Factors Affecting Colored Dissolved Organic
Matter in Aquatic Environments
of the Southeastern United States



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Richard Zepp and Christopher Shank (National Research Council Associate) - ORD/NERL/ERD, Athens G
Mary Ann Moran, Wade Sheldon and William Miller- Department of Marine Sciences, University of Georgia, Athens GA

Co-sponsored by USEPA and the Office of Naval Research

The sunlight-absorbing (colored) component of dissolved organic matter (CDOM) in aquatic
environments is widely distributed in freshwaters and coastal regions (Figure 1), where it influences the
fate and transport of toxic organic substances and biologically important metals such as mercury, iron,
and copper. CDOM also has significant effects on underwater visibility and ultraviolet radiation, and it is
an important component of remotely sensed ocean color. In this research, we have examined two key
factors, microbial degradation and sorption, that affect the loss of CDOM in rivers, estuaries, and coastal
shelf regions of the Southeastern United States (Figure 2). Sorption involves the sticking of CDOM to
particles that are in the water.

Figure 1. The Satilla River; like many rivers and estuaries in the coastal Southeastern United States
has a high concentration of colored dissolved organic matter (CDOM) that gives the river a black
appearance.

CDOM is generally resistant to microbial decomposition, but this research has discovered an important
pathway for its loss from aquatic environments that involves "photochemically stimulated" microbial
degradation. We discovered that absorption of sunlight by CDOM results in its breakdown to low-
molecular-weight substances that are readily degraded by microorganisms (Figure 3). Using measured
efficiencies for the effects of sunlight on microbial decomposition of CDOM, we have developed
procedures for forecasting its degradation as a function of time, location, and depth in coastal rivers and
shelf regions of the Southeast (Figure 4).

Figure 3. (A) Exposure to sunlight causes loss in the color and UV absorption by CDOM
(photobleaching); (B) The photobleaching is accompanied by large increases in the biodegradability of
the CDOM, indicated here by the observed increase in respiration for the photobleached CDOM.





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Figure 4. Depth dependence for CDOM photoreactions in the South Atlantic Bight (31o 43.43' N, 80o
38.83' W) on a cloudless summer day (June 28) in the southeastern U.S. These modeled results are
based on measured efficiencies for the effects of sunlight on microbial decomposition of CDOM.

CDOM also is lost by sorption to settling aquatic particulate matter (PM). We found that CDOM sorption
to aquatic PM is a reversible process that can be quantified by use of partition coefficients. The
research demonstrated that sorption is a major pathway for loss of CDOM in turbid waters, such as
coastal estuaries (Figure 5), and that this loss is accelerated when ocean water mixes with freshwater.
Measurements of changes in CDOM fluorescence showed that photo-stimulated microbial degradation
coupled with sorption of CDOM onto aquatic PM can result in substantial losses of CDOM within
estuaries of the coastal Southeastern United States (Figure 6).

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Figure 5. Sorption of CDOM to sediments is shown here to be a major pathway for loss of CDOM in
turbid waters, such as coastal estuaries and it controls the concentration of CDOM in pore waters of
bottom sediments.



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