Chesapeake Bay Research

[Abstract] [Supplemental Data] [Methods Photos]

For my dissertation work at VIMS (The Virginia Institute of Marine Science/School of Marine Studies - College of William and Mary), I conducted research on the history of eutrophication and anoxia in Chesapeake Bay. This work was done with my dissertation advisor, Dr. Elizabeth Canuel. The results of this work are published in Marine Chemistry (2000, v.69, p117). I collected sediment cores, dated these sediments, and used the organic geochemistry of the sediments to determine the amount and type of algae and bacteria living in the Bay over the last 500 years. That is, I reconstructed the history of eutrophication (algae growth caused by nutrient runoff) and anoxia (loss of oxygen in the water during the decomposition of organic matter by bacteria) during the period in which European settlers were making significant changes in the Chesapeake watershed.

Abstract: Historical Progression of Eutrophication in the Mesohaline Chesapeake Bay: A sediment geochemical record by Andrew R. Zimmerman and Elizabeth A. Canuel

An organic geochemical analysis of sediments in three cores from the mesohaline Chesapeake Bay (CB) was carried out to reconstruct the progression of eutrophication and anoxia/hypoxia over the past five centuries. Evidence of eutrophication was found in the stable isotopic and lipid biomarker signatures of organic matter (OM) in sediments of all three cores beginning in 1790 and continuing to the present. Enrichments in the carbon and nitrogen isotopic signature of these sediments likely result from enhanced primary productivity and nitrogen recycling, respectively. Coincidentally, fluxes of total organic carbon (TOC) increase and episodic enrichments (relative to TOC) of algal and bacterially-derived lipid biomarker compounds signal a change in the sources of OM to the sediments. During the 19th century, an increase in diatom abundance relative to other algal groups and zooplankton was indicated by decreases in the ratio of dinosterol, sitosterol and cholesterol (non-diatom plankton-derived sterols) to 24-methylenecholesterol (from diatoms). More extreme and enduring change occurred after 1915 with further isotopic enrichments, a 2- to 4-fold increase in TOC accumulation and 2- to 10-fold enrichments in algal and bacterially-derived lipid biomarker compounds. Increases in dinoflagellates and other non-diatom algae relative to diatom biomass occurred during this period of advanced eutrophication as indicated by the above lipid biomarker compound ratios. No change in the relative contribution of terrestrially-derived OM to the sediment was indicated in any of the cores. Sulfur speciation was used as an indicator of redox conditions in CB bottom waters. Increases in the ratio of acid volatile sulfur to chromium reducible sulfur indicate the occurrence of anoxia/hypoxia and are recorded in sediments deposited as early as 1790 at the deepest site (26 m), and in 1915 at a 15 m depth site. There was no record of anoxia in sediments deposited at a shallower site (8 m depth). An examination of both qualitative evidence and quantitative models of degradation rate indicates that diagenesis cannot account for the observed trends. Instead, increases in the total amount and labile quality of OM deposited during the 19th and 20th centuries occurred. A diagenetic model was used to estimate the amount of OM that may have been lost to degradation over time. Using this approach, it is estimated that both algal and bacterial production has increased by 100 to 200% relative to pre-Colonial times with a temporal progression similar to the history of anthropogenic alteration of the watershed.

Supplemental Data

1) Bulk Composition of RD, M3 and RR sediments
2) Lipid Composition of RD, M3 and RR sediments
3) TOC degradation correction calculations
4) Lipid compound degradation correction calculations

Supplemental Figures
1) X-ray radiographs of RD, M3 and RR Cores
2) Dating Information - Tables and Figures
3) TOC degradation correction (Intermediate Calc.) Figures
4) Organic Source Enrichment Profiles (Normalized, Degradation-Corrected Lipid Biomarker Conc.)
5) Chesapeake Bay watershed population and fertilizer consumption history

 Methods Photos

Kasten Core	Kasten Core		Lipid Biomarker Extraction	Sulfur speciation analysis