Geoscience Reference
In-Depth Information
DOM showed a similar red shift from 455 to 462 nm. The differential emission spectra
for each DOM type show that the majority of photobleaching occurs at the peak emission
wavelength (
Figure 7.7C
). The difference in emission peaks between these samples likely
reflects their different chemical compositions (terrestrial vs. aquatic DOM). The common
degree of red shifting (ca. 6 nm) could be due to partial oxidation of this organic matter.
Increasing the amount of carboxyl and hydroxyl groups on each DOM type would shift
fluorescence to longer wavelengths (Senesi and D'Orazio,
2005
).
Fluorescence lifetime measurements provide additional information on DOM and
humic materials' optical properties and photoreactivity. Many workers have studied decay
rates (
τ
, in nanoseconds) and have modeled multicomponent lifetimes at less than 1, 2-5,
and 6-14 ns (Clark et al.,
2002
and references therein). These studies suggest three broad
fluorophore components based on fluorescence lifetime range. Clark et al. (
2002
) photob-
leached humic Shark River (Florida) water using light at 280 nm, and found a significant
decrease in the two shorter lifetime component (at less than 1 and 2-5 ns). Interestingly,
after photobleaching Shark River DOM at a longer wavelength of 334 nm, Clark et al.
observed that the shortest lifetime component was significantly more reduced (the second
lifetime component's bleaching remain unchanged). Thus photobleaching results in meas-
urably shorter lifetimes in the resultant DOM. It remains to be seen if these lifetimes cor-
relate to specific humic peaks, though the fluorescence lifetime detection for Clark et al.'s
study was ex337/em430, which falls in the C peak region (Coble,
1996
).
Environments where ample sunlight exposure to DOM can occur will promote the great-
est fluorescence photobleaching. A clear example is the seasonal stratification that occurs
in surface waters. Gibson et al. (
2001
) observed a loss of DOM fluorescence in lakes in
the Canadian Arctic that occurred seasonally during periods of surface water stratification
and mixing that constantly bleaches the upper water column of stratified lakes. Vodacek
et al. (
1997
) and Del Vecchio and Blough (
2002
) showed substantial loss of fluorescence
in stratified coastal waters of the Middle Atlantic Bight. Similarly, Ma and Green (
2004
)
examined the EEM fluorescence effects of photobleaching in Lake Superior, finding that,
while humic fluorescence decreased with sunlight exposure, blue-shifted increases in DOM
fluorescence were observed. They suggested that new chromophores were being formed,
an effect that was also observed in seawater by Biers et al. (
2007
).
In coastal environments, mixing is important for diluting a “concentrated” DOM solu-
tion and for increasing the sunlight exposure as terrestrial runoff essentially mixes and
spreads out in a thin layer atop a denser seawater bottom layer. This creates ideal conditions
for substantial DOM photodegradation. Del Vecchio and Blough (
2002
) have shown this to
be an effective removal mechanism for DOM fluorescence, noting that the photobleaching
depth to vertical mixed depth ratio will provide an ultimate index to photobleaching extent.
Plotting shifts in emission peak intensity at fixed excitation wavelengths can indicate
whether substantial blue shifting or red shifting has occurred. These results may then be
used to infer if DOM photobleaching or dilution has occurred (Coble,
2007
; Conmy et al.,
2009
). Ultimately, these modeling constraints can then be applied to coastal observations
