Coral reefs are found in the tropical waters of the world and reach their highest development in oligotrophic waters where nutrients and not light energy appear to be limiting. There have been bioherms in the sea since the Precambrian. Modern reefs consist of a framework that is constructed principally by corals and infilled with carbonate sediment produced by a long list of biological and geological processes. Hermatypic corals have evolved a complex symbiotic relationship with zooxanthellae, yellow brown dinoflagellates which act as internal nutrient recyclers and harvest solar energy which enhances coral calcification. Without zooxanthellae, corals calcify but at rates that are approximately 1 to 2 orders of magnitude slower. This light requirements of the symbiotic relationship places limits on the depths to which active coral growth can occur, usually less than 75m. Thus, a signal that can be remotely sensed to depths of 75 meters would include the majority of reefs if the world.
At present we do not have a good estimate of the global abundance of corals reefs as most occur in remote places and, in many instances, the current navigational charts date from surveys carried out by the sailing ships. There is a need to accurately and precisely determine the abundance of corals reefs for a variety of reasons:
1. Cartography and navigation: Where are the reefs and how many are there?
2. Ecosystem management: Reefs are coming under increasing pressure from humans
3. Reef studies: Reef ecosystems are the oldest and most diverse shallow water ecosystems in the sea and there is much to learn
4. Reefs effect oceanic processes: Reefs alter the flow of water in the oceans at a variety of scales. For example, they protect shorelines and the island wake effect can cause upwellings result in increased oceanic production. Reefs store carbon and calcium in the form of carbonates.
5. Harbingers of global change: Reefs are very sensitive to environmental conditions. They are adapted to extreme oligotrophic conditions. Changes in coral health and vitality (disease, algal overgrowth, bleaching, etc.) may be more sensitive indicators of changing environmental conditions that current measuring technology.
For passive optical remote sensing the important optical properties of the reef are provided by the photosynthetic pigments fixed in the benthic organisms. These include members of all groups of algae, principally red, green, and the yellow brown zooxanthellae. The upwelling signal from these organisms is influenced by the overlying water column as the signal is reflected upward through it to the surface. Therefore there is a need to develop algorithms based on depth and water optical partition to effectively partition the signal by "stripping off the water" and leaving bare the benthic reflection.
Satellite imagery provides a scale that can be used to determine reef from non reef. Satellite data can also be used to provide information on oceanic processes and current patterns around reefs, as well as SST. But for satellites to become more useful to reef studies we will have to wait for the next generation of high resolution scanner with increased spatial and spectral sensitivity.
Aerial photographs can provide a much finer scale but without the spectral resolution and measure of absolute radiance. Using a digital scanner, air photographs can be converted to RGB digital imagery at a resolution that is compatible with the purpose of the survey. Multispectral cluster analysis then transforms the image into the GIS (Geographical Information System ) domain and with appropriate ground truthing, and georeferencing, the coverage of classes can be calculated. We have done this with 1:40,000 air photographs to a resolution of approximately 5 meters and with handheld 35mm photographs of selected patch reefs with resulting spatial resolution of less than 25 cm. Biomass estimates from these photographic images compares favorable with line transects and underwater quadrat mapping.