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Thesis - NSU Access Only
M.S. Marine Biology
Samuel J. Purkis
John C. Brock
Richard E. Dodge
Aerial photographs from 1937-2000 of Bahía Salina del Sur on Vieques, Puerto Rico were analyzed to detect and describe spatial changes in the areal cover of seagrass beds in Bahía Salina del Sur. The images were pre-processed to minimize noise and unsupervised classification was used to detect areas colonized by seagrass. The number of individual seagrass patches, direction, and characteristic of growth were quantified and described. Through the 64 years encompassed by this study, an 85.84% increase in seagrass coverage was observed. The time course of increase was best described by a 2nd order polynomial function (R2 = 0.91). Between 1937-2000, the spatial expression of the seagrass patchiness went through discrete episodes characterized by the expansion in the number and spatial extent of small patches, which was subsequently followed by an increase in patch size and agglomeration of small patches to form large homogeneous areas. Patch growth was limited only by proximity to boundaries (i.e. coastline and reef structures) and a fluctuating physical environment. This study suggests that the increase in seagrass cover is linked to the synergy of hurricane impacts, decrease in grazers, and the protective geomorphology of Bahía Salina del Sur. Conversely, a decrease in areal cover only occurred in concert with known anthropogenic impacts. This study shows that the analysis of a shallow-water benthic system using airborne remote sensing can provide a robust understanding of a biota’s variation in time and space that is useful for both environmental managers and decision makers.
Since the late 1970’s Acroporid coral communities declined drastically throughout the Caribbean. Their decline was to such an extent that both Acropora palmata and A. cervicornis were placed on the Candidate Species List of the Endangered Species Act. The demise of Acroporids in Vieques has been attributed primarily to an epizootic event of White Band Disease, and hurricane impacts. A synergy between hyperspectral AISA imagery and a SHOALS-LIDAR bathymetry dataset was utilized to detect areas presently or previously occupied by A. palmata. Classified hyperspectral imagery was draped over LIDAR bathymetry to create a three-dimensional model of the seafloor. The maximum holding-capacity of the environment for A. palmata was calculated from bathymetry using literature values for their vital and optimal ranges, defined as reefal areas of 1 to 5 m and <1 to 17 m in depth respectively. It is established in the literature that the 1970’s represented a period of climax for A. palmata in the Caribbean, at which point it is believed to have occupied 49.8% of its optimal range on Vieques and extensive mono-specific thickets of A. palmata were widespread. Since then, area coverage of A. palmata has decreased by as much as 99.99%. Most presently living A. palmata occur in waters deeper than their optimal range. Critical habitat for A. palmata in Vieques was defined as those areas of 2 to 17 m in depth. Our findings are in concert with the well-documented Caribbean-wide coral demise over the last two decades.
Luz Raquel Hernandez-Cruz. 2006. An Optical Remote-Sensing Evaluation of the Status of Acropora palmata in Vieques, Puerto Rico. Master's thesis. Nova Southeastern University. Retrieved from NSUWorks, Oceanographic Center. (277)
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