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Activiities of ASTER Working Groups |
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Tokyo Univ.
Hajime Kayane
NIES
Tsuneo Matsunaga
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On tropical/subtropical shores, a biologically characteristic coastal landform called coral reefs is found. Aside from their aesthetic value and beauty derived merit as tourist resources, the importance of coral reefs is also reconsidered in terms of diversity of life species as well as carbon circulation.
The coral composing coral reefs (not one in the precious stone group) has a calcareous skeletal frame work, which piles up to form a huge landform of coral reefs. Coral also allows a tremendously large amount of Algae to live together in symbiosis within its body and, through their photosynthetic production, maintain an exceedingly high level of production in generally less productive tropical waters. Coral reefs enjoy the greatest diversity of maritime life species by offering a landform suitable for habitation and feeding them with life partly serves as food resources for people living in tropical areas.
It is probably because of such diversity and their unique landform why coral reefs are precious tourist resources. And among the variety of materials produced by these many life species, there may possibly be a lot of things useful for us including medicinal substances and periphyton-repellants, calling our present notice to the importance of coral reefs as a place to store genetic resources. Furthermore, coral reefs are also involved in carbon circulation through their photosynthetic production and calcification, inviting current studies to develop ecosystem-based carbon dioxide fixation technology by controlling such involvement.
It is pointed out that coral reefs, whose importance has recently been reconsidered, are now endangered toward extinction due to population concentration and resulting rapid developments in coastal areas of tropical/subtropical zones. If coral dies not only directly due to landfilling of coral reefs and/or dredging for harbor construction but also when earth and/or nutriment flow into coastal waters as a result of inland developments, the numerous life species it has supported also die.
A certain researcher predicts possible near extinction in ten years or less of one-third coral reefs across the world including those around the Ryukyu Islands of Japan and other southeast Asian areas, Florida and Greater Antilles.
Coral reefs are also endangered due to global-scale environmental changes including global warming and the consequent rise in sea level. The upper limit of water temperature at which coral can grow is about 30℃ and, if a water temperature higher than this continues, coral releases symbiotic Algae from its body to become white and soon die. At a type of coral reefs called "atolls" in tropical zones, people live on islands of 2~3m above sea level at highest. Even today, these islands often suffer damages such as due to typhoons and storm surges, raising concern over an increase in the severity of damages in case global warming pushes up the sea level.
It can be said that coral reefs in the most recently developed tropical zones are now subjected to the severest environmental changes at a regional level as well as on the global scale.
In 1994, the U.S. proposed the "International Coral Reef Initiative" which advocates promoting the preservation, management, and sustainable exploitation of coral reefs. Under the initiative, international cooperative efforts are in progress to develop the Global-Scale Coral Reef Monitoring Network. Japan is also expected to play an active core role in the monitoring, security, and appropriate development of coral reefs, especially, in the Southeast Asian region. Coral reefs have, however, so far been monitored by putting together the results of observation done by diving or other means such as at a coastal station. The information obtained by diving on the site is spatially limited. And what is worse, no such information is available from coral reefs without a coastal station. A problem to first consider is that there are still many coral reefs in tropical zones having no exact map of their regions.
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With such importance of coral reefs and their monitoring in mind, the EOS Specialized Data Utilization Committee plans to actively use ASTER for preparing/monitoring a distribution map of coral reefs. Review has so far been under way, using existing LANDSAT TM and SPOT data on how effectively coral reefs can be monitored by ASTER.
Coral reefs are characterized by a landform of shallows zero to several meters deep below the sea surface (called reef plain?). The distribution of reef plains can be extracted from aerial pictures or by remote sensing through a satellite. ASTER can also map the distribution of coral reefs mainly by the use of visible (VNIR) bands. ASTER has a demerit of having no blue band highly permeable in seawater. On the other hand, its visible band with a high space resolution of 15m is quite effective to exactly grasp the patchily complex landform of coral reefs fringing indented coast lines in a width of hundreds of meters. A short-wave long infrared (SWIR) band can also be used to precisely detect clouds and land areas.
It is further possible with visible-range images to monitor the distribution of coral and other biotic communities on coral reefs as well. If you think of a coral reef, only coral distribution all over may come into your mind, but it is actually a complex composition with spatial combination of seaweeds/marine algae, sand from pieces of materials including coral, rocks exposed due to the death of coral, and so on.
Ecosystem WG is considering to monitor the distribution of such biotic communities on a coral reef as well as its change by using ASTER-derived data. The greater the water depth, the smaller becomes the reflective brightness because of dispersion due to sea water. Hence, the ratio is taken between 2 ASTER visible bands to determine water depth independent bottom quality indices, thereby finding different bottom qualities and communities. It is relatively easy to distinguish between three items of coral/seaweed/marine algae communities, sand, and rocks but considerably difficult to tell coral from seaweeds/marine algae. Their detail classification requires actual survey of how these communities are distributed on the site. Fig. 1 Distribution Map of Coral Reefs on Kayangel Atoll of the Palau Islands
Fig. 1 shows a distribution map and biotic community compartments on Kayangel Atoll of the Palau Islands prepared with SPOT data. The outer black area represents the ocean, several inner black patches on the righter side are land, and colored regions stand for coral reefs. It is found that the distribution of coral reefs has been extracted through its distinctive demarcation from the ocean and land. In addition, using 2 visible bands, the above-mentioned distribution of bottom quality indices is differently colored. In this figure, orange color is for coral reefs, yellow, for rocks, green, for sand, and light blue or blue, possibly for lagoons.
ASTER can thus identify the distribution of coral reefs and rough geographical compartments. It provides the most basic data for enhanced preservation and management of coral reefs. As for strongly required- monitoring of coral reefs including those currently endangered toward extinction and those whose special preservation is needed, a detail distribution map of biotic communities can be prepared as well, combinedly using current data, for continual observation. At that time, it is also possible to monitor the inflow of sand and nutriment from land which may destroy coral reefs. Precise monitoring of coastal line shapes permits us to predict the influence of a rise in sea level. ASTER can further be used to quickly monitor any changes in seawater temperature in coral reef waters.
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Fig. 1
Distribution map and biotic community compartments |
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For the efficient monitoring of coral reefs widely scattered over the ocean, it is necessary to submit an active request for data acquisition after having extracted coral reef locations and identified their path/row correspondence with ASTER prior to satellite launching. In line with the above consideration, a satellite data base has been developed for coral reefs on an entire globe scale.
First, the entire globe is gridded by 0.5-degree latitudinal/longitudinal lines, thereby extracting grid ranges of coral reef locations, with the coral reef data base (ReefBase), marine charts, and other materials as reference. A total number of grids extracted was 2,753 (of which 1,433 grids were assured to contain coral reefs). Next, these grids were corresponded to so-far obtained satellite path/row data. The satellites used were LANDSAT-1, 2, 3, 4, and 5, MOS-1 and 1b, SPOT-1, 2, and 3, and JERS-1. Then, several data bases were accessed to input how satellite images (such as the date of observation, amount of cloud, and picture quality) had been acquired.
Thus-obtained distribution maps of coral reefs in the Pacific region as well as the data acquired from SPOT under such circumstance were shown in Fig. 2 only for the cloud amount of zero after selection. Apparently, for those islands scattered around the middle of the Pacific, there are almost no data so far obtained via satellite.
These islands are covered with solid masses of cumulus cloud and, if only images with a small amount of cloud are selected, the majority will be for coral reefs whose good-quality data are still unavailable. Satellite coral reef data bases prepared in this way are under review for the linkage to "ReefBase" as a coral reef data base derived such as from local observations and marine charts.
EOS Specialized Data Utilization Committee plans to prepare a coral distribution map and practice the monitoring mainly of coral reefs such as in the northwestern Pacific including the Ryukyu Islands and southeast Asia. It is, however, almost impossible to monitor all coral reefs on the globe. And another purpose of monitoring is expected incorporation of all the algorisms and coral reef data bases used for distribution maps so far prepared into the ASTER data base developed by ERSDAC. I wish researchers of coral reefs and as many people involved in their preservation, management, and appropriate exploitation as possible to effectively use ASTER data.
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Fig. 2
Distribution of Coral Reefs in the Pacific
and State of SPOT Data Acquisition |
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( Eco/Oceanography and Limnology WG ) |
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