Geoscience Reference
In-Depth Information
moieties of DOM change configuration at higher ionic strengths, similar to proteins in bio-
logical systems (Boyd et al.,
2010a
), and quench or inhibit fluorescence.
Anions such as halides, hydroxyl, nitrite/nitrate, and carbonate, all prominent in nat-
ural waters, have been examined for their ability to quench the fluorescence of aromatic
compounds by heavy atom effects and electron and charge transfer mechanisms (Watkins,
1974
; Shizuka et al.,
1980
; Treinin et al.,
1983
; Mac,
1995
). Thus, ionic strength effects
on DOM fluorescence could result from quenching effects of these solutes. Ghosh and
Schnitzer (1979) identified decreases in excitation spectra intensity of soil HA and FA
as NaCl concentration was increased from 0.001 to 0.1
M
. They suggested that increased
molecular “coiling” (e.g., Conte and Piccolo,
1999
) and/or decreased ionization of pheno-
lic hydroxyl moieties (Senesi,
1990
) caused the observed fluorescence quenching. Thus, a
major synergistic effect of increased ionic strength on DOM fluorescence could result from
suppression of functional group ionization, coincident with the metal-ligand quenching
previously described. It is also possible that photochemically formed and short-lived radi-
cals or excimers (an excited state dimer) or exciplexes (excited state complex), especially
when aromatic compounds are the excited species (Mac et al.,
1993
) can modify, or inter-
fere with, fluorescence (Senesi,
1990
). Further, alkyl halides formed via the halogenation
of DOM can also alter DOM fluorescence as halogen substitution could also produce an
internal “heavy atom” effect (Senesi,
1990
; Senesi and D'Orazio,
2005
). This effect could
explain the blue shift in hydrophobic HS fluorescence (including a narrowing of the emis-
sion bandwith) observed in chlorination reactions (Korshin et al.,
1999
). Similar mecha-
nisms during water treatment activities probably reduce fluorescence as DOM is oxidized
(e.g., Henderson et al.,
2009
).
Despite these examples, scant data exist to support a large effect of ionic strength on
DOM fluorescence in laboratory studies. Mobed et al. (
1996
) examined EEMs of a peat-
derived FA from 0 to 1
M
KCl and found visual changes that were not statistically sig-
nificant, based on a matrix correlation method. One way to evaluate an effect of ionic
strength on DOM fluorescence (which would exclude mixing of water masses such as in
the coastal ocean) is to examine saline lakes situated in arid regions and hydrologically iso-
lated (
Figure 7.5
). Although the ionic compositions of saline lake water can differ markedly
from seawater, these systems can serve as models for the progression of DOM properties
during evaporation (though often their salinities can fluctuate seasonally).
Studies of DOM from saline lakes and wetlands suggest that autochthonous DOM is
dominant in these systems, partially attributed to their eutrophic to mixotrophic nutri-
ent states (Leenheer et al.,
2004
; Ortega-Retuerta et al.,
2007
). Recently, DOM fluores-
cence spectra from saline lakes in the Great Plains of the United States were analyzed
with PARAFAC (Osburn et al.,
2011
). A component from their PARAFAC model resem-
bled the prominent INT peak from
Figure 7.5
, a fluorescent signal also prominent in a
saline lake from Antarctica dominated by autochthonous production (McKnight et al.,
2001
) and in saline ponds situated in the Brazilian Patanal wetland (Mariot et al.,
2007
).
Compiling results from several studies, Osburn et al. (
2011
) also found a consistent slope
value (0.534 ± 0.127) for log-log regressions of DOC on conductivity for a range of inland
