Project Title: Impacts of Climate Variability and Anthropogenic Perturbations on Source Contributions of Organic Matter to Estuarine Sediments Using Multiple Biomarkers and Stable Isotope Analyses
Mendenhall Fellow: Antonio Mannino, NASA - Goddard Space Flight Center, Code 971.1, Greenbelt, MD 20771, (301) 286-0182, amannino@pop900.gsfc.nasa.gov
Duty Station: Reston
Start Date: January 28, 2001
Education: Ph.D. (Marine Science), University of Maryland, 2000
Research Advisor: William H. Orem, (703) 648-6273, borem@usgs.gov
Project Description: The goal of the research is to examine the effects of climate and anthropogenic perturbations on source contributions and microbial diversity within the Chesapeake Bay sedimentary record of the past 2000 years using multiple chemotaxonomic indicators, primarily lipids and stable isotope analyses. Defining the composition and sources of sedimentary organic matter within estuaries can provide a historical account of past ecosystem structure and predictions for the future. Source contributions and diversity in surface sediments are also being compared across several coastal systems (Chesapeake Bay, Delaware Bay and Mississippi River plume sediments). Hypotheses to be tested: (1) climatic change (such as occurred during the Medieval Warm Period and the Little Ice Age) and (2) anthropogenic activity (such as eutrophication) affect source contributions and microbial diversity in estuarine sediments.Estuaries along the Atlantic coast of the United States have been heavily impacted over the past four centuries by human activity (deforestation, agricultural expansion, industrialization and urbanization of watersheds). Anthropogenic perturbations have resulted in increased land erosion, higher sediment loads in rivers, inputs of heavy metals and organic contaminants, eutrophication and increased hypoxia/anoxia within estuaries such as Chesapeake Bay. As a result, estuaries such as Chesapeake Bay have experienced a dramatic ecological shift from benthic based ecosystems dominated by submerged aquatic vegetation and oyster reefs to planktonic ecosystems which are dominated by small phytoplankton. Furthermore, water column stratification and microbial decomposition of extensive algal blooms produce seasonal bottom water hypoxia/anoxia (in late spring and summer) in mid-bay waters of Chesapeake Bay. Migration of the oxic/anoxic boundary to the water column influences the microbial diversity and biomass preserved within the sedimentary record. Zimmerman and Canuel observed an increase in organic matter preserved within mid-bay sediments of Chesapeake Bay during the 20th century with a shift in source contribution and composition. Yet, direct comparisons of anthropogenic perturbations or natural climatic variability, such as temperature and precipitation changes, with microbial diversity and source contributions to coastal sediments are sparse.
Contributions of organic matter to estuaries is complex; sources include (1) terrestrial material transported by river and groundwater flow, land runoff and atmospheric deposition, (2) in situ primary production (phytoplankton, benthic algae, seagrasses), (3) material from the coastal ocean, (4) anthropogenic sources - such as wastewater treatment effluent, industrial and agricultural inputs. Sedimentary organic matter in coastal systems is a composite to varying degrees of terrestrial plant debris, submerged aquatic vegetation (for example, seagrasses and macro-algae), phytoplankton, prokaryotes and small portions of protozoan and animal biomass. As the major primary producers within oceanic and coastal ecosystems, phytoplankton production generally comprises the bulk of organic matter in the water column and is considered the major source of organic matter to sediments. Since terrestrial ecosystems are highly productive their contribution to estuarine sediments can be substantial due to the refractory nature of terrestrial organic matter.
Lipids have been used extensively as biomarkers for terrigenous and aquatic organic matter in various sedimentary environments. For example, polyunsaturated fatty acids can be attributed to phytoplankton, branched C15 and C17 fatty acids to bacteria, sterols to specific groups of algae and long chain (C24 to C32) saturated fatty acids, n-alkanes and alcohols to terrestrial plants. Microbial diversity and biomass contribution to sediments can be inferred using biomarker lipids from prokaryotes and algae. Eubacteria synthesize phospholipid membranes composed of ester-linked fatty acids. Some of these fatty acids are specific to eubacteria or certain groups of eubacteria (for example, sulfate reducers) and comprise the most common bacterial lipid biomarkers. However, the predominant bacterial fatty acids, the saturated C14, C16, and C18, are also common in algae. Additional geochemical measurements which can provide evidence of source contributions of sedimentary organic matter are the stable carbon and nitrogen isotope ratios. Bulk stable carbon and nitrogen isotopes are used to distinguish among marine plankton, terrestrial plants (C3 and C4 CO2 fixation pathways) and C4 aquatic plants within coastal food webs.
This research will contribute to the understanding of how climate variability has influenced Chesapeake Bay in the past 2000 years and how it may change ecosystems in the future. Comparison of source contributions and microbial diversity with eutrophication and anthropogenic contaminants within the sedimentary record can provide insight into past ecosystem structure, factors influencing current estuarine ecosystems, and how these ecosystems will change in the future with climate fluctuations and anthropogenic activity.
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