Geoscience Reference
In-Depth Information
The wide variation in physical and chemical properties of natural waters imparts wide
variation in the attendant effects on DOM fluorescence. Freezing and thawing may exten-
sively modify DOM, especially if it becomes polymerized, but cyclical dehydration and
rehydration appears to be more critical, making aridity a considerable influence on soil
DOM fluorescence. Changes in DOM fluorescence can occur from changes to ionization
potential in phenolic and carboxylic fluorophores, as well as intramolecular rearrange-
ments. Therefore, solution pH exerts a major control on DOM fluorescence. In addition,
metal-ligand complexation (primarily by salicylic or phenolic moieties within DOM) gen-
erally causes fluorescence quenching and is strongly influenced by pH. However, some
fluorescence intensity increase can occur from Mg(II) and Al(III) metal ions when they are
complexed with DOM. Changes to ionic strength appear to have a lesser effect on DOM
fluorescence than pH and metal-ligand interactions. A major synergistic effect of increased
ionic strength on DOM fluorescence could result from suppression in functional group
ionization, coincident with the metal-ligand quenching previously described. Moreover,
intramolecular rearrangements and ion suppression are likely mechanisms leading to fluor-
escence quenching in high ionic strength solutions. Photodegradation, or photobleaching,
of DOM fluorescence occurs after exposure to sunlight and generally results in diminished
emission intensity and blue shifting of fluorescence to shorter wavelengths, and to some
degree is a function of molecular size, pH, and ionic strength.
This chapter explores the physical and chemical effects on DOM fluorescence in nat-
ural waters, with a focus on the changes in fluorescence induced by changes in pH, ionic
strength, metal-DOM interactions, temperature, and particle size (colloidal versus non-
colloidal). First, we discuss the topic of quenching as a mechanism with which to examine
physicochemical effects on fluorescence. This is followed by an examination of the effect of
molecular weight and size of fluorophores, prior to discussing, in turn, the effects of tempera-
ture, pH, metals, ionic strength, and particles. Finally, the interaction of these environmental
effects and photobleaching is discussed. The aim is to provide the reader with an idea of how
the physicochemical environment changes the fluorescent properties of DOM through either
fluorescence quenching or enhancement. We have attempted to draw from the freshwater,
marine, and soil DOM literature to connect these topics. A key literature review on DOM
fluorescence and environmental effects studied by emission spectroscopy and synchronous
fluorescence (SF) spectroscopy will be introduced and supplemented by current literature
utilizing excitation-emission matrix (EEM) fluorescence analyses where possible. For this
discussion, we use the convention of ex
λ
/em
λ
to denote excitation wavelength and emission
wavelength,
λ
, respectively. For SF, we use SF
λ
to denote excitation wavelength and indicate
the offset value (Δ
λ
) where appropriate. For the EEM discussion, we follow the typical B,
T (protein); C and A (terrestrial humic); M (marine humic); and N (phytoplankton-derived)
peak assignments developed elsewhere (Coble,
1996
; Stedmon et al.,
2003
).
7.2 The Quenching of DOM Fluorescence
A consideration of environmental effects on fluorescence is centered on how the physico-
chemical environment either increases or decreases DOM fluorescence through a variety
