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CDOM PRODUCTION BY MANGROVE LEAF LITTER AND SARGASSUM COLONIES IN FLORIDA KEYS COASTAL WATERS
Citation:
SHANK, G. C., R. LEE, R. G. ZEPP, AND E. BARTELS. CDOM PRODUCTION BY MANGROVE LEAF LITTER AND SARGASSUM COLONIES IN FLORIDA KEYS COASTAL WATERS. Presented at 2006 Ocean Sciences Meeting, Honolulu, HI, February 20 - 24, 2006.
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 the importance of leaf litter from red mangroves (Rhizophora mangle) and living Sargassum plants as sources of chromophoric dissolved organic matter (CDOM) to the coastal ocean waters and coral reef system of the Florida Keys. The magnitude of UVB exposure to coral reefs in the Florida Keys exhibits substantial temporal variability controlled almost exclusively by the level of CDOM present in the water column. Previous research indicated that seagrasses are important local sources of CDOM, while tidally-exchanged Florida Bay water is the primary allochthonous source. Our data indicate that CDOM (using absorption coefficient at 305 nm as proxy) is released from both mangrove leaves (0.002-0.067 m-1g-1h-1) and Sargassum (0.004-0.012 m-1g-1h-1) at similar rates and that CDOM production increases sharply with temperature. Measured CDOM fluxes from both the mangrove litter and Sargassum plants were comparable to previous flux measurements for seagrasses in the Florida Keys. Release from mangrove litter depended greatly on the stage of senescence of the leaf as freshly fallen leaves exhibited the highest CDOM production while older decayed leaves released very little CDOM. Production of CDOM by Sargassum was highest in plants exposed to natural sunlight indicating that large Sargassum colonies may be important sources of CDOM to open ocean waters. The photobleaching half-life of CDOM produced from mangrove litter was ~24 h. We are currently investigating the photobleaching rates of CDOM released by the Sargassum colonies.