Citation:

ZEPP, R. G., G. SHANK, AND C. ROSENFELD. THE IMPACT OF CDOM PHOTOBLEACHING ON UV ATTENUATION NEAR CORAL REEFS IN THE FLORIDA KEYS. Presented at ASLO 2007 Aquatic Sciences Meeting, Santa Fe, NM, February 04 - 09, 2007.

Impact/Purpose:

The overall objective of this task is to develop quantitative relationships for assessing the vulnerability of aquatic ecosystems (freshwater and coastal) and their services 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. and Brazil These activities are contributing to two APGs in the ecosystems focus area of the Global Change Research Multiyear Plan: the 2008 APG (APG 2) on developing information and tools that managers will use in their decision-making about how to adapt to the effects of global change on aquatic ecosystems; and the 2010 APG (APG 3) on providing information and models that will support development of biocriteria for corals. 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 in the Florida Keys. This collaboration will contribute to one ERD APM in 2006 and three joint NERL-NHEERL APMs in the 2008 - 2010 period. Other research is examining the effects of changing climate and UV on microbial activity in waters close to beaches in the U.S. Work is being completed on the interactions of land use and climate changes with the ecological functioning of streams in watersheds of the southeastern U.S. The task also includes two sub-tasks that are funded mainly by funds-in IAGs. One sub-task funded by NASA involves research in central Brazil that is part of the Large Scale Biosphere Atmosphere Experiment (LBA). This work involves a close collaboration between EPA and a group of scientists from the Department of Ecology, University of Brasilia, Brazil. 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. Another sub-task funded by the Office of Naval Research is examining interactions between nitrogen and organic substances in aquatic ecosystems that produce the colored dissolved organic matter (CDOM) that controls penetration of solar UV radiation into coastal waters.

Description:

We have investigated how the loss of chromophoric dissolved organic matter (CDOM) in the water column due to photobleaching allows for increased penetration of UV radiation near coral reefs in the Florida Keys. Extended exposure to UV may contribute to coral bleaching episodes. CDOM serves as the primary control on UV exposure of corals in this region because it strongly absorbs UV radiation, especially damaging UVB wavelengths. An important fraction of the CDOM pool in Florida Keys coastal waters is transported from Florida Bay, but local sources including seagrasses, mangroves, and Sargassum colonies may also be substantial. CDOM samples collected along transects near the reefs and from mangrove leaf and Sargassum incubation experiments were exposed to simulated solar radiation for up to 96 hours. Calculated photobleaching rates (k305) of CDOM produced by mangrove leaf litter and Sargassum colonies (approx. 0.02 hr (-1)) were an order of magnitude greater than rates measured for the water column samples (0.002 hr (-1)). However, our experiments indicate that photobleaching of CDOM in natural waters near the reefs can still be substantial during summer months and may allow UVB levels at 4 m depth (typical depth of fringing reefs) to increase by as much as 20%. Corals located in shallower waters (2 m) along the reef line may experience up to a 40% increase in UVB exposure due to loss of CDOM.

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