Geoscience Reference
In-Depth Information
of fluorescent materials from global rivers. In a subsequent study, Yamashita and others
(
2010
) performed EEM analysis on samples from two other Pacific Ocean sites and mod-
eled fluorescence composition using PARAFAC. They found two humic-like components
(M and C), one protein-like component (B), and an unknown component. The humic-like
components reached maximum values in the mesopelagic zone and decreased below these
depths. Both components likewise were correlated with AOU in the bathypelagic layer
and the ratio of the two components showed a relative narrow range of values in the deep
waters. The implications of these findings are that both humic-like C and humic-like M are
produced by bacterial respiration in the deep ocean in relatively constant proportions, and
neither is significantly altered by biogeochemical processes subsequent to formation until
returned to the surface ocean.
Increased interest in the Arctic Ocean as a site of rapid climate change has stimulated
observations of CDOM in this region. Arctic rivers are high in DOC, with concentrations
in the range of 1-10 mg L
-1
(Retamel et al.,
2007
; Hessen et al.,
2010
). Warming due to
climate change is already melting permafrost and is expected to increase freshwater dis-
charge (ACIA, 2004). Several early studies showed that CDOM fluorescence was a tracer
of DOC in the Arctic (Guay et al.,
1999
; Amon et al.,
2003
). Terrestrial CDOM dominates
the surface waters and can be traced 50-400 km offshore in plumes (Retamel et al.,
2007
),
but there is also evidence for autochthonous CDOM. A strong correlation exists between
CDOM fluorescence and lignin phenol concentration (Amon et al.,
2003
; Walker,
2009
).
Walker et al. (
2009
) identified six PARAFAC components across the Arctic region. Four
of these were related to lignin phenols, indicating a terrestrial source, while two others
appeared to be of autochthonous origin. Three distinct water masses were identified using
PARAFAC components, illustrating the potential usefulness of fluorescence in studies of
Arctic Ocean circulation.
3.3 Fluorescence in Freshwater
The investigation of dissolved organic matter (DOM) fluorescence is now widely under-
taken in the freshwater sciences. Such studies utilizing fluorescence techniques focusing
on freshwater ecosystems have a range of aims such as examining the optical properties,
composition, source, redox state, and biological and photochemical reactivity of DOM.
Two discrete DOM fluorescent groups have been reported in freshwater ecosystems, with
one group having properties similar to humic-like material and the other group with prop-
erties similar to protein-like material. Currently the exact chemical compounds respon-
sible for DOM fluorescence are still unknown, however, a great deal of knowledge can
be obtained about their environmental role in freshwater ecosystems by tracing their
dynamics.
The incorporation of fluorescence measurements into DOM studies in freshwaters can be
used to provide new insights into DOM biogeochemistry (Jaffe et al.,
2008
; Fellman et al.,
2010
). Here we highlight a number of examples in which studies have utilized fluorescence
spectroscopy in freshwater ecosystems to examine the biogeochemical role of DOM.
