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THE ROLE OF WETLANDS
IN GLOBAL AND LOCAL ECOLOGY
Charles E. Shaw, Ph.D., Geologist
Wetlands, as the name indicates, are areas of the Earth where the ground-water table intersects the surface of the land. Lakes, rivers, inland swamps coastal marshes and mangroves are all part of the world's wetlands. The wetland may be small, less than a hectare, or very large, such as the wetlands within the Sian Ka'an Biosphere Reserve in the Yucatán Peninsula, the Florida Everglades or the Great Lakes of North America. Wetlands play a major role in buffering the carbon budget of the global ecosystem and the uptake of contaminants in local ecosystems.
Globally, wetlands occupy only two percent of the Earth's surface, but account for two-thirds of the carbon that is removed from contact with the atmosphere (Dean and Gorham, 1998). The other one-third of the carbon removed is stored in the oceans as coral skeletons, shells of invertebrate animals and as gas hydrates trapped within sediments on the continental shelves (Haq, 1998). The oceans, however, occupy 71 percent of the Earth's surface, over 35 times the area of the wetlands. These figures show that wetlands are, by far, the most efficient systems for storage of excess carbon. They occupy such a small percentage of the Earth and perform such a vital service to the global ecology that every hectare needs to be preserved.
Wetlands are natural buffers that slow the ongoing global warming. Global warming is caused by the build-up of carbon dioxide and water vapor in the atmosphere. This warming is called the greenhouse effect. If our understanding of the climatic changes of the last 100,000 years when the Earth went from an interglacial (greenhouse world) to a glacial (non-greenhouse world) and back to an interglacial climatic regime (present day greenhouse), is even approximately correct, the present rate of warming will bring major climatic changes by the middle of the 21st century. It is difficult to imagine the magnitude of the misery that will fall upon our children and grandchildren as an unstable climate brings major disruptions of agricultural production at a time when the human population of the Earth is expected to reach 10 billion persons (Broecker, 1997, 1999). If nothing is done by our generation of humans, we risk leaving a tragic inheritance for our children.
The mechanisms involved in this tragedy work as follows. Carbon that is released by the decomposition of organic material and by the burning of fossil fuels combines with oxygen in the atmosphere to form carbon dioxide. The carbon dioxide acts like an insulating blanket that slows the radiation of heat from the surface of the Earth. The result is a steady warming of the atmosphere as the carbon dioxide content increases. The warming allows more water vapor to accumulate in the atmosphere, which generates the warming several-fold. The planet Venus, for example, is in the grip of a runaway greenhouse with an atmosphere that is largely carbon dioxide. Temperatures at the surface of Venus are on the order of 400 degrees Celsius.
Organic material that is under water in the Earth's wetlands is removed from contact with the atmosphere and does not decompose by combination with the atmospheric oxygen. Instead, a slow decomposition takes places through the action of sulfur-fixing bacteria and a deposit of peat is formed which, over time and with burial, becomes coal. Carbon, therefore, is put in storage and removed from the cycle. The present greenhouse warming is being dangerously accelerated by burning of fossil fuels, most of which were stored in swamps during the Carboniferous Period, 300 million years ago. Vast quantities of this stored carbon have been released over the past two hundred years by the burning of coal and, over the last century, by burning petroleum. The sword has two edges as we burn fossil fuels to heat homes and buildings, to generate electric power and to power automobiles while simultaneously filling wetlands for parking lots and construction of buildings.
Wetlands perform other services in addition to storing excess carbon. A wetland is an outcropping of the ground-water table where plants have access to the water. Plants growing in the wetlands, such as in the mangrove wetlands along the Caribbean coast of the Yucatan Peninsula, avail themselves of nutrients, phosphorous and nitrogen, the common plant food from soaps, detergents and mineral sources. The nutrients are stored in the plant tissue which, in turn, are stored in the wetlands for long periods. Likewise, contamination from human wastes, which can be sources of disease-carrying bacteria and viruses, is purified in the wetlands by microorganisms that consume the pathogens. If the inflow of contaminants is not so large as to overwhelm the natural system, pathogens and nutrients are locked up in the wetlands and do not reach the ocean where they would fertilize the growth of algae. Algae presently are overwhelming the Mexican Caribbean coral reefs, an indication that the wetland buffer is overloaded.
Mangrove wetlands are an essential part of the coastal ecosystem of the Mexican Caribbean. Their destruction is a disaster for the coral reefs offshore and adds to the global greenhouse, a disaster of global proportions. We do not know for certain the scenario to expect, because past greenhouse events emerged after the Pleistocene glaciations. This time humans are adding to a global greenhouse while we living in a warm period (Broecker, 1999). There is no precedent. We walk a most dangerous path.
References
Broecker, W.. 1997, Will our ride into the greenhouse future be a smooth one?: GSA Today, v. 7, no. 6, p. 1-6.
Broecker, W. 1999, What if the conveyor were to shut down? Reflections on a possible outcome of the great global experiment: GSA Today, v. 9, no. 1, p. 1-7.
Dean, W. E. and Gorham, E., 1998, Magnitude and significance of carbon burial in lakes, reservoirs and peatlands: Geology, v. 26, p. 535-538.
Haq, B. U., 1999, Gas hydrates: Greenhouse nightmare? Energy panacea or pipe dream?: GSA Today, v. 8, no. 11, p. 1-6
To learn more about mangroves please click here.
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