Wetlands of tropical South America

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The climate of tropical South America is characterized over large areas by a high annual precipitation, varying from 1,000 mm to more than 5,000 mm per year. A pronounced seasonality in rainfall results in the periodic flooding of large areas covered by forests or savanna vegetation. Therefore, most of the wetlands in this area belong to the category of seasonal wetlands with a pronounced dry period.

Flooding may occur by lateral overflow of rivers and streams or by sheet flooding due to excess rain and insufficient drainage. The floodpulse is mono-modal and predictable in the savannas and the fringing floodplains along the large rivers whereas it is polymodal and unpredictable in the floodplains along small streams.

Plants and animals respond to this pulsing with a large set of morphological, anatomical, physiological, and ethological adaptations. Inspite of the physiological stress of the change between aquatic and terrestrial conditions, species diversity is comparatively high. Floodplains of tropical South America may be considered as areas of speciation, contributing to the great species diversity in the area. On the other hand, the floodpulse results in a periodic exchange of biological information between the wetlands and the drainage system, often over long distances. Therefore, many species have a wide range of distribution.

Nutrient status of the wetlands varies from extremely low levels in areas flooded by nutrient-poor water (e.g. from rains or black water rivers) to high levels in the fringing floodplains of white water rivers, rich in fertile sediments and electrolytes. Consequently, productivity varies from low to very high, reaching maximum values up to 100 t dry material per hectare per year in the floodplain of the Amazon River. There is a complex exchange of nutrients and energy between the terrestrial and the aquatic phase and between the floodplains and the connected river systems.

Further wetland types occur mainly along the coast of the Atlantic ocean, partly in the form of mangroves or salt marshes. Peat bogs, cushion bogs, and reed swamps occur, to a small extent, in the wet Paramos of the high Andes, salt pans occur in the dry Puna. There is no exact information about the total wetland area in tropical South America, partly due to the seasonal character of the wetlands which have been poorly studied and are often not recognized as wetlands. It is estimated, that more than 2,000,000 km2 may belong to the wetland category corresponding to about 20% of the area.

In recent times, all wetland types are becoming increasingly influenced and modified by man. In floodplains agriculture and husbandry are the main anthropogenic factors, modifying natural vegetation by deforestation and use of fire for weed control. Some river-floodplains are becoming strongly affected by the construction of flood-control measures and hydroelectric power schemes. Water pollution due to the input of sediments, agro-industrial wastes, agrochemicals, and mercury is becoming a serious threat. Mangroves are probably the most endangered wetlands due to industrial pollution, colonization projects, timber extraction, and large scale fish- and shrimpcul-ture.