Melissa works on her journal at the Mangrove Inn, Bocas, Panama!
|
|
Along oceans and other saltwater sources a unique breed of plant has adapted, the halophyte (Halophytes and Xerohalophytes). This plant type is distinguished from others by one main feature; the ability to survive in saline habitats. The cation sodium and the anion chlorine covalently bond to form salt, a toxic compound to most plants. These plants are often categorized by their salt tolerance adaptations. Plant that have glandular cells able to secret excess salt from plant organs are excretives, and plants that increase water holds in their vacuoles to reduce salt toxicity are succulents (Halophytes and Xerohalophytes). Probably the most famous halophyte (actually a facultative halophyte) is the mangrove.
Mangroves are subtropical or tropical C3 shrubs or trees that dominate intertidal zones (Tomlinson, 1986). ItÕs so widespread that estimates of its occupation are from 17 million hectares to 22 million hectares (Saenger, 1983). This number has steadily decreased as a result of human activity. There are about 65 species of mangrove recognized presently. These range from Avicennia sp. To Bruguiera sp. to Rhizophora sp. (Dawes, 1981). Scientists hypothesize that these magnificent plants originated in the Indo-Malayan region (Quarto, 2000). Far more species are present in this area with decreasing specie numbrs radiating out from this area. Because of their unique seeds and dispersal mechanisms the woody plants have colonized a great deal of the tropical shorelines, from the latitudes 32 degrees north to 38 degrees south.
The benefits of mangroves are extensive, but generally can be classified as abiotic and biotic benefits. The biotic benefits can be observed at molecular levels all the way to global levels. For starters nutrient assimilation leads to the colonization of epiphytic algae (Quarto, 2000). Plankton feed off the smaller algae and in turn larger organisms feed off of them. ItÕs a very complicated food chain that integrates the seawater, the mangrove, and the air. The underwater roots can form overhangs that serve as perfect space for coral, sponges, anemones, and many other organisms that feed off of microorganisms. The growth of these organisms ushers in small fish, gastropods, mollusks, etc. Some crabs even utilize the above seawater parts of the plant by eating the leaves, and other crabs eat them (Hogarth, 1999). The dense variety of small organisms underwater makes way for an incredible amount of larger organisms. From sharks, crustaceans, barracuda, jellyfish, even manatees (Hogarth, 1999). Almost 75% of commercially caught fish spend time in the mangrove ecosystems once in their lives, whether seeking shelter, food, or mating grounds (Robertson, 1992). One prime example is the barracuda. Many times this juvenile fish will use the mangrove ecosystem for shelter and food until it is large enough to makes its way to open water. On the flip side, the terrestrial part of the ecosystem is just as diverse. Holding numerous birds, epiphytic plants, large animals, arachnids, uncountable insects, this terrestrial ecosystem is of great importance. The capacity of mangroves ecosystems to provide all these organisms with necessary means to live make it a superproductive community.
From a greater perspective mangroves do much more than just house organisms. The actual physical presence of these plants has shaped much of our land. Like inland trees and shrubs the mangroves are superior erosion blockers. With their wide expanses of roots added with their relative density mangroves diminish waves which would eventually eat away at the coasts and eventually cause siltation to occur in surrounding waters (Hogarth, 1999). This siltation, which is evident today in many areas, can cause algae blooms which in turn decrease sunlight amounts reaching marine floors. This sunlight inhibition causes the death of countless organisms of the coral reefs and other areas like sea grass beds. In the worst case scenerio the effects would travel all of the way up food webs and eventually have some affect on nearly all organisms.
Basically, as indicated earlier, mangroves have two basic systems of coping with salt, either they dispose of it after itÕs incorporated into the system, or not let it in at all. One mechanism is to prevent salt from entering the system entirely. Some plants are efficient enough to exude over 90% of seawater salt. ÒIn salt excluding mangrove species, the mangrove root system is so effective in filtering out salt that a thirsty traveler could drink fresh water from a cut root, though the tree itself stands in saline soil (Qtd. Quarto). Another mechanism, now widely accepted, is the morphological feature of salt glands that excrete salt in the leaves. Many times these glands can be seen directly below the leaf blade on the petiole (Hogarth, 1999). It is not uncommon to actual see and taste the salt that is excreted out of and onto the leaves. A third way to cope with the salt is the most extreme and constitutes a concentration of the salt into the bark or older leaves that fall off and take the salt with them. Some mangroves only utilize one of these mechanisms, but most operate two or more at once. In addition to these mechanisms the leaves have an extremely thick cuticle which cuts down on transpiration and thus water loss, helping the plant stay healthy and not die from water loss due to high salt concentrations (Tomlinson, 1998).
Although mangroves occupy areas with substantial salt concentrations they donÕt actually require salt for growth and reproduction (Mangroves---more than just mozzies!). But the ability to deal with salt and the evolutionary energy put into these adaptations is small compared to the energy saved in competition. Only a small handful of other plants have adapted to these conditions, all of these plants share continued success because energy expenditures on living space and offspring that die from competition are minimum.
In a case study of the moderately salt tolerant specie Bruguiera gymnorrhiza, ÒPhysiological and biochemical responses to salt stressÓ were monitored to find the maximal growth of the plant (Takemura, 2000). The lab grown plants were found to have a maximum leaf area and growth rates at 125mM NaCl. When plants were transferred from water to solutions of higher salt content transpiration increased. This indicates that the stomata of the plant opened due to the change in salinity. In the presence of salt photosynthesis was adversely affected. Photosynthetic rates decreased, as salt concentrations were raised from 0-500mM salt. In conclusion, the researchers found that photosynthetic rates of the plants in water were nearly twice as high as the photosynthetic rates of plants in seawater (Takemura, 2000).
In another case study one of the most salt tolerant mangroves, Avicennia marina, photosynthetic performance was compared at two sites with different salt concentrations. The two locations were Durban Bay with salt concentrations of 35%o and Beachwood with a concentration of <12%o (Tuffers, 2001). Carbon dioxide exchange, the amount of carbon dioxide that diffuses through the stomata, and chlorophyll fluorescence, ability of a pigment to bounce a photon back at a different wavelength, were monitored (). The results indicate a very clear and intriguing conclusion about this specie. Carbon dioxide exchange and stomatal limitation were lower at Durban bay, which had the higher salinity. This means that the photosynthetic performance of the plants is decreased within low salinity sites like Beachwood as a result of potassium and nitrogen deficiencies in the leaves, according to the researchers (Tuffers, 2001).
From these case studies and many others made every year it is evident that mangrove species can tolerate a very wide range of salinities, which once again naturally gives them an advantage in their environments. Although mangroves flourish they have a small predisposed weakness. This is their susceptibility to photoinhibition. Photoinhibition simply describes the damaging process that occurs when excess light hits leaves. Basically, itÕs anything that causes the plant to cease carbon fixation, which in turn backs up the light reactions causing the formation of oxygen radical that can damage the group of proteins that make up photosystem II. With the fluctuating conditions and salinity the mangroves often have to shut down their light reactions, thus inflicting photoinhibition.
Mangrove structure is very similar to most other plants except for its adaptations to cope with salt, and thus water holding features (Dawes 1981). The main difference, and usually the most prevalent, are the root formations of the plants. There are several kinds that are very distinct. Probably the most recognizable and most associated with mangroves are the prop roots. These roots grow out from the stem and curve down into the water and eventually into the ground (Note: the roots of mangroves never grow to deep because of high salinity and anaerobic conditions.) (Hogarth, 1999). Another root type simply drops down into the water and soil from branches above and is accordingly named the drop root. Pnuematophores are roots that grow up out of the water from a straight ÒcableÓ root that originates from the base of the trunk. The pnuematophores are very recognizable also. As with most all other plants these specialized roots put out anchoring and feeding roots as well (Mozzies).
Mangrove reproduction is actually one of the most simple features of the plant. In general, the plants put out relatively normal flowers and are pollinated by basic animals such as bats, insects, birds, etc. (Bawa and Hadley, 1990). But the seeds produced by fertilization have several adaptations that make them unique. Some mangroves produce lone seeds but many produce the propagule, a long pencil like tissue that encases the seeds. Both forms of seeds have two adaptations that help them out compete other plants. A simple but essential feature is the ability of the seeds to float. Many times the seeds/propagules will drop into the sea and go out to sea and journey far away (they are also viable for up to a year) (Robertson, 1992). But the most important feature is the phenomenon called viviparity. This term describes the ability of the embryo to germinate while still attached to the tree. This adaptation allows them to immediately begin growing when the seed hits soil, instead of waiting extended periods of time to germinate (Dawes, 1981). The newly embedded embryos, that have already germinated, can grow up to two feet in a single year. By the age of ten the pioneer mangrove can produce a whole mangrove community, given the proper conditions (Saenger, 1983).
Throughout the age of man, mangroves have intrigued many and were shunned just as quickly. Some people used to think of mangroves as muddy, crocodile infested swamps with mosquitoes that were better off destroyed, and many times were. Now that our minds know the capability and many benefits of these ecosystems much has been done to conserve them. But with increased industrialization there are still many negative effects on the mangroves. Basically all of their problems arise from human activity. Whether it be destroying forests for human residential areas, fishing areas, resorts, etc. Also the long debated greenhouse effect may someday cause the ice caps to melt increasing the global water level and thus killing the mangroves under acres of water. But presently, they form one of the most diverse and productive ecosystems on Earth, and probably anywhere else.
REFERENCES
Hanagata, Nabutaka; Takemura, Taro; Karube, Isao; Dubinsky, Zvy. "Salt/Water Relationships in Mangroves." Israel Journal of Plant Sciences. 47 (2) 2 1999, 63-76. Abstract. Duke, N.C.; A.E. Schwarsbach. "Life on the edge: Past and Future of Mangroves." Wetlands Ecology and Management 9: 159, 2001. Hanagata, Nabutaka; Takemura, Taro; Karube, Isao; Dubinsky, Zvy. "Physiological and Anonymous. "Halophytes and Xerohalophytes." Anonymous. "Mangroves ---- more than mud and mozzies!" Hogarth, Peter J. The Biology of Mangroves. New York: Oxford University Press, 1999. Robertson, A.J.; D.M. Alongi (eds.). Tropical Mangrove Ecosystems. Washington DC.: American Geophysical Union, 1992. Quarto, Alfredo. ÒThe mangrove forest: background paper.Ó The Ramsar Convention on Wetlands, 2000.http://ramsar.org/about_mangroves_2.htm. Dawes, Clinton J. Marine Botany. New York: A Wiley-Interscience publication, 1981. Bawa, K.S.; M. Hadley, eds. Reproductive Ecology of Tropical Forest Plants. Volume 4. Boston; UNESCO & The Parthenon Publishing Group, 1990. Tomlinson, P.B., 1986. The Botany of Mangroves. Cambridge University Press, Cambridge. Saenger, P., Hegerl, E.J. and Davie, J.D.S. 1983. Global status of mangrove ecosystems. Environmentalist 3 (Suppl. 3): 1-88.
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 It is 1:06:08 AM on Monday, November 23, 2009. Last Update: Wednesday, December 10, 2008
Biochemical responses to salt stress in the mangrove, Bruguiera gymnorrhiza."
Aquatic Botany 68 (2000) 15-28.
Tuffers, A.; Naidoo, G.; Willert, D.J. "Low salinities adversely affect photosynthetic performance of the mangrove, Avicennia marina." Wetlands Ecology and Management 9:225-232, 2001.
Next Article
Previous Article
Return to Topic Menu
Here is a list of responses that have been posted to your discussion topic...
Important: Press the Browser Reload button to view the latest contribution.
If you would like to post a response to this topic, fill out this form completely...
DOWNLOAD the Paper Posting HTML Formating HELP SHEET!
Listen to a "Voice Navigation" Intro!
(Quicktime or MP3)
WEATHER & EARTH SCIENCE RESOURCES
OTHER ACADEMIC COURSES, STUDENT RESEARCH, OTHER STUFF
TEACHING TOOLS & OTHER STUFF
