owing to the inability of ice crystal
structures to accommodate large compounds. This ultimately had the result of diminished
humic acid fluorescence after separation into <30 kDa and <5 kDa fractions. Notably, some
of these effects could be due to pH or metal ion concentrations changing with brine forma-
tion. Amon et al. (
2003
) was able to use DOM fluorescence to track brine formation in the
Arctic Ocean during sea ice formation and determined this process to be a major mechan-
ism for organic carbon transport (and transformation) in the Arctic. The large amount of
terrigenous, highly fluorescent DOM delivered by high-latitude rivers to the Arctic Ocean
