Geoscience Reference
In-Depth Information
3.3.1 Temporal Variation in DOM Source and Dynamics
The examination of DOM fluorescence in freshwater ecosystems with respect to temporal
variability has been undertaken at a range of time scales from diurnal to seasonal. Seasonal
variation in humic-like and protein-like fluorescence has been used to show increasing
autochthonous inputs (i.e., peak in protein-like fluorescence), increasing allocthonous
inputs (i.e., peak in humic-like fluorescence), and thus the dominance of biological or
hydrological controls on the ecosystem (Jaffe et al.,
2008
; Miller and McKnight,
2010
).
Simple DOM measurements such as the fluorescence index (ratio of
λ
em
470 to 520 nm at a
λ
ex
of 370 nm; McKnight et al.,
2001
; Cory et al.,
2010
) has been linked to aromaticity and
indicates the relative contribution of low molecular weight (LMW) nonaromatic DOM ver-
sus high molecular weight (HMW) aromatic (see
Chapter 9
). Therefore, fluorescence index
(FI) has been used to track autochthonous versus allocthonous changes with respect to
seasonal DOM inputs in freshwater ecosystems (Hood et al.,
2005
; Miller and McKnight,
2010
). FI has also been shown to change due to increased flow path, residence time, and
thus greater microbial mineralization of DOM, as well as due to extensive flushing of
organic rich horizons (i.e., source materials) during wet periods in a tropical ecosystem
(
Figure 3.7
; Spencer et al.,
2010
). The relatively lower FI during the flushing period shown
in
Figure 3.7a
highlights that DOM during this period is enriched in HMW compounds
and aromatic moieties compared to other times of the year due to the leaching of fresh
organic rich layers (Spencer et al.,
2010
). This is further supported by the linear correlation
observed between lignin carbon-normalized yields (
Λ
8
; which indicates the contribution of
vascular plant-derived material to the DOM pool) and FI shown in
Figure 3.7b
(Spencer
et al.,
2010
).
The impact of short-term events such as storms and salmon runs has also been exam-
ined via DOM fluorescence characterization. Hood et al. (
2007
) presented data from an
Alaskan (USA) salmon spawning stream and showed that DOM derived from salmon car-
casses during the spawning period is elevated in protein-like fluorescence. The DOM in the
stream at the time of salmon spawning is therefore distinct from the humic-like material
that dominates the stream DOM load during the rest of the year and that is derived pre-
dominantly from wetlands (Hood et al.,
2007
). Stormflows can represent a substantial por-
tion of seasonal and annual catchment DOM export as concentrations of DOM typically
increase with increasing discharge. Therefore, understanding the quality of DOM exported
during stormflows is often critical for understanding DOM biogeochemical processes in a
catchment. Utilizing DOM fluorescence measurements Fellman et al. (
2009a
) tracked the
contribution of protein-like fluorescence and humic-like fluorescence during storm events
and highlighted as hydrologic flowpaths changed to near surface soil horizons the DOM
transported into streams was similar to that in soil solution. Recent developments with
respect to
in situ
fluorescence techniques are now opening up the possibility of moni-
toring storm events at the temporal resolution required to capture these highly dynamic
events with respect to DOM quantity and quality (Saraceno et al.,
2009
). The potential of
high temporal resolution
in situ
monitoring of DOM fluorescence to capture diurnal DOM
