You are here:
EFFECTS OF SUNLIGHT ON CARBOXYL CONTENT OF DISSOLVED ORGANIC MATTER IN THE SATILLA RIVER OF GEORGIA, UNITED STATES
Citation:
Xie, H., O. C. Zafiriou, W. J. Cai, AND R G. Zepp. EFFECTS OF SUNLIGHT ON CARBOXYL CONTENT OF DISSOLVED ORGANIC MATTER IN THE SATILLA RIVER OF GEORGIA, UNITED STATES. Presented at 225th American Chemical Society National Meeting, New Orleans, LA, March 23-27, 2003.
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:
A study examined the effect of sunlight-initiated photo-degradation of dissolved organic matter (DOM) on its carboxyl content, and the role of oxygen and iron in this process. Solar-simulated irradiations were performed on 0.2-mm filtered water samples collected from the highly colored Satilla River in Georgia, USA. Prior to the irradiations the samples were purged with air, pure oxygen or pure nitrogen to obtain three different initial oxygen levels: air-, oxygen- or nitrogen-saturated, respectively. The influence of iron was studied by comparing samples with and without addition of deferrioxamine mesylate, a strong, photochemically inert, iron-complexing ligand. The carboxyl contents of DOM before and after irradiations were determined using potentiometric titrations. Photochemically induced loss of absorbance and dissolved organic carbon (DOC), production of carbon dioxide (CO2), oxygen consumption, and variations of pH and [Fe(II)] were also monitored. For all the treatments, the carboxyl content in mmoles per liter of sample did not change significantly over the time course of irradiation up to 46 hours. However, the DOC-normalized carboxyl content (mmol/mg C) after 46 hour of exposure increased by a factor of 1.6 and 2.6 in the air- and oxygen-saturated samples, respectively. The variations of the DOC-normalized carboxyl content in all other treatments were within the experimental uncertainty. The results unequivocally indicate that the prolific photoproduction of CO2, which has long been hypothesized as a result of photodecarboxylation of DOM, cannot be accounted for by the changes of carboxyl content observed. In addition, methylation of the carboxylic groups on DOM reduced the rate of CO2 production only by 25%. This study, therefore, strongly suggests that the primary mechanism for photoproduction of CO2 was not photodecarboxylation or that carboxylic groups were photochemically regenerated.