Date of Award

1996

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

F. Quay Dortch

Abstract

Sediment cores were collected to study the development of eutrophication and hydrological changes in coastal Louisiana. The cores were taken at marsh sites hypothesized to be influenced primarily by riverine inputs (Fourleague Bay-FLB), localized activities (Terrebonne Bay-TSMP), and both riverine and localized processes (Airplane Lake-APL). The sediment cores represented time frames ranging from 330 years (APL) to 70 years (TSMP) BP according to core dating results. Sediment core samples were analyzed for diatoms, other microfossils, pigments, biogenic silica, and sediment characteristics. Additionally, land use records were utilized as a proxy of anthropogenic activities, and to aid in the interpretation of the paleoindicators. Analysis of the data indicated that eutrophication developed at all three sites since the 1950s, most rapidly since the early 1970s, and was directly related to fertilizer use in the region. The diatom community changed over time, reflecting a species shift towards smaller, more lightly silicified forms in response to growing eutrophication. A diatom-based trophic index, biogenic silica measurements, sediment chlorophyll-a content, and several diatom species were the paleoindicators that most strongly responded to the development of eutrophication at the three sites. A diatom-based salinity index displayed trends similar to long-term salinity measurements taken in the region. Salinities decreased at the FLB and APL sites, in response to increased river discharge, whereas salinities increased at the TSMP site, probably due to the construction of the Houma Navigation Canal. Increased water levels, observed through tide gauge measurements around coastal Louisiana, were not evident in this study. Several acute events left signals in the sediment, including Hurricane Andrew and the flood of 1927. Although several events were evident, they were generally distinctive enough to have no effect on the eutrophication and hydrology studies. A canonical discriminant transform function was derived from analysis of a sediment layer left by Hurricane Andrew, which was shown to be reworked sediment displaying characteristics of the surrounding freshwater, estuarine, and marine environments. This transform function was standardized to distinguish preserved event horizons in any coastal sedimentary environment, although further development and refinement is needed before the function can be utilized elsewhere.

ISBN

9780591288742

Pages

302

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