Date of Award:

1998

Document Type:

Thesis

Degree Name:

Doctor of Philosophy (PhD)

Department:

Biology

Department name when degree awarded

Biology Ecology

Advisor/Chair:

W. David Liddell

Co-Advisor/Chair:

James a. Gessaman

Abstract

The composition, recruitment rates and fine-scale spatial dynamics of the sessile benthic community occupying hard substrata off Lee Stocking Island, The Bahamas, was determined through photography and the deployment of artificial substrata from September 1993 to September 1996. The study spanned 10 to 250 meters in depth. The benthic community exhibited a pronounced bathymetric zonation. Filamentous and macroalgae dominated the shallow communities , but were replaced by corals, sponges and endolithic algae with increasing depth. Living cover was high above 100 m, but declined significantly below this depth, being replaced by increasing amounts of bare and sediment-covered rock surfaces. Benthic zonation was shifted downward with respect to other locations in the western Atlantic, possibly due to the clear waters of Exuma Sound allowing relatively deep light penetration.

Recruitment experiments were deployed during two time inten :als from 1994 to 1995. The experiments enabled the analysis of the effects of five treatments which differed in depth, orientation and degree of exposure. Recruitment of all sessile taxa declined sharply with depth. Peak recruitment for most taxa occurred between 20 m and 50 m. Recruitment experiments retrieved from sites located above 50 m exhibited high cover of :filamentous and macroalgae on exposed surfaces. Sponges, corals and other invertebrates dominated the cryptic surfaces. Dominance by polychaetes characterized the deeper sites.

Fine-scale changes in community structure were determined through repeat photography of natural substrata and artificial settling panels. Numerical change indices were generated for changes in cover and additions and losses of individual organisms occurring over time. Each change index began at a relatively low value at the 10 m depth, increased to a maximum value at 30 m or 50 m and then declined with increasing depth. Numbers of fish grazing scars were positively correlated with the loss index and negatively correlated \\ith algal cover, suggesting that fish contribute to fine-scale changes in the sessile community. Important contributors to fine-scale spatial changes within the benthic community shift from algae to corals to sponges with increasing depth.

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