HEALTH OF CORAL - SENSITIVE BIOINDICATOR OF GLOBAL & LOCAL
Mineo OKAMOTO Ph.D., Motohiko MOHRI M.D. and Hirotsugu TAKEUCHI
Marine Ecosystems Research Department
JAMSTEC (Japan Marine Science and Technology Center)
2-15 Natsushima Yokosuka 237-0061, Japan
Tel.: 81-468-67-5536; Fax.: 81-468-66-5747
E-mail : firstname.lastname@example.org
Coral and coral reef are widely distributed in tropical and subtropical seas. Rapidly growing corals are in symbiosis with Zooxanthellae, single-celled micro algae, in their polyp, and they are called reef-building corals (Hermatypic corals) . The areas where reef building corals are distributed widely are called coral seas. Coral (reef building coral) polyp lives in a skeleton made of calcium carbonate, possessing several million Zooxanthela in a square centimeter. By using organisms produced by photosynthesis of symbiotic algae, coral grows rapidly in comparison with non-symbiotic coral. Coral that has various shapes (tabled, massive, branching and so on) is created from one settled larva by growing in chip direction or concentrically circular. The assembled polyp and skeleton are called a colony.
Coral and coral reef are considered as potentially important biological indicators of global climate change and fluctuation of the oceanic environment (Wilkinson et al.,1994; Sea Grant HAWAII, 1997). Their health may provide an index for marine pollution by land drainage and mud inflow (Jones, 1986; AIMS, 1995). Coral reefs are also important fishery areas and sources of biodiversity (Done. et al., 1996). Recently, Kayane (1992) discussed the role of coral reefs in reference to the global climate warming caused by the increase of carbon dioxide.
To make use of the coral as a bioindicator of environmental changes, monitoring of the coral health condition and its changes is needed. There are two coral monitoring methodologies. One is broad scale, rough monitoring; and the other precise scientific monitoring.
In this paper, we summarize the various popular measurement techniques to use coral as biological indicator. A new health measurement technique of the coral of Okamoto (1998) and its preliminary field results in the Sekisei Lagoon are reported.
2. CORAL HEALTH MEASUREMENT TECHNIQUE
To apply the coral health as a bioindicator of environmental changes, it is necessary to carry out the coral health measurements from various points of view. The outlines of the measurement techniques are summarized below.
(1) Coral distribution
Coral is widely distributed in subtropical and tropical seas, and its distribution depth band is concentrated near surface within 50 meters, since light is necessary to photosynthesize symbiotic algae. Coral grows well in the place of hard bottom material, coral reef and rocks, since it takes hard substrate and long time to grow. The water temperature below 18° C slows down coral growth, and high water temperature above 30° C causes coral bleaching. Coral grows in a clean water, thus its growth is inhibited by turbid and polluted waters, low tidal exchange and inflow of minute mud, etc. Therefore, the outline of environmental conditions can be deduced by knowing the pattern of the coral distribution. There are more than four hundred coral species in the world.
(2) Live coral cover
Coral grows thick when the environmental condition is favorable, especially at the edge of coral reef. However, the coral growth and cover are affected by both long-term and the short-term environmental perturbations. Therefore, it is possible to evaluate the effect of the environmental impact by monitoring the coral cover at the edge of coral reef. For measuring coral distributions, manta-tow survey method is commonly used (AIMS, 1994). The manta tow is a manned observation method (Done, 1982; Dahl, 1986; AIMS, 1994; Done et al., 1996). An observer is towed at the sea surface along the reef edge at slow speed to examine the extent of coral cover in fixed intervals. Coral cover is divided into six stages from 0 to 100%. This method is convenient since no special measuring device is needed, and can cover a wide area rapidly.
(3) Coral distribution depth band and composition of coral community
In general, coral grows when the water temperature range 18 - 30° C. When temperature rises above 30° C the coral at the shallow depth is impacted, and similarly when temperature falls below 18° C the deeper coral too is affected. When the seawater becomes muddy, the surface light does not reach the deeper depth sufficiently, and the coral of the deeper place is effected. The increase of the ultraviolet ray inhibits the coral growth of the shallow place. Therefore, diagnosis of coral health in conjunction with environmental changes becomes possible by investigating coral cover and composition of coral community of the typical water-depth layer.
As for the survey technique, the line intercept transect method and the quadrate method are commonly used (AIMS, 1994). The line intercept transect estimates coral cover along the traverse line set on the seafloor. The quadrate method is used for detailed studies in small areas. Data acquired vary among different observers who use video recording, still photographs, and writing down visial observations.
(4) The health of coral colony
Coral species have various distinct colors such as red, green, and blue. The color of coral changes depending on the condition of the health, even if it is a homogeneous colony in the same place. Generally, healthy coral has deep color, and becomes light when unhealthy. The coral that photosynthesizes actively radiates the fluorescence by the chlorophyll of the Zooxanthela. The photosynthesis of Zooxanthela decreases and it escapes from coral polyp when the environment deteriorates. Then coral becomes whitish by the decrease in Zooxanthela, and this phenomenon is called bleaching. The coral dies when such condition continues long and when drastic environmental deterioration occurs. Therefore the health condition of such coral colony is thus possible by measuring photosynthesis activity and color change of the coral.
3. DEVELOPMENT OF THE NEW CORAL HEALTH MEASUREMENT TECHNIQUE
We have been examining the coral ecosystem at Sekisei Lagoon that is located between Ishigaki-jima (Ishigaki Island) and Iriomote-jima (Iriomote Island), Okinawa Prefecture (Figure 1). The width of the lagoon is approximately 25 km east to west and 20 km north to south. The area is surrounded by islands and reefs. The reef is exposed to the air at low tide, and the flow of seawater to the lagoon is obstructed. To the west of the lagoon is Iriomote-jima. The south side of the reef has almost collapsed except around Aragusuku-jima and Kuro-shima. Channels occur at several places in the reefs through which strong tidal flows pass. On the outside of the reef there exists reef frameworks to depths of 20 to 25 m. Inside the reefs, the lagoon is shallow and flat, up to 10-m depth. The bottom material is calcareous sand derived from coral. There are many patch reefs in the lagoon. Our study is conducted around the whole area in the lagoon and the three typical patch reefs of varing environmental conditions (Figure 1; Stations A, B, C).
(1) Research method
A. Coral distribution and cover
For surveying distribution and cover of coral at the whole Sekisei Lagoon, cruising survey was conducted by using a simple tow-survey system CS-2 (Coral Surveyor-2, Figure 2), which consists of a tow and onboard equipment. Tow consists of underwater TV camera (Q.I. Inc., digital 8-mm video camera with 10 x zoom lens with 300 W light), an underwater 35-mm still camera (Q.I. Inc., Nikon F4s with 28-mm f 2.8 lens, and 250 exposure type film cartridge), an echo-sounder (Kaijyo, PS-20R, 200 kHz) and an aluminum frame which contains these instruments. The system was hung at 50-cm depth during the cruising survey, and controls and recorders of them were set onboard. The TV camera and echo-sounder are operated continuously during the survey, and picture recording and depth record of the sea bottom image are acquired. The pictures are taken once every 15 seconds and by choosing a moment when the oscillation of the ship is low. With normal cruising speed of 1-2 knots, still pictures are obtained at a distance of around 7-17 m interval. Position of cruise line was measured once a minute. By this method, it is possible to survey the traverse line of about 2-4 km length in about 30 minutes. Surveys of forty-five traverse lines were successfully completed in the Sekisei Lagoon in February 1996, December 1996, and October to November 1997.
The degree of coral coverage was obtained by analyzing the 35-mm still pictures. Each negative film (Fuji-Color G400) was enlarged for printing, and converted to a photo CD. The coral coverage was analyzed from photo CD by using image analysis software (Adobe photoshop 3.0J). Coral was chosen from each picture, and the coral coverage was estimated as the proportion of the screen. The ground area of each picture was calculated from the bathymetry record and the angle of field of the 28-mm lens. Water depth was obtained by correcting bathymetry record by tide to the standard water surface. These data were diagrammed with the data of the water depth. On each traverse line, average coral coverage of the each water depth band was obtained.
B. Minute Coral Survey
A diving survey technique was developed to document coral conditions more accurately than the cruising survey technique. A tape measure of 50-m length was stretched over the seafloor and feature of the bottom and condition of coral were recorded along the tape. The camera used was an NIKONOS with a 15-mm f 2.8 lens. One 36-exposure type ISO 400 negative color film was used for each site. The 35-mm still pictures were taken from a height of 1 to 2 m above the tape measure, each with some over-wrapping. The height is lower where corals had complex features and in shallow water depth. By this method, the investigation was carried out at 135 sites in the Sekisei Lagoon in February 1996, December 1996, and October to November 1997.
C. Examination of coral health at typical patch reef
a. Selection of study site and mapping
Three permanent study sites (Stations A, B, C in Figure 1) were selected that had typical coral reef patterns of Sekisei Lagoon, healthy, moderate and unhealthy reefs. Two of them (Stations A, C) were measured for their shapes precisely, but the remained Station B was too large for mapping.
b. Preliminary survey of coral health
Composition of coral community, coral cover, and visual coral health measurement were conducted at the two patch reefs of Stations A and C. Along several transect, mosaic pictures were taken by using 50-m diving survey technique.
c. Evaluation of the chlorophyll fluorometer
The Diving-PAM underwater chlorophyll fluorometer (made by WALZ) was evaluated as for the possible measuring instrument of coral health. It is to measure the efficiency of the photosynthesis of the Zooxanthellae (Walz,1998). This equipment puts the strong pulsed radiations of 650 nm to the plant, and measures fluorescent yield (F) and largest yield (Fm), then calculates photosynthesis yield (Y= F/Fm).
d. Evaluation of the spectral colorimeter
The Spectra Scan PR 650 colorimeter (Photo Research Inc.) was applied for coral color measurement by containing underwater housing.
4. RESULT AND DISCUSSION
(1) Outline of coral cover at Sekisei Lagoon
The coral distributions obtained by diving survey and cruising survey are summarized. Coral growth on the south side affected by the open ocean waves is poorer than on the north side. On the outside of the outer reef, the rocky reef slope spreads down to 25-m depth, and outside of it changes to sand. Coral grows well from the edge of the reef to 25-m depth of this rocky stretch; and in deeper sandy areas, coral is found on the boulders down to 29-m depth. In places where the outer reef have collapsed, the coral is distributed to below 20-m depth where the rocky stretch has remained. It is limited to boulders in the sandy areas. On the reef flat, coral is quite rare on sand, and shallow areas occasionally exposed to air. In shallow, narrow reef ponds between the reefs and the islands, coral is sparse. In the lagoon, the coral grows both on patch reefs, and in some places it is distributed widely across the flat sea-bottom floor at depth of 10 m or less. The results obtained by cruising survey along Line 6 and Line 13 are shown in Figures 3 and 4, respectively, examples. Line 6 is located at more open area in comparison to Line13, and coral cover of the former is much more than the latter. These differences are considered to be caused by the extent of seawater exchange by tide and current.
(2) Coral health measurement at typical patch reef
A new coral health measurement technique have been developed at typical patch reefs (Stations A and C; Figure 1) . The outline of these reefs and the coral community structure are as follow.
A. Outline of two typical patch reefs
Statiom A is located at an open area close to the outer reef line; its biota are various and distributed densely. It is a typical table-mountain shape patch reef that is located at approximately 14-m water depth and has 40 x 25 m flat top at the sea surface (Figure 5; a). The dominant coral species are Acropora hyacinthus, Acropora cytherea, Acropora digitifera and Acropora selago.
Station C is located at near the center of the lagoon, and the minute silt is contained in the bottom material. It is gentle mountain-shaped small reef located at 6 - 7 m water depth and 25 meters in diameter (Figure 5; b). Branching coral is distributed much more here than compared to Station A, and also biota and coral coverage is poor in comparison with Station A. Dominant coral species are Acropora hyacinthus and Acropora selago.
B. Coral health monitoring by using visual monitoring and still pictures
At the Sekisei Lagoon, the coral color was felt somehow thin and coral looked not so healthy during the coral spawning study in May 1998. Then, coral bleaching problem around Okinawa Islands and Ishigaki-jima occurred around July 1998. Before these phenomena, large-scale coral bleaching at Great Barrier Reef in Australia was reported in spring 1998.
Development of coral health measurement technique at the patch reefs was started in October 1998. At that time, the health condition of one dominant coral Acropora hyacinthus was observed by visual inspection. Coral health was distinguished into three grade: Normal, healthy colored; Bleaching, whitening due to the Zooxanthellae escaping; Dead, the coral skeleton becoming white by the death of polyp. The results at Stations A and C on 13 October 1998 are as follows:
Station A: Subject 100 colonies; Normal 91%; Bleaching 8%; Dead 1%.
Station C: Subject 89 colonies; Normal 12%; Bleaching 74%; Dead 13% .
Water temperature at the 3-meter depth of Station C at the study time was 28~29° C as shown in Figure 6. The temperature gradually fell down to 20° C in February 1999, then rose again. The temperature in summer 1998 was estimated around 30° C or higher, which is a critical situation for coral. The pictures at the same place of Station C on 6 October 1998 and 9 March 1999 are shown in Figures 7 and 8, respectively. The tape shown has 7 cm in width and scaled by 1 cm interval. All the coral colonies in the picture are bleached on 6 October 1998, and almost all of them recovered by 9 March 1999. Then, the massive coral family Faviidae spawned two hours after the sunset on 3 May 1999, and table-shape family Acropora spawned two hours after the sunset on 4 May 1999 as shown in Figure 9.
The large-scale coral bleaching and recovery process in the Sekisei Lagoon is summarized as follows. During summer 1998, coral bleaching problem occurred in Sekisei Lagoon. Approximately 90% of coral bleached or died at the place where the environmental condition was not good, and 10% at the good environmental condition remainded. This event was almost gone by October 1998, and then coral health recovered on account of dropping water temperature, and the coral could spawn in May 1999. The cause of this bleaching and difference of its impacts by site are deduced mainly by high water temperature in summer and differing coral health condition by siltation, for example.
C. Coral health measurement technique by using photosynthesis yield
The visual assessment of the coral health appears comparatively easy for a scientist. However, the coral variety is numerous, and even one species has many colors and shapes. In order to carry out the health examination easily, it is necessary to use the technique that can digitize the measuring results and can be carried out by an even non-expert. For this purpose, we applied the measurement of photosynthesis of Zooxanthellae. Example of its data is shown in Figure 10. The coral Acropora cytherea was 2.3 m in diameter, and its left side was bleaching. The measurement was conducted by 10-cm interval from left to right along the measure of 2-m length. In this figure, M (maximum florescence yield) is a fluorescent maximum value utilizeable by the coral, which is approximately equal to the largest photosynthesis quantity. Y (photosynthesis yield) shows the degree of utilized light for the photosynthesis with the absorbed light. As shown in the picture, bleaching is slight at the left end of the coral, then much more bleaching at around one meter to rightward, and almost normal at right half. The value of M and Y also correspond with this, and there is the high possibility to use the parameter of the photosynthesis activity to estimate the coral health by a digital means.
D. Coral health measurement technique by using color spectrum meter
The coral health can be judged from its color by visual observation as mentioned in B (given above). Therefore, coral color was measured by using sector color meter to learn of its underwater efficiency. Results, obtained from Acropora hyacinthus and Acropora digitifera each at the 8.6-m water depth are shown in Figure 11. The measurement was conducted at several points of the two colonies and white standard reflector board under natural light condition. The photographing distance was set at about 10 cm. The curve that has peak at 480~500 nm is the result of standard reflector board, and others are of coral at several places. The white board become yellow green to blue, peaked blue green, and the near infrared and red color are disappeared. As for the coral, two species have same peaks from the blue green to yellow green and is slightly different near the yellow green, Acropora hyacinthus having a peak at 590 nm and Acropora digitifera at 580 nm. The coral has different colors at very shallow water or under the light, but color changes depend on the increasing depth by color filtering effect of seawater. Therefore, it seems difficult to use the coral color spectrum for the comparison of species or its health under natural light at present. To apply this for coral health measurement, measuring the reflected light in irradiating standard light source will be necessary at least.
The study of coral to use as a bioindicator of global and local environmental changes has just started beginning April 1998 by the authors. In this study, quantitative measurement and monitoring of the coral biomass, and digitization of the coral health were our first goal. We were fortunate to have an opportunity to examine both coral bleaching and recovery processes when we begin the study, which is a very precious and rare experience. Around Sekisei Lagoon, the water temperature in the summer 1998 was known as relatively higher than normal, but the coral bleaching problem was not expected. However, some unusual condition of the coral was noticed by the authors in May 1998. Then the large-scale coral bleaching occurred in the ensuing summer, and coral recover started after October 1998 by the falling water temperature, and the coral started spawning on May 1999. All these processes seemed to occur according to the fluctuation of water temperature. In summary, the high water temperature observed seems to cause the coral bleach observed. Nevertheless, the coral was in an unhealthy condition in May 1998 during which the water temperature did not rise. It means, there may be some other causes that induced coral bleaching. Thus the processes governing coral bleeching appears complex beyond the high temperature factor.
The authors are planning to develops diagnostic technique of coral health and monitoring coral biomass and health in conjunction with many biological, physical and chemical parameters in the whole Sekisei Lagoon and some patch reefs. Also, we plan to expand our monitoring sites to other typical coral seas, such as Florida Keys, U.S.A.; Great Barrier Reef, Australia; Malku Islands, Indonesia, to elucidate the coral health as a bioindicator for global environmental change.
We thank Professor Satoshi Nojima of Kyusyu University for his consent to use his precice survey results regading coral bleeching and spawning, and Dr. Paul Kilho Park, retired U.S. Oceanographer, for his consultation.
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Figure 1 Sekisei Lagoon
Figure 2 Tow survey system CS-2:
Left to right; onboard TV control, echo-sounder, tow (transmitter of echo-sounder, still camera, video camera)
Figure 3 Coral coverage and bottom profile of Line 6
Figure 4 Coral coverage and bottom profile of Line 13
Figure 5 Outline of
two typical patch reefs
a: Station A, b: Station C
Figure 6 Water temperature at 3 meters deep at Station C
Figure 7 Coral colonies at 3 meters deep at Station C on 6 October 1998:
Figure 8 Coral colonies at 3 meters deep at Station C on 9 March 1999:
Figure 9 Spawning of Acropora selago at Station A on 4 May 1999:
Figure 10 Photosynthetic yield measurement of Acropora cytherea:
Length of measure: 2 m;
M: Maximum florescence yield; Y: Photosynthesis yield
Figure 11 Coral
Left: Acropora hyacinthus, right: Acropora digitifera