Colored Dissolved Organic Matter in the Coastal Ocean:
An optical tool for coastal zone environmental assessment and
management
Paula Coble, Chuanmin Hu
College of Marine Science, University of South Florida, St. Petersburg, FL
USA
Rick Gould
Stennis Space Center, Mississippi USA
Grace Chen
Michelle Wood
Paul Bissett?
[no need for an abstract?]
Introduction
“Deep blue sea” is a phrase so common in English usage that all three
words are, individually, synonymous for “ocean” in common English usage.
Poems, song titles and most recently, a movie, have used these words to conjure
in our minds an image that few people actually observe firsthand. The real “deep
blue sea” can typically be seen only hundreds of miles offshore [not really. In
Miami you'll see the Gulf Stream a few miles offshore]. The part of the ocean
most people on the planet are able to observe are our coastal waters – rarely
“deep,” and only in the cleanest, clearest regions of the world, such as the coast
of Australia, do these waters appear blue. Soil runoff from rivers, algal blooms,
and suspended sediments make coastal waters appear to be various colors –
black, brown, red, blue or green.
The color of the ocean, when measured in full spectral detail, tells
scientists the composition and concentration of dissolved and particulate
materials in seawater, and thus it is a valuable tool for water quality monitoring as
well as for studies of carbon dynamics. The very fact that ocean color varies
from one part of the ocean to another can also be used to study the dynamics of
mixing in coastal regions. This is of great value to coastal managers interested
in the spread of harmful algal blooms, coastal pollution, and distribution of fish
populations and is also vital to successful naval operations and navigation in
coastal regions as visibility in water is modulated by the color.
Sunlight striking the ocean surface penetrates into the water column and
interacts with the dissolved and particulate material in the water as well as with
the water itself. Part of the light that is not absorbed by these materials is
reflected back out. This reflected light represents the “ocean color signature” of
the water. The most important components contributing to ocean color, other
than the water itself, are phytoplankton pigments (mainly chlorophyll), non-living
particles, and colored dissolved organic matter (CDOM).
Separation of the ocean color signature into discrete components is
accomplished using a relationship called an algorithm. Details of algorithm
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development and application are provided in other articles in this volume (Arnone
et al., Schofield et al.) In this article, we focus on the properties, sources, sinks,
and distribution patterns [in what areas?] of the major dissolved component of
ocean color – CDOM. Through an understanding of the complex characteristics
and interactions [with what?] of the CDOM pool, we can gain valuable insight into
a variety of physical and biogeochemical processes occurring in coastal and
shelf regions.
Why is CDOM important?
Changes in the concentration and [optical?] properties of CDOM in a
coastal region can be used to trace physical circulation and water mass history.
CDOM also is a very good scavenger of compounds like trace metals and
polyaromatic hydrocarbons (PAHs), removing them from solution and thereby
decreasing their toxicity to organisms. Interactions between sunlight and CDOM
have several consequences for the ecosystem. Absorption of sunlight can serve
to protect sensitive organisms in surface waters, much as sunscreens can
protect us from sunburn. In particular, CDOM may reduce the amount of light
(especially in the UV region) reaching the bottom. This provides a protection
mechanism for UV-sensitive organism such as corals, but on the other hand also
hinders plant (e.g., seagrass) growth. The CDOM itself is eventually destroyed by
sunlight (Kieber and Mopper, 1987), but the process releases organic
compounds (Miller and Moran, 1997) that are required for growth of some
phytoplankton and bacteria. Other essential elements, such as nitrogen (ref)
and trace metals (ref) are also released. In the presence of sunlight, CDOM can
catalyze destruction of organic pollutants (ref). Thus, CDOM plays an important
role in carbon cycling and the biogeochemistry of coastal waters.
What is CDOM and where does it come from? [this para should
proceed the above one]
CDOM is a mixture of compounds that are byproducts of plant and animal
decomposition. It has both terrestrial and marine sources. When present in high
concentrations, it imparts a brown or yellowish color to the water. In coastal
areas, most of the CDOM comes from rivers containing organic materials
leached from soils. Figure 1 shows two views of the Mississippi River just
offshore of where it enters the Gulf of Mexico. The brown color one observes
from a boat or from the coastline (Fig. 1A) is intense enough to also be visible in
satellite images (Fig. 1B) at certain times of the year. Some of the variation in
the shade of brown is due to suspended particles that scatter light, giving the
brown water it’s “café au lait” color. [Fig. 1b and 1c should use the same dates
and lat lon range and image size. We should regenerate these images]
In addition to rivers, CDOM is also produced in the ocean by release of
organic molecules from organisms during lysis, excretion, and grazing (Coble et
al., 1998; Nelson et al., 1998; Steinberg et al., 2000). Open ocean surface
waters often show a depletion of CDOM because of sunlight bleaching.
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How is CDOM measured?
CDOM abundance is typically measured from water samples with optical
means, as the absolute concentration is extremely difficult to determine
(technically, it is difficult to determine on the molecular level that which DOC
molecules are "colored" and which are not. [is this understanding correct?]) The
water sample is first filtered to remove suspended particles, typically of >0.2 m
in diameter, and then analyzed with standard absorption and fluorescence
techniques. Optical instrumentation has advanced tremendously in recent years.
Absorption and fluorescence instruments now in routine use provide a direct
means to characterize the CDOM abundance in the water, as the latter is
supposed to be proportional to both the absorption and fluorescence signals.
Rapid and synoptic assessment of CDOM distributions in coastal areas is
only feasible with remote sensing techniques, as ocean color sensors from
space, such as the SeaWiFS, MODIS, and MERIS, provide global data on a
daily, operational basis. These satellite sensors provide estimates that can be
used to optically track water masses and features at time scales comparable to
the processes controlling the distribution patterns (mixing, photo-bleaching,
phytoplankton growth, advection, particle settling and resuspension). Often,
however, due to the optical complexity in coastal waters, the satellite data need
to be ground-truthed and fine-tuned with the field samples.
Why is CDOM a good tracer?
Because rivers are the main source of CDOM in coastal regions, we often
find an inverse correlation between CDOM concentration and salinity – CDOM is
high where salinity is low ( Blough et al., 1993; Carder et al., 1993; Ferrari and
Dowell, 1998; Benner and Opsahl, 2001; D'sa et al, 2002; Hu et al., 2003; Hu et
al., in press,). Chlorophyll concentration can be uncorrelated with salinity or
CDOM in these regions. Figure 2 shows relationships between salinity and the
major ocean color parameters – CDOM, chlorophyll and particles. Data were
collected at 5 m by the Ocean Physics Laboratory (OPL) HyCODE mooring [Lat
=; Lon=], which was deployed about 25 km offshore of Tuckerton, NJ in 24 m
water depth. These data show that CDOM absorption is the only bio-optical
property that is significantly correlated with inverse salinity. R 2 values for
chlorophyll concentration, total scattering (b), and total attenuation (c) with
salinity are close to zero, compared to 0.77 for CDOM. [the red solid lines in this
figure should be removed – they don't mean anything except for perhaps CDOM]
The relationship between salinity and CDOM means that ocean color
satellite images might be used to map freshwater intrusions into the coastal
ocean, reflecting changes in river discharge rates as well as variation between
CDOM from different rivers within a given coastal region. An example of one
such map is shown in Figure 3 for the northern Adriatic Sea in February 2003. In
this study, salinity and CDOM data were collected using a continuous underway
flow-through measurement package. Note that in offshore areas away from the
influence of the Po River discharge, the relationship is weaker or non-existent
(blue points along cruise track and in scatter plot). This is due to long exposure
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of CDOM at the surface to sunlight, which destroys the inverse correlation with
salinity. Also, CDOM from these offshore regions is perhaps of marine origin
where terrestrial influence is minimal. Thus, the relationship between CDOM and
salinity is temporally and spatially variable. This has been discussed in detail for
the NE Gulf of Mexico, which is under significant river influence (including the
Mississippi), using field data collected between 1998 and 2000 (Nababan et al,
submitted). It has been demonstrated that although in some cases predicting
salinity from satellite ocean color imagery via CDOM may be feasible (e.g., D'Sa
et al., 2002), in general it still remains a challenge as the relationship between
CDOM and salinity varies both spatially and temporally (Nababan et al.,
submitted).
In coastal regions lacking a strong river input, highest concentrations of
CDOM are found in subsurface waters, below the limit of light penetration. In
areas where upwelling occurs, this can cause the relationship between CDOM
and salinity to be reversed, such that CDOM is high at higher salinities. One
such example was observed in and around Monterey Bay, California during a
period of strong northwesterly winds (April 2003, Figure 4). The blue points at
salinity values higher than 33.2 represent the most recently upwelled waters.
Data represented by red points show influence of mixing with fresh water and
photobleaching as upwelled water persists at the surface. [where are the blue
and red points?]
What impact does CDOM have on the ecosystem?
Some types of phytoplankton appear to preferentially thrive where CDOM
concentrations are elevated above seawater levels. This could be due to a
variety of factors. Input of CDOM to coastal waters, whether from river runoff or
upwelling of deep waters, often is accompanied by input of nutrients that
stimulate the growth of phytoplankton. The red-tide species, Karenia brevis (also
known as G. breve) in the coastal waters of West Florida Shelf, appears to
aggregate under protection of elevated CDOM. Sun-sensitive organisms, such as
corals, may be damaged by the solar UV light without the photo-protection of
CDOM. Photo-degradation of CDOM can release limiting elements such as
nitrogen and trace metals.
An interesting example of the role of CDOM on coastal ecosystems is
shown in Figure 6 for a region of the West Florida Shelf. [where is figure 5?]
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References
Benner, R., and S. Opsahl. 2001. Molecular indicators of the sources and
transformations of dissolved organic matter in the Mississippi river plume.
Organic Geochem. 32:597-611.
Blough, N.V., and O.C. Zafiriou, and J. Bonilla. 1993. Optical absorption spectra
of waters from the Orinoco river outflow: Terrestrial input of colored organic
matter to the Caribbean. J. Geophys. Res. 98:2271-2278.
Carder, K.L., R.G. Steward, G.R. Harvey, and P.B. Ortner. 1989. Marine humic
and fulvic acids: Their effects on remote sensing of ocean chlorophyll. Limnol.
Oceanogr. 34:68-81.
D’Sa, E.J., C. Hu, F.E. Muller-Karger, and K.L. Carder. 2002. Estimation of
colored dissolved organic matter and salinity fields in case 2 waters using
SeaWiFS: Examples from Florida Bay and Florida Shelf. Proc. Indian Acad. Sci.
(Earth Planet Sci.), 111(3), September 2002, pp. 197-207.
Ferrari, G.M., and M.D. Dowell. 1998. CDOM absorption characteristics with
relation to fluorescence and salinity in coastal areas of the Southern Baltic Sea.
Estuarine, Coastal and Shelf Science 47:91-105.
Hu, C., Z. P. Lee, F. E. Muller-Karger, and K. L. Carder, Application of an
optimization algorithm to satellite ocean color imagery: A case study in
Southwest Florida coastal waters. SPIE Vol. 4892 (Ocean remote sensing and
applications), R. J. Frouin, Y. Yuan, and H. Kawamura eds. SPIE, Bellingham,
WA, 2003. pp 70-79.
The South-West Florida Dark-Water Observation Group (SWFDOG), Satellite
images track “black water” event off Florida coast. EOS. Transactions, AGU,
83(26):281,285, 2002.
Hu, C., F.E. Muller-Karger, D.C. Biggs, K.L. Carder, B. Nababan, D. Nadeau, and
J. Vanderbloemen. 2003.
Comparison of ship and satellite bio-optical
measurements o the continental margin of the NE Gulf of Mexico. Int. J. Rem.
Sensing 24(13):2597-2612.
Hu, C., E. T. Montgomery, R. W. Schmitt, and F. E. Muller-Karger. (in press). The
dispersal of the Amazon and Orinoco River water in the tropical Atlantic and
Caribbean Sea: Observation from space and S-PALACE floats. Deep-Sea
Research II.
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A
B
31.0
Latitude
30 May, 2003
adg(412)
0
0.2
0.4
(m-1)
30.0
C
29.0
-90
-89
-88
-87
-86
-85
Longitude
Figure 1. Three views of the Mississippi River plume – one from boat and two from
space. A. Photo of lighthouse at the entrance to the main channel. B. True color
image from SeaWIFS taken March 15, 2001. Red line in bottom image show
approximate location of the shoreline. C. False color image of absorption
coefficient at 412nm of dissolved and particulate gelbstoff taken May 30, 2003.
Note that the two satellite images are not for the same day or location.
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Figure 2. Salinity versus ag (top left), chlorophyll (top right), backscatter (bottom left, and beam
attenuation coefficient (bottom right). [the red solid lines should be removed – they really don't mean
anything]
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46
20 February, 2003
adg(412)
0
0.05
0.10
0.15
(m-1)
Latitude
45
44
43
42
11
12
13
14
15
16
17
Longitude
39
Salinity
38
2
R = 0.704
37
36
0.00
0.05
0.10
ag(412) m
0.15
-1
Figure 3. A. Northern Adriatic Sea cruise track, 3-21 February 2003, overlaid on SeaWiFS adg(412)
image from 20 February. Continuous underway flow-through measurements included CTD salinity
and CDOM absorption (ag) collected with a 0.2 m-filtered ac9 instrument. B. Salinity vs. ag at 412
nm extracted from the underway flow-through data stream at the locations indicated in panel A.
Symbol colors correspond in A&B. Red points indicate coastal locations where salinity and a g are
inversely related; blue points are more offshore locations where there is no relationship.
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37.5
22 April, 2003
0
0.05
0.10
0.15
(m-1)
37.0
Latitude
Monterey
Bay
36.5
adg(412)
36.0
-123.0
-122.5
Longitude
-122.0
-121.5
34.0
Salinity
33.5
33.0
32.5
32.0
0.00
0.05
0.10
0.15
ag(412) m
0.20
0.25
0.30
-1
Figure 4. A. Monterey Bay cruise track, April 22, 2003, overlaid on SeaWiFS a dg(412) image
from the same day. Continuous underway flow-through measurements (white line) included
CTD salinity and CDOM absorption (a g) collected with a 0.2 m-filtered ac9 instrument. B.
Salinity vs. ag at 412 nm extracted from the underway flow-through data stream. The linear
relationship observed in Figure 1 is now positive rather than negative.
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aCDOM(412)
Oct 19, 1998
(JD292)
Figure 6.
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