Geoscience Reference
In-Depth Information
6.2.2 Evolution of DOM Field Sensors
In the 1940s von Kurt Kalle (
1949
) discovered that seawater exhibited a blue fluorescence
when irradiated by UV light. It wasn't until the 1990s that
in situ
fluorometers were modi-
fied to measure this fluorescence of organic matter in seawater. It is important here to
clarify the pools of natural organic matter (NOM), which is comprised of particulate, dis-
solved, colored (POM, DOM, COM, respectively) organic matter, and non-colored con-
stituents. The term “dissolved” indicates a filtrate that passes through a filter of some pore
size (normally between 0.2 and 1.0 μm) and the term “colored” indicates the property of
absorbing light. Field sensors are traditionally designed to collect measurements in unfil-
tered mode and include both the dissolved and particulate OM signatures, of which the
sum has been termed COM. In many aquatic systems, the dissolved signal dominates over
the particulate portion, so essentially CDOM ~ COM (Chen,
1999
; Belzile et al.,
2006
).
However, contribution by particles in some waters can be significant, as demonstrated by
operating field sensors in unfiltered and filtered modes (Downing et al.,
2009
). Beyond
just the presence or absence of particles, there is also a need to subdivide the colored pool
because some fraction of COM also fluoresces and can be termed fluorescent organic mat-
ter (FOM). So it follows that all FOM is colored but not all COM fluoresces, and it is the
former that
in situ
fluorometers are actually measuring. However, nomenclature used in the
literature does not accurately reflect this because often “CDOM” is reported when in fact it
is the nonfiltered fluorescent material that is truly measured. In some instances, FOM can
be accurately reported as (1) FDOM if a filter was used to collect
in situ
measurements or
(2) FDOM or CDOM if the sensor was calibrated to discrete samples of fluorescence inten-
sity or absorption coefficients, respectively. For practical purposes when discussing cited
literature within this chapter, nomenclature will be consistent with originally published
literature. However, in an effort to avoid confusion about which pools are reported in the
future, the authors recommended standardization of nomenclature within the community
through the use of FOM, FDOM (when filtered
in situ
or calibrated to discrete fluores-
cence samples) and CDOM (when calibrated to discrete absorption samples) for
in situ
fluorescence data.
The first reports of
in situ
fluorescent DOM were published in 1991 (Coble and Gagosian,
1991
; Coble et al.,
1991
). Investigations of CDOM in the Black Sea utilized a pumped
profiling system, where water was pumped from depth through on-board continuous flow-
through fluorometers customized for the fluorophore of interest (excitation/emission [ex/
em] wavelengths described in
Figure 6.3
). The sampling design focused on measuring
chlorophyll, DOM and flavins to search for material from photosynthetic bacteria. Before
hydrocasts, the baseline of the DOM fluorometer was set to zero in the surface waters to
observe relative changes throughout the water column, so these uncalibrated values are
reported as relative fluorescence (
Figure 6.3
).
The first submersible sensor of DOM fluorescence was the Yellow Matter Meter devel-
oped by Sea Tech (Corvallis, OR). The instrument employed right angle detection and
interference filters centered on ex/em = 330/450 nm with bandwidths of 80 and 65 nm,
