Citation:

ZEPP, R. G., G. C. SHANK, A. VAHATALO, E. BARTELS, AND R. JONES. PHOTOBIOGEOCHEMISTRY OF SARGASSUM: A POTENTIALLY IMPORTANT SOURCE OF CHROMOPHORIC DISSOLVED ORGANIC MATTER IN THE UPPER OCEAN. Presented at 2008 Ocean Sciences Meeting, Orlando, FL, March 02 - 07, 2008.

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:

Sargassum is a genus of widely distributed, generally planktonic macroalgae that often is found near the surface of the ocean. Here we report that a consortium of commonly occurring species in the Gulf of Mexico, Caribbean Sea and North Atlantic, Sargassum natans and Sargassum fluitans, are a source of chromophoric dissolved organic matter (CDOM) that contributes to the optical properties and photobiogeochemistry of the upper ocean. The production and light induced degradation of CDOM from sargassum was examined under field conditions and controlled conditions in the laboratory. The laboratory studies measured the temperature and light-dependence of CDOM production from sargassum and photodegradation of the spectral (UV-visible, fluorescence) properties of CDOM produced. These studies indicate that CDOM (based on absorption coefficients at 350 nm) is released from sargassum (0.003-0.009 m-1g-1h-1) at significant rates and that CDOM production increases sharply with increasing temperature and with exposure of the sargassum colonies to natural sunlight. The CDOM produced by sargassum is photodegraded under simulated sunlight with half-lives of 25-30 hours, and the spectral slope coefficients of its UV-visible absorption spectra (about 16 µm-1) are in the range of S-values generally observed in ocean water. Sargassum derived CDOM thus likely contributes significantly to remotely sensed observations of ocean color and to attenuation of solar UV radiation in the upper ocean.

Record Details: