Date of Award

12-2006

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Ecology and Environmental Sciences

Advisor

Laurie Osher

Second Committee Member

Christopher Cronan

Third Committee Member

Mohamad Musavi

Abstract

Seagrasses are vascular plants that grow in shallow coastal waters. They are essential for protection and sustainability of coastal and marine ecosystems because they support food webs, provide critical habitat and physically stabilize substrates. The productivity and health of seagrass communities are largely dependent on water clarity, because clear waters have the greatest light availability. Changes in coastal ecosystems that result in increased light attenuation, including turbidity and eutrophication, reduce seagrass growth and abundance. Regional studies of seagrass coverage along the Atlantic coastline estimated that present populations represent a 70 to 90% decline in seagrass coverage from populations of 50 years ago. The major cause of these declines is attributed to the increase of nutrients entering coastal waters, and the light attenuation associated with eutrophic systems. Other causes of seagrass declines include storm events, disease outbreaks caused by seagrass-specific pathogens, and grazing by biota. Lack of data for most portions of the US coastline makes it difficult to quantify loss or determine the rate of disappearance of seagrasses for specific locations. Recently, the Maine Department of Marine Resources measured an 85% decline in seagrass coverage in Taunton Bay, Maine, over a six-year time period. Eelgrass (Zostera marina) is the predominant seagrass in this estuary, and its apparent decline has raised questions regarding causes of this phenomenon and the history of eelgrass fluctuations in the bay. How does the current distribution of eelgrass compare with the 'normal' percent crown cover for this population in the last 50 years? Is there a possibility that the eelgrass population in Taunton Bay has previously reached such a low percent crown cover and recovered? To address these questions, visible eelgrass beds were delineated in 11 sets of aerial photographs collected from 1955 to 2005. Using GIS software, photomosaics were created for each aerial photo set and maps were generated identifying the eelgrass beds in each photo mosaic. Using these data, the change in eelgrass area and crown cover density were quantified over time in the Taunton Bay estuary was evaluated. Changes in area and crown cover were calculated for the entire bay and also on a quadrant by quadrant basis. Eelgrass cover is estimated in absolute terms (total area and percent crown cover) and in terms of total percent crown cover density as a percent of the potential area where eelgrass could grow. Results indicated that the eelgrass area as well as percent crown cover in Taunton Bay has declined gradually between 1955 and 1996. In contrast, eelgrass loss from 1996 to 2005 occurred at a much faster rate that was three times higher than the loss rate from 1955-1996. In 1955 and 1996, eelgrass crown cover occupied 25 and 17 percent of the potential eelgrass habitat. By 2005, eelgrass crown cover occupied only 6 percent of the potential habitat. As has been observed elsewhere where seagrasses have declined, the remaining populations are fragmented rather than growing in large, continuous beds.

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