24 January 2009

How vital are mangroves?

Mangroves (and seagrass meadows) apparently play a critical role in healthy reefs and perhaps even the health of the planet.Api api (Rhizophora sp.)One study in 2002 found mangrove forests and seagrass meadows an indispensable and stable nursery ground for coral reef fish in the Caribbean. Fish did not enter the nursery grounds by chance before subsequently moving to a coral reef. The coral fish deliberately chose a certain place. The fish use the mangrove forests as a shelter from predatory fish. Seagrasses not only serve as a safe haven but also as a feeding place. Due to changes in their diet, carnivorous species move from the nursery ground to the coral reef whilst still immature. The herbivorous fish mostly depart due to becoming mature.

Another study in 2006 found mangroves to be far more important to the global ocean's biosphere than previously thought. Mangroves provide more than 10 percent of essential dissolved organic carbon that is supplied to the global ocean from land. The researchers speculate that the rapid decline in mangrove extent threatens the delicate balance and may eventually shut off the important link between the land and ocean, with potential consequences for atmospheric composition and climate.

Coralreef fish desperately needs mangrove forests and seagrass fields
EurekAlert 15 Nov 02
Biologists from the University of Nijmegen have demonstrated that some coral fish really do choose nursery grounds before heading for the coral reef. According to the researchers, managers of the waters around the Caribbean islands must devote more attention to the coast as a whole and not just to the protection of coral reefs.

Up until now scientists suspected that seagrass fields and mangrove forests (trees which can survive in salt water) were nursery grounds for coral fish because young fish were often found there. However, there was a lack of scientific evidence to support this. The conclusions from the Nijmegen research have changed this. They lend support to the idea that mangrove forests and seagrass fields are an indispensable and stable nursery ground for coral reef fish around the Caribbean islands of Curaçao, Bonaire, Saba and Belize.

The Nijmegen PhD student Elroy Cocheret de la Morinière studied the nine most common fish species in the nursery grounds around Curaçao. He examined how each of these nine species behaved around mangrove and seagrass fields as well as on coral reefs.

It turned out that the nine species of fish did not enter the nursery grounds by chance before subsequently moving to a coral reef. The coral fish deliberately chose a certain place. An examination of the fishes' stomach contents together with chemical analyses revealed that the fish use the mangrove forests as a shelter from predatory fish. The seagrass fields not only serve as a safe haven but also as a feeding place.

Due to changes in their diet, carnivorous species move from the nursery ground to the coral reef whilst still immature. The herbivorous fish mostly depart due to becoming mature.

The coral fish are important for both commercial fishing and local subsistence fishers in the tropics. Until now mangroves have often been chopped down, to build jetties for example. The seagrass fields are frequently polluted. The researchers state that the mangroves and seagrass fields must be protected. Without the nursery grounds, a number of very common and economically important coral fish species will not survive.


The critical importance of mangroves to ocean life
Eurekalert 27 Feb 06;
WASHINGTON -- Mangroves, the backbone of the tropical ocean coastlines, are far more important to the global ocean's biosphere than previously thought. And while the foul-smelling muddy forests may not have the scientific allure of tropical reefs or rain forests, a team of researchers has noted that the woody coastline-dwelling plants provide more than 10 percent of essential dissolved organic carbon that is supplied to the global ocean from land, according to a report to be published 21 February in Global Biogeochemical Cycles, a publication of the American Geophysical Union.

Thorsten Dittmar at Florida State University in Tallahassee reports that mangrove plants, whose finger-like roots are known to protect coastal wetlands against the ocean and as important fish habitats, cover less than 0.1 percent of the global land surface yet account for a tenth of the dissolved organic carbon (DOC) that flows from land to the ocean. Dittmar and his colleagues at several German research institutions analyzed the carbon output from a large mangrove forest in Brazil and suggest that the plants are one of the main sources of dissolved organic matter in the ocean.

The researchers note that the organic matter that is dissolved in the world oceans contains a similar amount of carbon as is stored in the skies as atmospheric carbon dioxide, an important greenhouse gas. Dissolved organic matter is an important player in the global carbon cycle that regulates atmospheric carbon dioxide and climate.

"To understand global biogeochemical cycles it is crucial to quantify the sources of marine dissolved organic carbon," Dittmar writes. "Here we show that mangroves play an unexpected role in the global carbon cycle."

Dittmar reports that the mangrove root system slows carbon-rich leaf litter running from continental land and allows it to settle into shallow sediment, where dissolved organic matter is leached in large quantities into the coastal waters. The daily rise and fall of the tides then flushes the dissolved carbon into the open ocean (like a teabag being dipped in an out a cup). Once in the ocean, however, the intense tropical sunlight destroys some of the most delicate dissolved organic carbon molecules. But more than half of the dissolved organic matter survives the attack from sunlight or bacteria.

The authors measured the chemical signature in water samples from the massive mangrove forest in northern Brazil, using natural carbon isotopes and nuclear magnetic resonance spectroscopy--an established and common technique for determining the structure of organic compounds--to determine that mangroves are indeed a main source of dissolved organic carbon in the open ocean. In total, they concluded that the carbon exported from mangroves is approximately 2.2 trillion moles of carbon per year [2.2 x 10(12), similar to the annual Amazon River discharge], nearly triple the amount estimated from previous smaller-scale estimates of the carbon released into the ocean.

Mangrove foliage, however, has declined by nearly half over the past several decades because of increasing coastal development and damage to its habitat. As the habitat has changed, ever-smaller quantities of mangrove-derived detritus are available for formation and export of dissolved organic matter to the ocean. The researchers speculate that the rapid decline in mangrove extent threatens the delicate balance and may eventually shut off the important link between the land and ocean, with potential consequences for atmospheric composition and climate.

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