Date Awarded

2000

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

Elizabeth A. Canuel

Abstract

To evaluate the effects of anthropogenic alteration of the Chesapeake Bay (CB) watershed since European settlement, the historical progression of eutrophication and anoxia in the mesohaline region of CB was reconstructed. Lipid biomarker and carbon and nitrogen stable isotopic and elemental composition of CB surficial sediments were examined seasonally in order to identify the present sources of organic matter to CB sediments and the processes controlling their distribution. Temporal variability in surficial sediment composition could be linked to seasonal changes in phytoplankton community composition and biomass while spatial variation was dominated by the delivery of alloclithonous versus autochthonous sources of organic matter. Three cores (3 to 4.5 m in length) collected from the mesohaline region of Chesapeake Bay were dated using a combination of tools including 210Pb and 137Cs radioisotopes, anthropogenic Pb and pollen indices. Enrichments in the carbon and nitrogen isotopic signature of sediments of all three cores deposited between 1790 and 1915 indicated enhanced primary productivity and nitrogen recycling, respectively. at the same time, increases in the flux of total organic carbon (TOC) and episodic enrichments (relative to TOC) of algal and bacterially-derived lipid biomarker compounds signaled a change in the sources of OM to the sediments. More extreme change occurred after the 1915's with further isotopic enrichments, a 1.5 to 2.5-fold increase in TOC deposition and 2 to 5-fold enrichments in algal and bacterially-derived lipid biomarker compounds. No change in the contribution of terrestrially-derived OM was indicated in any of the cores. Changes in sulfur speciation identified the initial occurrence of anoxia/hypoxia in 1790 at the deepest site (26 m) and in 1929 at a 15 m depth site. An examination of both qualitative evidence and quantitative models of degradation indicates that diagenesis cannot account for the observed increases in the total amount and labile quality of OM deposited during the 19th and 20th century in CB. Using diagenetic models, 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.

DOI

https://dx.doi.org/doi:10.25773/v5-v753-eq57

Rights

© The Author

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