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INTERACTIONS OF SOLAR ULTRAVIOLET RADIATION AND DISSOLVED ORGANIC MATTER IN FRESHWATER AND MARINE ENVIRONMENTS
Citation:
Zepp, R G. INTERACTIONS OF SOLAR ULTRAVIOLET RADIATION AND DISSOLVED ORGANIC MATTER IN FRESHWATER AND MARINE ENVIRONMENTS. Presented at Lehigh University - Invited Speaker, Bethlehem, PA, March 8, 2001.
Impact/Purpose:
The overall objective of this task is to develop quantitative relationships for assessing the vulnerability of aquatic resources to global change. The task will contribute experimental and modeling tools for assessments of the interactions of global climate and UV changes with coral reefs and selected watersheds and estuaries in the U.S. These activities are contributing primarily to two APGs in the ecosystems component of the Global Change Research Multiyear Plan: the 2006 APG (APG 3) on building the capacity to assess global change impacts on coastal aquatic ecosystems, including coral reefs and estuaries and the 2004 APG (APG 2) on building capacity to assess and respond to global change impacts on selected watersheds. One major task objective is to assess interactions of global warming and UV exposure that are contributing to the observed coral bleaching and disease. Our lab is working with scientists at the NHEERL Gulf Ecology Lab to characterize UV exposure and effects at several coral reef sites. Other research in this task is examining the interactions between UV-induced breakdown of refractory organic matter in estuaries and coastal areas that enhance UV penetration into the water and concurrently form biologically-labile nitrogen-, phosphorus- and carbon-containing substances that stimulate productivity and microbial activity. This task also involves research in central Brazil that is part of the Large Scale Biosphere Atmosphere Experiment (LBA). The objectives of this project are to assess the impacts of land use and climatic changes on soil nutrient cycles and microbiota, trace gas exchange and water quality in the Brazilian cerrado. This work involves a close collaboration between EPA and a group of scientists from the Department of Ecology, University of Brasilia, Brazil. Other objectives of this task are to assess the interactions of land use and climate changes with the ecological functioning of streams in watersheds of the Piedmont region of the southestern U.S.
Description:
Solar radiation provides the primary driving force for the biogeochemical cycles upon which life and climate depend. Recent studies have demonstrated that the absorption of solar radiation, especially 'm the ultraviolet spectral region, results in photochemical reactions that can have significant effects on the environmental cycling of carbon, oxygen, sulfur, and various trace metals in the environment. Other research has shown that photoreactions help cleanse the environment of the waste materials derived from human activities. In this presentation, selected current research results on the photochemical reactions in aquatic environments and on environmental surfaces are presented. First, photochemical reactions of organic matter and metal-organic complexes are considered. Then, rate equations and mechanisms for these photoreactions are described, including recent studies of the behavior of carbonyl excited states in aqueous media. Finally, modeling approaches are presented for the extrapolation of laboratory and field experiments to larger regional and global scales. The presentation is not intended to provide a comprehensive review of environmental photochemistry but rather to provide a taste of the exciting studies that are taking place in this area and the types of measurement approaches and concepts that are being applied.
A significant portion of the photochemistry that occurs in aquatic environments is associated with the colored component of dissolved organic matter, referred to as CDOM. CDOM, which is a mixture of lignocellulose-derived polyelectrolytes that are derived mainly from the decay of terrestrial vegetation, constitutes the majority of the organic carbon in many lakes, rivers, and coastal waters. Photodegradation of CDOM results in loss of its UV and visible absorbance and fluorescence, a process referred to as "photobleaching," changes in the biological availability of its carbon- and nitrogen- containing constituents, and production of carbon dioxide, carbon monoxide, volatile hydrocarbons, and sulfur-containing gases (Figure 1). Photochemical production rates of dissolved inorganic carbon (DIC) from CDOM generally are at least an order of magnitude greater than those of other known photoproducts. However, a recent study has shown that the rates and quantum efficiencies for formation of biologically-labile photoproducts (compounds that are readily assimilated by bacteria) from CDOM are about the same as those observed for DIC photoproduction (Miller et al, personal communication).
Organic complexes with metals (e.g. iron, copper, mercury) also are involved in environmental chemistry in aquatic environments, either via direct photoreactions of the complexes or reactions of the complexes with reactive oxygen species that are produced photochemically. Iron is involved in the photooxidation of CDOM in some freshwater ecosystems, such as the rivers that drain into the Atlantic Ocean and the Gulf of Mexico in the coastal U.S.A. The addition of strong FE(IH) chelating ligands, such as fluoride, to such river waters can significantly reduce CDOM photodegradation rates, presumably by reducing concentrations of photoreactive FE-CDOM complexes that participate in photoredox reactions or that catalyze free radical oxidation of the CDOM. Moreover, the environmental cycling of mercury is affected affected by photochemical reactions in the photic zone of aquatic environments.