Geoscience Reference
In-Depth Information
As analyses proliferate, care should be taken in use of terminology to describe both model
and spectroscopic results. Designation of a new peak from spectroscopic data requires
demonstration that it can be observed as a distinct peak in an EEM in the presence of
previously identified peaks. Lacking that verification, our recommendation is to use the
term “region” to refer to newly identified areas of interest. It is the ultimate goal of many
researchers eventually to discover the chemical compound identities for all fluorophores
in natural water samples. Only when this goal is attained will the underlying distinctions
among peaks, components, and regions be clarified.
3.2 Fluorescence in Seawater
3.2.1 Introduction
The distribution and properties of CDOM in seawater has been the subject of several
reviews. The original review by Blough and Green (
1995
) and its update by Blough and
Del Vecchio in
2002
covers observations of CDOM optical properties in coastal regions
collected worldwide since 1981. Nelson and Siegel (
2002
) reviewed CDOM in open ocean
areas, including remote sensing applications, photochemistry, and photobiology. The
review of Del Castillo emphasizes remote sensing applications for CDOM in the coastal
ocean, including the chemical basis of the optical properties, the role of CDOM in ocean
color and ocean optics, and development of empirical algorithms for separation of ocean
color spectra into discrete components. The most recent review by Coble (
2007
) covers the
topic of CDOM in both coastal and open ocean areas between 2004 and 2007, including the
application of PARAFAC and other multiple-component analysis techniques for resolving
EEMs into individual components.
The following section summarizes the key characteristics and distribution of CDOM in
seawater and highlights a few case studies of recent developments and observations.
3.2.2 CDOM in Coastal Ocean and Estuaries
The distribution of CDOM in seawater is dominated by high concentrations along coastal
margins contributed by inflowing freshwater from rivers. This CDOM has a terrestrial sig-
nature, consisting of humic-like fluorescence derived from soils. Open ocean regions have
a CDOM concentration on the order of 1 ppb quinine sulfate equivalents (QSE), which is
two or more orders of magnitude lower than in rivers; hence dilution of freshwater with
seawater is the major process controlling nearshore distributions. The preponderance of
studies of CDOM distribution in coastal regions have reported a robust negative correlation
with salinity, and thus CDOM is largely considered to be a conservative tracer for mixing
on the time scale of weeks for the surface ocean. This distribution is robust enough such
that CDOM has been used as a tracer of coastal mixing in numerous past studies, and more
recently enabled the development of ocean color algorithms for salinity and freshwater dis-
charge (Del Castillo and Miller,
2008
).
