Geoscience Reference
In-Depth Information
variability has also been described and thus the ability to examine DOM processing in real-
time (Spencer et al.,
2007
).
3.3.2 Anthropogenic and Land Use Impacts on DOM
Anthropogenic sources of pollution to freshwater ecosystems can be examined via fluores-
cence characterization of DOM. For example, Baker et al. (2002) highlighted the influence
of wastewater from a large tissue paper mill through elevated protein-like fluorescence
intensities and a fluorescence center attributed to fluorescent whitening agents. Similarly,
Baker and Inverarity (
2004
) described an urban catchment with elevated wastewater inputs
and showed protein-like fluorescence correlated to biological oxygen demand (BOD) and
inorganic N and P concentrations. DOM fluorescence has also been used to investigate dif-
fuse agricultural pollution resulting from slurry spreading (Naden et al.,
2010
). A strong
linear relationship was found between fluorescence intensity and slurry concentration, and
fluorescence ratios were described that allowed slurry impacted waters to be discriminated
from uncontaminated waters (Naden et al.,
2010
). Such studies examining anthropogenic
pollution in freshwater systems will benefit greatly from the development and utilization of
in situ
fluorescence sensors and portable monitoring devices that allow real-time monitor-
ing of DOM fluorescent components (Baker et al.,
2004
; Spencer et al.,
2007
).
As well as examining point and diffuse anthropogenic sources of DOM, fluorescence
characterization has also been utilized to investigate the current and future effects of land use
and land management practices on freshwater biogeochemistry (Wilson and Xenopoulos,
2009
; Williams et al.,
2010
). A study examining a land use gradient of increasing cropland
coverage and decreasing wetland coverage showed that the structural complexity of DOM
decreased as the ratio of continuous croplands to wetlands increased, as evidenced by the
FI and humification index (Zsolnay et al.,
1999
; see
Chapter 9
;
Figures 3.8a,b
). Therefore,
simple fluorescence ratios can be used to examine how land use changes have impacted on
the biogeochemical role of DOM in freshwater ecosystems.
3.3.3 Transformations and Reactivity
The redox state and the rates of oxidation/reduction of DOM can be examined via the
redox index (RI; derived from
Q
red
/(
Q
red
+
Q
ox
) where
Q
red
and
Q
ox
are the sum of the load-
ings of the reduced and oxidized and quinone-like components respectively as defined by
Cory and McKnight [
2005
]) in freshwater ecosystems. For example, Miller et al., (
2006
)
investigated nutrient cycling in a wetland-stream ecosystem using RI to show that DOM
in a wetland adjacent to the stream was reduced. In addition, dissolved iron concentra-
tions were elevated and ammonium was the primary species of inorganic nitrogen in the
wetland. Thus, as water from the wetland entered the stream, DOM was rapidly oxidized
with a corresponding shift in RI, ammonium was converted to nitrate, and dissolved iron
concentrations decreased (Miller et al.,
2006
). Therefore, fluorescence measurements can
