The Amazon Estuary
The eastern wetlands near the Amazon River mouth region consist of a complex maze of archipelagoes, a giant island, an enormous and largely freshwater bay and tidal forests; they are the meeting area of Amazon and Atlantic waters. In the battle between fresh water and saltwater, the fresh water largely dominates the ecology of the eastern region. Amazon River discharge forms a bulwark against marine water and the coastal wetland savannas are dominated by rainwater. The marine influence is most easily detected by the presence of mangroves near the ocean.
The Amazon estuary centers on the giant island of Marajó (40,000 km2) around which Amazon River water flows. The main Amazon River channel meets the Atlantic north of Marajó Island but part of the river also flows south and then northeast through Marajó Bay. Due to the Amazon River’s huge discharge its estuary is largely fresh water. The Amazon coastal region stretches from the Oiapoque River at the Brazilian-French Guiana border to just southeast of Marajó Bay. Most of this area is not hydrographically in the Amazon Basin. However, from the viewpoint of Amazon wetlands management, it should be considered as ecologically connected. The presence of migratory freshwater catfish attests to its ecological connection with the main river system.
The dominant geographical features north of Marajó Island are the huge quantities of mud that are strewn along the Amapá coast and extensive mangroves. The muddy coastal area receives most of its sediments from Amazon River discharge. There has been little development along the Amapá coast and most of the mangrove forests are relatively undisturbed. Amapá’s mangroves are used for crabbing, but little commercial fishing is done in the muddy habitats. Most commercial fishing takes place offshore. Cattle ranching activities are moving eastward from the Amapá savannas and beginning to encroach on some of the tidal streams and adjacent mangrove areas.
The northern Atlantic side of Marajó Island is heavily influenced by Amazon River fresh water. Sand dunes and extensive bamboo thickets characterize the northeastern coast, although there are also areas of extensive mangroves. The sand dunes contrast sharply with the mud flats at the mouth of the Amazon River. Annual fires set by cattle ranchers have radically modified Marajó’s northeast coast and the extensive bamboo thickets are probably artifacts of fire. The nearshore coastal waters of northeastern Marajó are important nursery habitats for commercial catfish, other fish species and crustaceans.
Geographically, Marajó Island is naturally divided into two halves: its eastern half with relatively large savannas and its western half with sweeping forest. Most of the giant island is subject to seasonal or tidal flooding. Numerous tidal rivers less than approximately 100 km long slice through the western, forested region. The eastern interior of Marajó forms an enormous inland basin that collects rainwater, which inundates the area for about six to eight months annually. The huge flooded forests of Marajó are related floristically to the Amazon River floodplain forest. Marajó Island’s western tidal forests have been heavily logged for select species such as for virola (Virola surinamensis) and assacu (Hura crepitans). Mangroves, which probably account for less than 10 percent of all floodable forest on Marajó, are found mostly along the northern coast and inland along Marajó Bay.
Indigenous peoples, Portuguese colonists, and Brazilian ranchers have all had major impacts on the landscapes of Marajó, especially in the eastern savannas. Marajó’s savannas have been burned for centuries, if not thousands of years. The Portuguese reported indigenous peoples using fire to corral turtles during the dry season and this practice continues. The savannas of eastern Marajó have expanded because of burning and many of the forests that have survived are dominated by swamp-loving palms that escape fire. With the expansion of modern ranching, however, cattle and water buffalo now trample through these swamp forests, destroying seeds, seedlings and saplings.
Arari is the largest lake on Marajó and the center of livestock ranching and fishing. The Arari River, whose mouth is just opposite Belém, connects the shallow lake to Marajó Bay. A canal on the northern part of Marajó, Canal da Tartaruga, was excavated in the 1950s to expedite shipping between Belém and Macapá near the Amazon River mouth. The canal, however, began to drain Arari Lake and it became necessary to build an earthen dam on the Arari River to save local fisheries. The Arari Lake region has been almost completely deforested and savanna grasses are maintained by fire. During the flooding season the Arari River is joined to one vast sheet of water that inundates the eastern interior of Marajó Island. A species of bottom-feeding armored catfish called tamoatá (Hoplosternum littorale) dominates the Arari Lake fishery. The high production of these catfish might be partly related to livestock feces washing into and fertilizing the Lake.
The Araguari was the first relatively large river in the Amazon Region to be dammed. The Coaracy Nunes Dam was constructed in 1972 to supply energy for a local multinational manganese industry and for Macapá, the capital of the state of Amapá. The Araguari was first impounded in 1972. Bordered on the south by the lower Araguari and on the east by Cabo Norte (Cape North), the Piratuba Lakes region is an immense low-lying swamp inundated mostly by local rainwater but also by tides at its eastern and southern borders. About one-half of the Piratuba wetland area is a federal conservation unit. With its large open-water bodies and vegetation-covered islands, it is reminiscent of central Brazil’s Pantanal.
Compared with the Amazon River, Marajó Bay discharges relatively little water and sediments. Nevertheless, Marajó Bay is always turbid because it receives large quantities of sediments from the Amazon River via the Breves Channel and the Pará River. The deepest waters of Marajó Bay are less than 30 m deep. River discharge through Marajó Bay is sufficient to ensure a largely freshwater environment, though during the low-water period of the Amazon River it can become brackish for approximately 80 km inland from the Atlantic.
The tidal forests southwest of Belém are an excellent example of the eastern Amazon’s richness in palms. The açaí palm is exploited for palm hearts and fruit used in juices and ice creams, and the recent international market for its fruits has led to some tidal forest deforestation to increase production of this species.
Marajó Bay is polluted to some extent by industrial and urban effluents from Belém and smaller coastal cities. The largest industrial facility on Marajó Bay is an aluminum-processing plant located on the island of Barcarena southwest of Belém. However, the Barcarena plant is modern, so slurry and tailings appear to collect in holding ponds.
Although the state of Pará considers Marajó Island a conservation unit, no protected parks or reserves currently exist there or even in the surrounding area.