Date of Award

2003

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Oceanography

Department

Oceanography

First Advisor

John King

Abstract

The health of the Rhode Island coastal lagoon ecosystem is of importance due to its critical role as a wildlife habitat and recreation destination. As human population continues to expand along the coast, this ecosystem will receive increasing pressure from contaminants. By examining how the lagoons have changed in the past, we can postulate what variables are linked to change. Current conditions need to be thoroughly studied to provide a baseline with which to study future change, as well as to determine areas of concern in the lagoons. In order to understand how the ecosystem responds to such pressures, a multi-disciplinary approach was employed to gauge the extent and impact of chronic, low-level pollution, examine how the ecosystem has changed and will change over time, and to investigate methods to further our ability to monitor change.

Three coastal lagoons along the southwest coast of Rhode Island were selected for study: Ninigret, Quonochontaug, and Winnapaug Ponds. They represent a spectrum of conditions in terms of size, volume, and human population densities, yet they are all under similar pressures regarding future population increase and tidal inlet modifications. The distribution of organic and inorganic contaminants established that these lagoons are not well mixed despite how shallow they are, and certain areas within the lagoons focus contamination (“hot spots”). Equilibrium partitioning theory and measurements of acid volatile sulfides suggest that currently, organic and inorganic contaminant concentrations are too low to cause any significant biological impact.

Paleoproductivity studies were conducted in Quonochontaug and Garden Ponds. Lithofacies associated with hurricanes were identified. The physical disruption or salinity alterations associated with hurricanes appeared to have minimal impact on long-term trends. However, sedimentation of the lagoon’s tidal inlet (due to storms or other natural processes) leads to a freshening of the lagoon and higher productivity. In contrast, a dredging project, which opened the tidal inlet, led to an increase in salinity within the lagoon and a decrease in productivity. Anoxia was documented during a period when the tidal inlet was closed, highlighting the sensitivity of the lagoon to eutrophication.

Lastly, side-scan sonar technology was optimized to accurately map critical geologic environments and biologic habitats in Quonchontaug Pond in order to provide a baseline to study future habitat alterations. The 500kHz side-scan survey was adequate to identify eelgrass beds. The eelgrass had a characteristic speckled pattern in the side-scan mosaic, which allowed precise mapping of the extent of the beds. Groundtruthing was conducted with underwater video and sediment grab samples. Areas with boulders were at times hard to differentiate from the eelgrass, especially when the swath curved at all, blurring the boulder images. This also limited the prospects for automating the process of identifying the eelgrass pattern in the mosaic. However, this method holds potential for developing accurate estimates of eelgrass bed areal coverage. Eighty acres of eelgrass were present in Quonochontaug Pond.

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