Geoscience Reference
In-Depth Information
from 450 to 440 occurred from 0 to 10 salinity for the 10-50 and >50 kDa fractions, sim-
ilar to the Chesapeake Bay results from Boyd and Osburn (
2004
), then remained stable at
salinity from 10 to 33. No change in emission peak position was found for the <10 kDa
size fraction. Only slight changes were observed in fluorescence intensity, yet fluorescence
efficiency decreased markedly in each size class from ~25 to 33 salinity.
Moreover, salinity appears to have contrasting effects on DOM removal mechanisms,
especially photodegradation by sunlight. Minor et al. (
2006
) observed no effect on DOM
photomineralization as a function of salinity, yet Osburn et al. (2009a) and Grebel et al.
(
2009
) found distinct effects of salinity and halides, respectively, on DOM photobleaching.
Grebel et al. (
2009
) specifically found a halide ion effect on Suwannee River DOM absor-
bance photobleaching but not on DOM fluorescence photobleaching. Halide ion abundance
may become a factor in natural systems such as estuaries, where halide-rich seawater mixes
with freshwater, and in regions where halide-rich groundwater mixes with surface waters.
Halide ion effects due to waste treatment chlorination have been shown to narrow DOM
fluorescence emission bands (termed “contraction”) and to blue-shift fluorescence maxima
(Korshin et al.,
1999
).
7.8 Effect of Particles
Particles such as Fe- and Al-hydroxides and clay minerals have the capacity to sorb
organic components, perhaps even more so when “coated” with humic materials (Zhou
and Rowland,
1997
). Particle association can influence DOM fluorescence in soils and
aquatic sediments (Kaiser and Guggenberger,
2000
). Two mechanisms are likely at play:
first, through humic sorption to clays (which does not appear to alter their physicochemical
properties; Zhou et al.,
1994
) and second, by proteinaceous DOM sorption. Humic materi-
als exist as macro-ions contributing varying sorptive capacity to clay minerals. In estuar-
ies, acidic functional groups are impacted by conditions such as pH (decreasing sorption
with increasing pH) and salinity changes (increasing sorption with increasing salinity)
(Zhou et al.,
1994
; Specht et al.,
2000
). Although OM removal in estuaries has been pos-
tulated to occur by “salting out” (solubility decreased due to increasing salinity), most
studies have demonstrated an increased sorptive capacity for minerals as salinity increases
(Means,
1995
; Zhou and Rowland,
1997
). Mechanistically, as DOM becomes less soluble,
it becomes more surface-active and preferentially sorbs to soil or aquatic particles. In ter-
restrial environments, especially in agricultural regions, salinization could cause similar
effects in soil DOM (Cilenti et al.,
2005
; Provenzano et al.,
2008
,
2010
).
Differential humic or proteinaceous material sorption to particles may impact fluor-
escence. Many studies have assessed the impact of cations at various concentrations - to
simulate estuarine mixing - on natural DOM fluorescence (cf. Antunes et al.,
2007
). In
the Alberts et al. (
2004
) study highlighted earlier in
Section 7.7
, the authors speculated
humic material coagulation caused a decrease in absorbance at higher salinities. The sal-
inity range causing the effect was similar to that reported (above) having maximal impact
on clay-DOM sorption. If humic materials were preferentially lost to particles during this
