Geoscience Reference
In-Depth Information
mixing experiment, a change in humic peaks (A and C) might reflect the phenomenon.
Indeed, a peak shift and change in relative fluorescence intensity were observed for the C
peak in response to increasing salinity.
Only recently have optical properties been linked to coagulation and flocculation associ-
ated with estuarine particulates. In a recent study, extracted aquatic humic acids were used
to assess partitioning on different sized estuarine particles (Sun et al.,
2009
). Particles were
collected from a natural estuary and thus likely contained previously sorbed organic mat-
ter (i.e., they were not pure clay minerals). Differential partitioning was observed between
fluorescence components by relating partitioning coefficients to the ratio of A to C peak
fluorescence intensity. Increased partitioning coincided with larger peak A to peak C inten-
sity ratios. This implies that larger, perhaps more aromatic humic fractions preferentially
sorb to estuarine particles. In another recent study (related to water treatment efforts), alu-
minum sulfate was used as an artificial coagulant (Gone et al.,
2009
). Below and above pH
5, coagulation was most pronounced (based on DOC measurements). At higher pH, more
net negative charge was offered as an explanation for lower DOM sorption. Coagulation or
natural DOM sorption in estuaries could produce a loss of fluorescence through increased
DOM removal via sorption and flocculation. Lead et al. (
2006
) found such an effect on sep-
aration of freshwater DOM (SPLITT; see earlier) by molecular weight, in which ca. 40%
of the T peak fluorescence was found in the >1 µm fraction - particles by operation defin-
ition. They also found no significant changes to A and C peak fluorescence with molecular
size and postulated that humic and fulvic DOM moieties existed in bound form to “clays,
biological cells, etc.” rather than in free form. In summary, modifications to DOM fluor-
escence via particle sorption phenomena may be important in environments undergoing
substantial changes in ionic strength (as in estuaries and coastal waters) as well as in fresh-
water systems.
7.9 Effect of Sunlight
Photodegradation, or photobleaching, of DOM fluorescence occurs after exposure to sun-
light and generally results in diminished emission intensity and blue shifting (Coble,
1996
).
In stratified water bodies (e.g., lakes, estuaries, and coastal waters, especially those influ-
enced by riverine discharge), the potential for DOM fluorescence bleaching is high owing
to the delivery of photoreactive fluorescent material and a residence time long enough for
photochemical bleaching reactions to occur. Light attenuation within the water column will
be important to determining overall rates (Miller,
1998
), as will light absorption by DOM
itself (cf. Osburn et al.,
2001
; Del Vecchio and Blough,
2002
; Osburn and Morris,
2003
)
Natural sunlight is polychromatic. Under polychromatic light exposure, emission
loss is broad and unstructured and extends throughout the entire spectral range, with the
bleaching always being more pronounced in the spectral region passed by the cutoff fil-
ter. Examples of fluorescence photobleaching indicate a general nonspecific decrease in
broad emission spectra (Koussai and Zika,
1990
; Kieber et al.,
1990
; Vodacek et al.,
1997
).
However, SF spectra from river DOM show some structure after photobleaching indicating
