Students take a "coral quiz", San Salvador, Bahamas.
|
|
Symbiosis is the living together of differently named organisms.10 It is often described as a relationship in which all the partners obtain a mutual benefit from the association in other words mutualism instead of parasitism where one organism benefits while the other suffers. Tropical corals such as Cnidaria, Mollusca, and Foraminifera obtain nutrition in two ways. The corals prey mostly on zooplankton but the nutrition gained by the corals feeding on organisms is minimal only about two percent of their nutrition intake.2 Each coral polyp contains autotrophic algal symbionts usually in the hostÕs digestive system. The autotrophic algal symbionts are in the genus Symbiodinium.7 The microscopic form of algae is called zooxanthellae because of the behavior of the algae.2 Zooxanthellae use photosynthesis to generate their food and ultimately pass up to ninety-eight percent of the food generated to the host.2 The algae forms its food from the coralÕs waste products carbon dioxide and nitrogen in the form of ammonia.2 Since the hosts acquire some of the zooxanthellae through their water column, they can live without the host but many are inherited maternally.10
When the zooxanthellae abandon their hosts the process is called bleaching. The bleaching occurs under stressful situations both environmental and thermal. The coral itself is clear and the zooxanthellae gives the coral its pigment so a sign of bleaching occurring is when the corals turn chalky white. Studies show a decrease in hard coral after a warm-water bleaching event occurs. When deprived of the algal symbionts, the corals lose a great deal of the nutrients gained by the photosynthesized food they generate and thus the corals basically starve to death.
Vitality of reefs is on the decline. Coral vitality is tightly linked to environmental factors. In three decades, eighty percent of coral cover has been lost in the Caribbean Basin and Western Atlantic.12 The causes of coral mortality can be linked to high amounts of fishing, predation, parasitism or boring, algal overgrowth, disease, sedimentation, toxin accumulation, thermal or climate change, or haline stress.1,3 Coral reefs are located in the topmost thermal limits and yet are sensitive to very small increases in seawater temperatures, and this makes the reefs among the most threatened ecosystems by climate change. Climate change also leads to changing water levels which can lead to dramatic drops in sea level leading to exposed corals above the water level to dry out and become land. A raise in sea level is less deadly to the reef but can pose a great problem to some species of coral. The corals that are fast growing and branching will match the sea levels and will not suffer from rising sea levels. The corals that form in deeper waters are more massive and grow more slowly as coral boulders will also survive but will change composition to survive. After the raise in water level, some corals simply stop growing together because they cannot keep up with the changing water.2 Climate change can also induce bleaching which overall results in discoloration of the coralÕs tissue and coral starvation. Algal overgrowth and the accumulation of sediment for example may cause tissue necrosis.3 The questions proposed are: What sort of environmental disasters are we potentially facing in the next several years (i.e. increased hurricane strength, temperature, more pollution, changing water levels, etc.)? How can reefs survive the storms, degradation, climate change, and the human race?
The planet changes all the time with varying temperatures from the ice age to todayÕs climate. Human pollution is causing the depletion of the ozone and great temperature increases that are melting the polar ice caps and glaciers, which is raising the sea levels and causing worldwide temperature increases. The intensity of storms today is also increasing, hurricanes are appearing more frequently and with a stronger force than our generation has ever seen. What can these disasters do to the coral reef environment and what are the possible outcomes to the reef environment without help? To help answer this a look at the devastation to the Rainbow Gardens Reef after several incidents occur such as El Nino, hurricanes, and water temperature change.
The site of Rainbow Gardens Reef showed how all of the damaging factors to corals can effect an ecosystem. From 1991-2004, Rainbow Gardens Reef in the Bahamas was examined . The reef spans 70m x 40m in the lee of Iguana Cay with a north by northeast orientation. Live cover was observed to have dropped from thirteen percent to three percent over the time span and the number of colonies likewise decreased from 295 to 240. The reef was observed to be Òless rich, equally diverse, more even, and the spacial structure of the reef had become more homogeneous.Ó3 In 1991, there were large conies present abundant with reef organisms. By 2004, most large colonies were absent because they were liable to stress related algal smothering, excess sediment, and endolithic boring. Further examination showed signs of extreme bioerosion in addition to a bright yellow coral skeleton. This skeleton agrees with the 1998 El Nino event, which is likely a major part of the decline of the Rainbow Gardens.3
The main degradation of the reef happened between the years of 1999 and 2001 which showed stony coral cover being reduced from eleven percent to two percent. This is because on the ebb tide water from the shallow Bahama Bank dammed in Iguana Bay.9,11 Warm and dense water poured into Rainbow Gardens Reef ultimately causing tissue necrosis and coral mortality of the soft corals within the days to follow the incident.9 Globally a great increase in bleaching and coral mortality were observed.4
The largest individuals of the colonies were lost over the thirteen year span. Twenty three species were encountered on the reef in 1991 and after thirteen years twenty four species were encountered. Some of the species in 1991 were not encountered in 2004 and vise versa. The biophysical stresses were lessened, no diseases or recently dead colonies were observed and also overall healthy colonies increased from thirty-five percent to eighty-two percent. But the physical stresses such as tissue damage, excessive sediment, algal smothering, boring, overgrowth, and competition were heightened.3
Symptoms of reef decline include coral mortality as well as loss of live coral cover. Over the thirteen year span, a decrease of seventy seven percent of live coral cover was reported. Two of the species of stony corals did not show decline but all others did with the greatest affected corals being A. agaricites, P. porites, and M. Annualaris the last was most affected with about an eighty-six percent loss. M. Annualaris was so abundant though that it in fact remains a major contributor to the reef structure while the other two species had all large patches destroyed and A. agaricites was almost completely eliminated. The surviving corals to this reef showed that they were highly eroded, fragmented, and loosely anchored to the substrate. Many great structures were reduced to carbonate rubble. Statistically the diversity remained unscathed the observers saw a much more homogenous environment with a vast increase in evenness and a vast decrease in richness, which is sad for a once beautiful and diverse world.3
Because of fragmentation the colony may have maintained abundance much better than it maintained cover. The fragmented pieces can regrow in a stable environment when time is given. This theory was observed by several examples of regrowth due to fragmentation on the reef. Some corals were cracked at their bases. They were found to have fallen over to begin growing in their new orientation. Hurricanes in the fall of 2004 were the cause of the fragmentation as well as boring and sediment overload. The fragmentation of the corals lead to the broken point being infected with organisms responsible for boring.5
A hurricane today will be much more destructive then that of a hurricane four years ago. The hurricanes are appearing much more in the present day due to changing climate then they once were, and they are coming with such a force that they leave devastation. The sediment left by the hurricanes in 2004 would be much less than that left by the hurricanes in the present year and those to come because of the increased number and strength. The number of level five hurricanes could completely bury a reef with sediment and leave reefs with the physical stresses of tissue damage, excessive sediment, algal smothering, boring, overgrowth, and competition of species. Not to mention leaving corals torn from the reef or so loosely anchored that they stand no chance of remaining attached.
The regular exposure of patch reef gatherings to a variation in salinity and temperature changes in comparison to deeper reefs may lead to physiological tolerances. This could in fact mean that shallow reefs could been a haven for coral populations and could eventually repopulate those reefs which have suffered heavier mortality.11, 12 The Exuma Cays patch reefs are an example since they encounter the warm waters with very high saline content during ebb tides and yet cooler oceanic water during floods.8 Bleaching was observed to be varied at a small spacial scale. The shallow reefs can be picked by environmentalist and marine biologist alike to determine the best locations for refuges. Then implementation of protection for these marine areas should be put in motion so that before the next disaster occurs the corals can be protected before the become extinct. A likely problem would be that the diversity of corals in reefs today will still suffer greatly because some corals do not live in the shallow environments that these havens provide.
To answer the second question posed above, the way that corals and coral reefs with survive pending disasters is with our help. Corals have survived storms for millions of years but pollution and large amounts of tourism to areas makes an impact on the corals and in mostly a negative way. Pollution is damaging enough to the reef but when climate change and increased storm intensity are added because of the pollution the overall damage from our carelessness is far worse than it originally seemed. Tourism money can pay for protective areas and national parks to protect the wondrous and delicate ecosystems which is a great benefit of tourism. The problem is with over fishing and pollution and well as the lack of knowledge for the corals and the reef environment that tourists bring when going a reef. As Osha Gray Davidson put it:
We are doing a rather good job of destroying coral reefs around the world. According to recent estimates, Ôabout 10 percent [of the worldÕs coral reefs] have already been degraded beyond recovery and another 30 percent are likely to decline significantly within the next 20 years.Õ In some areas, human activity has destroyed entire reefs converting them into algae-covered rubble.2
Because we have destroyed so much already we are obligated to do our part to protect what is left of this vast ecosystem. It might even mean that people will have to make the sacrifice of avoiding coral reefs until they can make a comeback from our destruction. Fishing limitations should be invoked and enforced. Pollution punishments should be made more severe to promote a healthier planet. Recycling should be mandatory and all cities should have it, which as of now is not the case.
In conclusion, a major bleaching incident could be catastrophic to corals and the reef environment. In many reefs, with a large enough temperature variation a bleaching incident can be caused such that it can effect the globe as it did in 1999 with the Rainbow Gardens Reef. If this were to happen, many of the corals and reefs around today could cease to exist. This could end much the way it did for Rainbow Gardens with an even, homogenous reef or with no reef at all. In addition to the temperature change, the increased strength of hurricanes today could destroy a reef ecosystem with excessive sediment poured on to the fragile plants. On a short term scale, this can lead to fragmentation of some plants and their survival but long term unless something is done for the corals and the reef the outlook in not so good.
Sources:
1) Ateweberhan, M., Maina, J. McClanahan, T.R., Mohammed, M.S., Muhando, C.A.(2007)Effects of Climate and Seawater Temperature Variation on Coral Bleaching and Mortality; Ecological Monograms, 77(4) pp. 503, 525
2) Davidson, O.G.(1998)The Enchanted Braid; John Wiley & Sons, Inc.
3) Dustan, P., King, A., Pante, E. (2007)Short-term Decline of a Bahamian Patch of Reef Coral Community: Rainbow Gardens Reef 1991-2004; Hydrobiologia 596: 121-132
4) Goreau, T., T.McClanahan, R. Hayes, & A. Strong, 2000. Conservation of coral reefs after the 1998 global bleaching event. Conservation Biology 14:5-15
5) Highsmith, R. C., 1982 Reproduction by fragmentation in corals. Marine Ecology Progress Series 7: 207-226
6) Jones, R.J. (2008) Coral Bleaching, Bleaching-Induced Mortality, and the Adaptive Significance of Bleaching Response; Mar Biol 154: 65-80
7) Lajeunesse, T.C., Pettay, D.T. (2007)Microsatellites from Clade B Symbiodinium spp. Specialized for Caribbean Corals in the Genus Madracis; Molecular Ecology Notes 7: 1271-1274
8) Lang, J. C., H. R. Lasker, E. H. Gladfelter, P. Hallock, W. C. Jaap, F. J. Losada, & R. G. Muller, 1992. Spatial and temporal variability during periods of ÒrecoveryÓ after mass bleaching on Western Atlantic coral reefs. American Zoologist 32:696-706
9) Lasker, H. R. 2005 Gorgonian mortality during a thermal event in the Bahamas. Bulletin of Marine Science 76:155-162
10) Leggat, W., Rees, T.A.V., Yellowlees, D. (2008)Metabolic Interactions Between Algal Symbionts and Invertebrate Hosts; Plant, Cell, and Environment 31: 679-694 Gardener, T., I. Cote, J. Gill, A. Grant, & A. R. Watkinson, 2003. Long-term region-wide declines in Caribbean corals. Science 301 958-960
11) McClanahan, T. & J. Maina, 2003. Response of coral assemblages to the interaction between natural temperature varitation and rare warm-water events. Ecosystems 6:551-563
12) McClanahan, T., J. Maina, C. J. Starger, P. Herron-Perez & E. Dusek, 2005. Detriments to post-bleaching recovery of corals. Coral Reefs 24:230-246
Next Article
Previous Article
Return to Topic Menu
DOWNLOAD the Paper Posting HTML Formating HELP SHEET!
We also have a GUIDE for depositing articles, images, data, etc in your research folders.
Article complete. Click HERE to return to the Pre-Course Presentation Outline and Paper Posting Menu. Or, you can return to the course syllabus
WEATHER & EARTH SCIENCE RESOURCES |
|
OTHER ACADEMIC COURSES, STUDENT RESEARCH, OTHER STUFF
|
|
TEACHING TOOLS & OTHER STUFF
 |
It is 11:51:02 AM on Monday, November 23, 2009. Last Update: Wednesday, December 10, 2008
Listen to a "Voice Navigation" Intro!
(Quicktime or