Comparison of Benthic Cover Trend Between Satellite and In Situ Datasets (1996-2002) for Reef Ecosystems of the Florida Keys National Marine Sanctuary
Palandro, D., Hu, C., Andrefouet, S. and Muller-Karger, F.E.
University of South Florida, Saint Petersburg, FL
Lybolt, M.,
Florida Marine Research Institute, Saint Petersburg, FL
The primary operational method to gather percent coral cover for coral reefs is to gather in situ data for the specific site. The Florida Marine Research Institute (FMRI), under the Coral Reef Monitoring Project (CRMP), has been gathering such data for 40 sites in the Florida Keys National Marine Sanctuary (FKNMS) since 1996, with the introduction of three more sites in the Dry Tortugas in 1999. This dataset is the largest of its kind and has provided for the basis of a change detection study to be made. The CRMP uses eight permanently placed stakes per reef site to construct four 20m x 2m transects. This provides approximately 160m2 of coverage for each reef site. Video transects are analyzed with PointCount® software to provide percent cover. The results of these yearly studies provide live coral percent cover, by species, as well as the percent cover of broader benthic categories (e.g., substrate, sponges macroalgae, octocorallia). Unfortunately in situ methods allow for only the study of a small part of the coral reef system, as the time and cost required to cover all would be immense.
Recent advances in satellite remote sensing technology and methodology have allowed for long-term change detection studies to be performed on benthic habitats. The Landsat series of satellites carrying the Thematic Mapper (TM, Landsats 4 and 5) and the enhanced Thematic Mapper Plus (ETM+, Landsat 7) sensors are the longest running continuous satellites that can be used for coral reef studies. Landsat provides 16-day repetitive coverage for sites at a 30m spatial resolution. Although the resolution is too coarse to yield specific coral species coverage, TM and ETM+ data allow for the study of overall benthic cover at the community level. This synoptic view of a reef system allows the researcher to study all sections of a reef, not just a small part, at various places in time over the past 20 years.
CRMP and Landsat datasets for 1996, 1998, 2000 and 2002 for six sites in the FKNMS and one site in the Dry Tortugas for 2000 and 2002, are used in this study. The sites were chosen for their location (two sites for each of the upper, middle and lower keys), their designation as a Sanctuary Preservation Area (SPA), and their depth (less than 12m). The sites are: Carysfort Reef Shallow and Molasses Reef Shallow (Upper), Alligator Reef Shallow and Sombrero Reef Shallow (Middle), Looe Key Reef Shallow and Sand Key Shallow (Lower), and White Shoal Reef (Dry Tortugas, non-SPA) (figure 1). Because of the difference in spatial resolutions, e.g. species level versus community level; the trend in the change in benthic cover becomes a paramount concern. Not only is the percent coral cover the most comparable between the two datasets, but also is of the most importance to local policy makers.
Preliminary results (1994-2000) show an excellent fit between the two datasets (figure 2). In one case for Carysfort Reef Shallow the values are nearly identical (2000). Other sites show more variance between the datasets. These positive results between remote sensing data and data gathered in situ highlight the fact that remote sensing can provide information about a reef that in situ data cannot, i.e., a view of the entire reef community, whereas satellite data cannot provide information on the species level. There will unlikely ever be a time when there is no need to visit a reef for either species information (unable to gather using current remote sensing technology) or to gather ground-truthing information (to provide quality control for satellite data). However, it is hoped that the positive results of this study might help push the use of satellite remote sensing into a more operational phase for coral reef monitoring.
Corresponding Author:
David Palandro
College of Marine Science, University of South Florida
140 7th Avenue South, Saint Petersburg, FL 33701
727-553-1186
727-553-1103 (fax)
Figure 1. Satellite image displaying the locations of the seven study sites.
Figure 2. Percent live coral cover for Carysfort Reef Shallow derived from both CRMP in situ data and Landsat satellite data.