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PHOTOCHEMICALLY-INDUCED ALTERATION OF STABLE CARBON ISOTOPE RATIOS (DELTA C-13) IN TERRIGENOUS DISSOLVED ORGANIC CARBON
Citation:
Opsahl, S. P., J W. Jones*, AND R G. Zepp. PHOTOCHEMICALLY-INDUCED ALTERATION OF STABLE CARBON ISOTOPE RATIOS (DELTA C-13) IN TERRIGENOUS DISSOLVED ORGANIC CARBON. GEOPHYSICAL RESEARCH LETTERS 28(12):2417-2420, (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:
Exposure of riverine waters to natural sunlight initiated alterations in stable carbon isotope ratios (delta C-13) of the associated dissolved organic carbon (DOC). Water samples were collected from two compositionally distinct coastal river systems in the southeastern United States-the Satilla River which has high DOC concentrations (10-35 mg/l), and the Altamaha
River which has relatively low DOC concentrations (3-9 mg/l). Approximately 21-26% of the DOC was remineralized to DIC. The delta C-13 of remineralized DIC was isotopically "light" (delta C-13 = -28.8 to -33.2 parts per thousand) relative to the initial DOC (delta C-13 = -27.4 parts per thousand), leaving a residual fraction of isotopically "heavy" (delta C-13 = -25.8 to
-26.6 parts per thousand) DOC. Photochemically-induced fractionation of C-13 results from selective degradation of certain biochemical constituents including lignin phenols. These results are consistent with shifts in VC of DOC observed during mixing of river and marine waters, suggesting that photochemically-induced alterations are a factor in determining these changes.