CDOM dynamics in natural waters sources sinks and transformations

We have seen that when CDOM absorbs UVR, several types of chemical reactions may occur. These reactions may be variable depending on the aquatic medium, which is also subject to wide variation. Aquatic ecosystems are dynamic systems, constantly influenced by biological activity, chemical reactions, and physical processes.

6.7.1 Chemical transformations of CDOM by UVR

As the chemical composition of the water changes, the amount of light energy available and the reactants available also changes. For example, terrestrial CDOM transported from a river into an estuary encounters a distinct change in the ionic composition of water due to increased salinity. This change in ionic composition may alter the solubility or conformation of certain C moieties and thus influence the optical properties of the CDOM. Osburn et al. [143] have shown that CDOM added from riverine and estuarine waters is more photoreac-tive in higher salinity CDOM-free permeates (Figure 9). Availability of 02 can affect the degree of photodegradation. Allard et al. [124] showed that rate and degree of photodegradation for a fulvic acid solution was reduced when exposed under nitrogen gas. Prior exposure to UVR may also reduce the photoreactivity of CDOM, reducing the efficiency of photochemical reactions. Miller and Zepp [29] and Miller and Moran [104] both reported correlation between higher production of C02 and CO and higher absorption coefficient loss at 350 nm for humic acids isolated from a salt marsh. Thus the photochemical reactions are strongly influenced by the chemistry of the aquatic environment.

Sources of CDOM to aquatic ecosystems fall into two broad categories. Allochthonous sources of CDOM are derived from carbon sources outside of the aquatic ecosystem. This term is generally synonymous with terrestrial organic matter sources and the products of terrestrial organic matter biodégradation (e.g., lignin, tannins, and flavonoids). These compounds can also be characterized as humic substances, owing to the presence of many aromatic moieties. Autochthonous sources of CDOM come from the organic matter produced within a particular aquatic ecosystem. This material is largely derived from algae, though macrophytes can contribute CDOM in freshwater ecosystems. Several workers have shown that autochthonous sources of CDOM exist, primarily due to the humification of compounds released by algal senescence [66,144-148]. The mechanism of this humification is unclear, but appears to be caused by oxidative linkage of fatty acids either by microbial activity or sunlight [149]. It is also

Permeate/CDOM

Permeate/CDOM

Figure 9. Change in photoreactivity (PRx) in a mixing manipulation of CDOM removed from sampling waters in the Chesapeake Bay by ultrafiltration and added back to the permeate, which is "CDOM-free." In each case where CDOM was added to an increased saline permeate, PRx increased. Interestingly, adding CDOM from a saline source to a fresher permeate (e.g., estuarine [E] CDOM in fresh water [F] permeate) decreased PRx.

Figure 9. Change in photoreactivity (PRx) in a mixing manipulation of CDOM removed from sampling waters in the Chesapeake Bay by ultrafiltration and added back to the permeate, which is "CDOM-free." In each case where CDOM was added to an increased saline permeate, PRx increased. Interestingly, adding CDOM from a saline source to a fresher permeate (e.g., estuarine [E] CDOM in fresh water [F] permeate) decreased PRx.

possible that mineral surfaces may cause this oxidative process.

Reche and coworkers [125] have investigated the effect of water chemistry and trophic status on the photobleaching of CDOM in lake ecosystems. They examined about thirty lakes in the United States and found a strong and significant correlation (r2 = 0.94) between acid-neutralizing capacity and loss of dissolved absorbance at 440 nm. Conductivity showed a less strong correlation (r2 = 0.74), as did cation concentration (Ca+ and Mg2++, r2 = 0.62), but both were still significant. Trophic status was estimated with chlorophyll a concentration, and showed a rather weak correlation (r2 = 0.15). They conclude that high ionic composition in lakes likely increased photobleaching efficiency by changing the conformation of chromophoric moieties.

6.7.2 Ecological implications of CDOM photochemistry

Ultimately for ecologists, the photochemical reactions involving CDOM are of interest for multiple reasons, most dealing with the exposure of organisms to UV-B radiation. Many of these reasons are dealt with in other chapters of this book, and here we briefly speculate on a sample of direct and indirect connections between CDOM photochemistry and aquatic ecology. From the ecosystem perspective, CDOM photodegradation to smaller biolabile compounds might strongly influence carbon transfer among trophic levels by providing bacteria with carbon sources. Thus bacterial stimulation by photoproduced C (perhaps evidenced optically by an increase in the acDOM(250):aCDOM(365) ratio) might enhance its movement through the "microbial loop" [33-38,95,103,106,108,128,146,150,151]. Alternatively, some reports have suggested that CDOM photochemistry renders C unavailable to bacteria [109,147]. Because many reports show that CDOM photomineralization produces DIC [29,56,118-121,123,152,153], it is likely that this CDOM is utilized by primary producers (or by bacteria in the case of CO, [11,35,154]) at some point. This speculation has not been researched thoroughly, and DIC photoproduction is inextricably linked to other chemical factors in aquatic ecosystems (e.g., pH, iron, and conductivity; [38,54,119,125,152,155-157]). Its importance to carbon cycling remains unknown.

From the physiological perspective, the link between ecology and CDOM photochemistry is important through both direct and indirect associations. A direct effect is the potential for production of high reactive oxygen species (see Chapter 8) which can damage cellular membranes, and to a minor degree the photo-activated toxicity of organic compounds (Chapter 7). However, these effects are likely quite small compared to the indirect ecological effect of CDOM photochemistry: the change in the underwater radiation field caused by CDOM photobleaching. We see that CDOM photobleaching alters its spectral properties, most evident by changes in the spectral slope (S), and the net effect of CDOM photobleaching might be enhanced UV-B flux relative to UV-A flux down through the water column. This would potentially expose organisms to higher amount of damaging UV-B radiation, even if vertical mixing mediates these changes through reduced time at the surface (see Chapter 4), thus reducing the roles of photorepair (Chapters 10 and 13) or influencing behavioral responses (Chapter 14). A reduction in overall photosynthetic capacity of aquatic primary producers may also occur in an enhanced UY environment (Chapter 11). Because CDOM is colored, a contrasting ecological effect could be shading, whereby CDOM competes with primary produces for available photosynthetically active photons. Perhaps to some degree, CDOM photobleaching stimulates primary production in strongly colored waters such as wetlands and humic lakes.

It is clear from the above summary that CDOM plays a central role in the interaction of UVR and aquatic ecosystem function, and that the effects and implications of CDOM photochemistry are not always straightforward. Moreover, the effects become exacerbated by climate change and anthropogenic modification of aquatic ecosystems (e.g., acid deposition; see Chapter 17).

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