Date of Award

1994

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

John Day, Jr.

Second Advisor

Robert Twilley

Abstract

This research investigated the effects of two suspension feeding bivalves, the infaunal Rangia cuneata, and the epifaunal Crassostrea virginica, on benthic-pelagic coupling in Fourleague Bay, LA, a river-dominated estuary. A continuous-flow microcosm system was developed to overcome recognized limitations of batch methodology and to facilitate manipulations designed to investigate the ecological role of benthic macrofauna. Experiments comparing traditional batch methodology to continuous-flow methodology indicated that rates of benthic processes were consistently greater when measured using continuous-flow methodology. Examinations of concentrations indicated that differences were due to concentration gradients which departed from ambient over time in the batch cores. Over an annual cycle, both species significantly increased rates and altered patterns of benthic exchange of both particulate and dissolved materials. The magnitude of the change relative to ambient sediment rates varied with season and with the faunal community. Weight-rate regressions indicated that increases were accounted for by individual bivalve metabolism in most cases. The effects of both species on benthic processes resulted in significant changes in stoichiometric ratios when compared to those of the sediment alone, potentially ameliorating water-column phosphate and nitrogen limitation at different times of the annual cycle. Differences between the effects of these species were not entirely due to differences in their respective functional group; results suggested a need for the inclusion of a relative measure of metabolic activity when predicting a species effect on its environment. A framework, in the form of a dynamic simulation model, was developed in order to evaluate these rates relative to other nutrient sources and sinks in the context of ecosystem function. Model simulations addressed the influence of the oyster community on NH$\sb4$ distributions in the lower Fourleague Bay. Results indicated that riverine input was the primary determinant of observed NH$\sb4$ distributions in the spring, while in fall, the release of NH$\sb4$ by the oysters was the most important factor determining water-column concentrations. As primary production rates peak in this area of the bay in the fall, it was suggested that the oyster community plays the primary role in supporting these high production rates.

Pages

211

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