Several reports have demonstrated that dissolved organic matter (DOM, principally dissolved organic carbon - DOC) is largely responsible for controlling the penetration of UVR in aquatic ecosystems [1-7, see also Chapters 1 and 3]. In addition, evidence suggests a strong coupling between the optical properties of natural waters and carbon cycling [8-12]. This understanding adds a decidedly ecological role to the photochemical reactions that DOM may undergo in the presence of natural solar radiation, by influencing the cycling of carbon. In this review, we will discuss "ecological photochemistry," or the photochemistry of DOM, especially the radiation-absorbing (chromophoric) fraction of DOM, termed CDOM. Our goal is to introduce the reader to the CDOM photochemistry that influences the optical properties of natural waters, by discussing the primary chemical reactions that influence the optical and chemical properties of DOM. While this will not be an exhaustive review of photochemical concepts related to natural waters (the reader is referred to refs. [12-16]), our review should provide some general reference to those approaching the topic for the first time, as well as providing a current "state-of-the-art" for those familiar with the photochemistry of CDOM. Moreover, we hope to introduce some basic concepts in predictive modeling of CDOM photobleaching in natural waters and suggest directions for future research.

6.1.1 The importance ofphotochemistry in the cycling of DOM

DOM is ubiquitous in natural waters, representing a substantial fraction of the total reduced carbon pool. Estimates of the total DOM concentration (as dissolved organic carbon, DOC) in lakes range from 1 to 10 mg C 1_1, while freshwater swamps, marshes, and bogs may have DOC concentrations ranging from 10 to 60 mg C 1_1 [17]. Sea water and groundwater have substantially less DOC concentrations, ranging between 0.5 and 0.7 mg C l-1 on average [17]. A large source of the DOM found in natural waters originates from the degradation of terrestrial biomass and is present in the form of dissolved humic substances, predominantly humic and fulvic acids. In general, this material is transported to freshwaters by runoff or groundwater intrusion and to marine waters by riverine discharge. In fact, the global loading of terrestrial DOM is sufficient to overturn the oceanic pool of DOC in a relatively short time [18,19]. However, it has been demonstrated that terrestrial DOM is a very small fraction of the total marine DOM pool and that its turnover may be shorter than marine DOM synthesized in situ [20-22]. Explanations for this paradigm include removal mechanisms such as DOM flocculation [23], microbial utilization [21,24,25], and photodegradation [26-31].

6.1.2 Definitions and terms

Throughout this chapter, several terms relating to the photochemistry of DOM will be used and we provide a short glossary of terms here. Photobleaching is the loss of absorbance by CDOM in natural waters, also termed fading [32]. Photobleaching is an optical term, which is more appropriately connected with the chromophores found in CDOM, and, from here forward, we use "CDOM" to describe the optically-active component of bulk DOM that undergoes photochemical reaction and photophysical reaction (e.g., fluorescence). Photodegradation refers to the process of breaking down DOM to smaller compounds, which usually results in smaller molecular weight DOM products that may be rapidly consumed by bacteria [33-38]. The actual cleavage of chemical bonds by a photon of light energy during photodegradation is termed photolysis. Photo-mineralization is the actual oxidation of certain moieties to dissolved inorganic C (DIC), usually in the form of C02 but also in the form of CO and COS. Photooxidation is a catchall term that, in current usage, suggests a mix of the aforementioned specific processes. IUPAC recommends avoiding this term and using the more specific terms defined above and we will follow that convention.

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