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)
palandro@seas.marine.usf.edu
http://www.imars.marine.usf.edu
Figure 1. Satellite image displaying the locations of the seven study sites.
Coral Cover for Carysfort Reef Shallow
Percent Coral
15
10
5
0
1994
1996
1998
2000
Year
Landsat
In Situ
Figure 2. Percent live coral cover for Carysfort Reef Shallow derived from both
CRMP in situ data and Landsat satellite data.