Geoscience Reference
In-Depth Information
provided valuable insight into the differences in DOM fluorescence properties between
freshwater and marine systems. However, we suggest that three environmental processes
will exert the largest effect on DOM fluorescent properties in surface waters. First, surface
water acidification that causes substantial decreases in pH below about 5 could induce
diminished fluorescence and blue shifting seen by Laane (
1982
), Mobed et al. (
1996
), and
Spencer et al. (2007b) among others. Evidence that decreasing pH or increasing pH (Reche
et al.,
1999
), as well as modifying ionic strength (Osburn et al.,
2009a
; Boyd et al.,
2010b
),
can promote photochemical reactions should also be noted, even if the effects to DOM
fluorescence specifically are not apparent (Grebel et al.,
2009
). Whereas pH can change
spectral shapes and lead to diminished fluorescence, it appears that metal quenching solely
leads to diminished fluorescence intensity. The exception is Al(III)-OM complexes, which
amplify DOM fluorescence in humic regions, therefore increasing fluorescence intensity in
addition to imposing red shifting in emission spectra.
Second, photobleaching can permanently alter DOM fluorescence by degrading fluor-
ophores. The photobleaching behavior under monochromatic and polychromatic light
exposure cannot be explained by a simple superposition of non interacting fluorophores.
Therefore, the interactivity of photobleaching on the molecular properties of DOM as
it relates to bulk DOM fluorescence properties should be addressed. Advanced mass
spectroscopy techniques such as Fourier transform-ion cyclotron resonance mass spec-
trometry (FT-ICR-MS) are currently being applied to these problems (e.g., Kujawinski
et al.,
2004
; Mopper et al.,
2007
; Gonsior et al.,
2009
) and should provide substantial
information regarding the molecular changes DOM undergoes as it is degraded. The
information from this technique could provide insight into the molecular classes in given
fluorophores.
Third, surface water metal speciation will offer multivalent elements that complex with
available DOM and either quench or enhance DOM fluorescence. As erosion in upland
catchments and aeolian rain and dust inputs increase, so should the metal content of surface
waters. DOM fluorescence will respond accordingly. A possible future direction of study
would be relating the regional mineralogy of a watershed (inland or coastal) to the DOM
fluorescence properties in its streams and catchments. Gradual changes in DOM charac-
teristics over time may therefore be related to the concentration and abundance of metals
released into solution via weathering or anthropogenic inputs.
Ionic strength effects are more difficult to resolve. Although salinity was shown to
increase DOM fluorescence to some degree, its overall quenching effect seems minimal,
especially in coastal waters where freshwater and seawater continuously mix. The saline
lake data provided a similar finding for DOM yet in a different chemical milieu than estu-
arine waters. Evapoconcentration of lake water does appear to alter DOM fluorescence
properties as ionic strength increases. By contrast, the salinization of soils dramatically
increased the ionic strength of soil solution and decreased the fluorescence as soil pore
water DOM structures coil (possibly inducing steric hindrance) and functional groups ion-
ize (Provenzano et al.,
2008
). Thus, DOM fluorescence offers the potential for new inves-
tigations in relation to climate and land use change.
