Subtropical and
Tropical Wetlands

Wetland Environments
F. Pavri & J.S. Aber

Table of Contents
Introduction Pantanal wetland
Okavango wetland Sunderbans wetland
Venezuelan Andes References

Introduction

Our discussion on low-latitude wetlands includes a wide array of wetland types across different geographic locations. In general, the term tropical implies that freezing conditions never take place, and this is normally associated with low-latitude and equatorial locations. However, glaciers exist on high mountains near the equator in Africa and Irian Jaya, and non-freezing conditions extend far northward in some places--south coasts of Ireland and England. For purposes of this discussion, we will consider the region of the globe between the Tropic of Cancer (23½°N) and Tropic of Capricorn (23½°S), where more than half of the world's human population lives.

We begin our discussions in South America with the Pantanal, arguably the world’s largest freshwater wetland. Africa is next on our list, and here we discuss the characteristics of the Okavango delta region of Botswana. The Sunderban mangroves of South Asia comprise a coastal tropical wetland system. Finally alpine wetlands in the Andes Mountains are described from Venezuela. Specific ecosystem properties differ for each wetland site identified, as do precipitation and temperature regimes, and human use and management.

Pantanal

The name Pantanal comes from “pantano” (in Spanish), which essentially means swamp or marsh. The Pantanal is part of the Paraná-Paraguay river basin (in Bolivia, Paraguay, Brazil) and Mato Grosso (Brazil). It extends over approximately four times the size of the Everglades’ original size and comprises riverine, palustrine and lacustrine wetlands among other terrestrial ecosystems. The Pantanal comprises alluvial plains with hydromorphic soils.

Skylab photograph (color-infrared) of the Paraná River on the border between Argentina and Paraguay. Pink-red areas indicate active vegetation in this late winter (dry season) view. Photograph taken 8/30/73. Image adapted from JSC Digital Image Collection, Press Release Images--see Paraná.

Seasonal climate variations impact the plant and animal life in predictable ways. The wet season spans across the months of October through March, while the dry season is roughly between April and September. The northern reaches of the Pantanal receive rainfall primarily between Jan.–March, while it might generally be May by the time peak flood is observed in the downstream sections. This flood season with increasing water levels and the inundation of adjacent land coincides with the primary fish spawning time. Retreating water during the dry months exposes areas for grazing animals and causes fish 'stranding,' which in turn provides a perfect habitat for wading birds. This region plays host to a myriad of wildlife including hundreds of bird species, mammals and reptiles, including jaguars, howler monkeys, capybaras, toucans, anacondas, caimans and tapirs.

While human inhabitation and population density has always been low to moderate in this region, economic activities including cattle ranching, crop farming (corn, sugar cane), agribusiness, mining (gold), fishing, and tourism, especially ecotourism, are common. In more recent decades increased human activity in the Brazilian section of the Pantanal has been recorded and as a consequence impacted the wetland ecosystem. Typical activities and impacts include deforestation and wetland drainage for cattle ranching and agriculture, agrochemical, urban and mining runoff, and major hydro projects, of which the proposed Paraguay-Paraná Waterway Project or “Hidrovia” is the most significant.

The Hidrovia project proposed a navigation plan for the Paraná-Paraguay rivers to allow year-round cargo transportation from the Brazilian and Paraguayan interiors to the Atlantic. Proposals included channelization, straightening, widening, damming and diking the river system. There was certain to be significant environmental impact from this project, and after much opposition the inter-governmental consortium proposing this project temporarily set it on the back burner. However, minor hydro projects continue within the Pantanal and will no doubt have long lasting impacts on this ecosystem.

Okavango

The Okavango is a seasonally filled inland delta for the Okavango River, which has its headwaters in western Angola and makes its way through Namibia and then disappears in the midst of Botswana’s Kalahari desert. The Okavango River leaves a maze of lagoons, channels and islands before disappearing into the desert.

Okavango map.

Space-shuttle photograph of the Okavango delta in northwestern Botswana. Dark green shows vegetation along the Okavango River valley and delta. Linear pattern to west and north is formed by sand dunes. Photograph taken 11/85; STS61A-43-29. Image obtained from Johnson Space Center--see JSC.
Space-shuttle photograph of the Okavango delta in northwestern Botswana. Clouds cover parts of the relatively humid delta wetland, which is surrounded by desert. Photograph taken 11/94. Image adapted from JSC Digital Image Collection, Press Release Images--see Okavango.

Like the Pantanal, the Okavango also experiences seasonal flooding starting toward the north in the summer (Oct.–Apr.) and ending in the winter (May–July) toward the southern end of the delta (seasons are reversed as the Okavango is in the southern hemisphere). The delta is almost permanently flooded in the north, but only seasonally flooded in the south. Reports suggest that about 96% of the water that enters the delta evaporates and what is left flows into salt pans. These rich soda deposits in turn support a unique ecosystem. The Okavango is famous for wildlife including elephants, large cats, hippos, and hundreds of bird species. Ecotourism is a major economic activity for the region and brings in considerable revenue; however, the wetlands are also facing pressure from the expansion of agriculture.

Sunderbans

The Sunderbans comprise the southwestern coastal area of Bangladesh and the coastal region south of the city of Calcutta in India. These are mangrove forests covering an area of approximately 10,000 km² across the Ganges delta region of the countries of India and Bangladesh. The Sunderbans are the largest mangrove forest preserved in the world. Indian mangroves comprise several genera and more than 40 species, most of which grow in the Sunderbans.

Sunderbans map.

Space-shuttle photograph of the Sunderbans region, delta of the Ganges River in Bangladesh and India. Note the heavy sediment (mud) content of the distributary channels and coastal waters. Photograph taken 11/94. Image adapted from JSC Digital Image Collection, Press Release Images--see Ganges.
Space-shuttle photograph (color-infrared) of the Sunderbans area, delta of the Ganges River in Bangladesh and India. The bright red zone is a region of protected mangrove swamps. Photograph taken 12/85; STS061B-50-007. Image obtained from Johnson Space Center--see JSC.

The top soil layers are comprised primarily of sediment deposits from the Ganges, Brahmaputra and Meghna rivers before they enter the Bay of Bengal. These ecosystems comprise intricate interconnecting waterways and 100s of dynamic alluvial islands, which are shaped by constant tidal action, erosion and deposition. The alluvial deposits are deep with intermittent layers of clay and sand. The climate of this region is heavily influenced by the monsoons, which bring in a tremendous amount of precipitation between June-Sept. Cyclonic storms are common in May and again in late October and in many cases cause severe tidal waves leading to much destruction and property damage.

The Sunderban name comes from the dominant tree species, Heritiera fomes, also known locally as “sundari” (which roughly translates into "beautiful") found across this region. Sunderbans are categorized as moist tropical seral forests including beach and tidal forests. They support a diverse fauna, including Panthera tigris, or the royal Bengal tiger, spotted deer, and wild boar among other mammals, and esturine crocodiles among numerous other reptiles. However, a long history of human activity in this region has resulted in a significant loss of habitat, and local extinctions include the Javan rhino last observed in 1870 and the one-horned Indian rhino. Frequent encounters between the local population and the Bengal tiger have earned this big cat the notorious title of “man-eater.” The Sunderbans exhibit one of the last refuges of Panthera tigris, and reports further suggest that this is the largest concentration of tigers in India.

The Sunderbans are tremendously valuable ecosystems. The Indian and Bangladeshi governments have made attempts to manage carefully these resources. For instance, the Sunderban Biosphere Reserve and the mangrove eco-park in Jharkhali are both steps in that direction. Yet, increasing populations within the region have led to significant pressures on these resources. Some of the major issues facing the Sunderbans include expanding agriculture in the northern reaches, increased flows of industrial and agricultural effluents, increased timber harvesting and hunting, and in Bangladesh, increasing salinization as fresh river water is diverted for human use. Today the Sunderbans are an important site for ecotourism and habitat preservation.


Bibliography


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ES 351 © J.S. Aber (2016).