Marajó Varzea forests
The Marajó Varzea forests are a unique biome located at the confluence of the Amazon River and the Atlantic Ocean, primarily covering Marajó Island and surrounding areas. The term "varzea" refers to flooded forests that experience seasonal inundation, particularly from white-water rivers, and the Marajó Varzea is distinct in that it is also influenced by tidal changes from the ocean. This biome is characterized by a rich amalgamation of both terrestrial and aquatic ecosystems, supporting a diverse array of species, including mammals, birds, and fish.
Notable species include the endangered short-tailed opossum, pink river dolphins, and various migratory birds that thrive in this dynamic environment. However, the Marajó Varzea forests face significant threats from climate change, which could disrupt the delicate balance of its ecosystem. Rising sea levels, temperature changes, and altered precipitation patterns pose risks to the flora and fauna that depend on the seasonal flooding and nutrient influx that characterize this unique habitat. Additionally, human activity has led to the conversion of parts of this biome for agriculture and urban development, further threatening its biodiversity and ecological integrity.
Subject Terms
Marajó Varzea forests
- Category: Forest Biomes.
- Geographic Location: South America.
- Summary: These flooded forests line the mouth of the Amazon and cover Marajó and numerous other islands, as well as the interface of the Amazon River and the Atlantic Ocean.
The word varzea means flooded forest; it typically refers to the seasonally inundated forests that flank the Amazon River and its tributaries. In particular, varzea refers to those areas flooded by white-water rivers, contrasted with areas flooded by black-water rivers, which are called igapo.
The Marajó Varzea Forests biome is also tidal—and therefore quite different from the seasonal varzea of the middle and upper Amazon floodplains, which are inundated annually by flooded rivers. The Marajó varzea represents the flooded forests within the Amazon River delta, where the largest river system in the world meets the Atlantic Ocean. Similar to other varzea biomes, this area is a seasonally flooded, riparian forest ecosystem characterized by both terrestrial and freshwater elements.
However, the Marajó varzea has one additional complicating factor: the ocean. The Marajó varzea is therefore both seasonally flooded and tidally influenced, with a gradient of marine influence that gradually decreases moving westward (upstream). Aside from seasonal inundations, the landscape floods twice a day when the ocean tide pushes river water onto the landscape to a height of up to approximately 10 feet (3 meters).
At its mouth, the Amazon River splits into a braided network of islands as it becomes a river delta system. The largest of these is Marajó Island, at some 18,500 square miles (48,000 square kilometers), which lies roughly in the middle of the massive delta. The Marajó Varzea Forests biome is dominated increasingly by mangrove ecosystems along the coastline and around eastern Marajó Island.
Specially Adapted Species
Varzea biomes are harsh environments, where flood levels can reach 20 feet (6 meters), while at the same time they are unique, in that they are characterized by their mixture of terrestrial and aquatic elements. These include pacu fish that act as seed dispersers for trees, and pink river dolphins that hunt between the giant buttresses of riparian trees. It is also not unusual to see a bull shark entering the flooded forest.
Because humans have aggregated along the river banks of the Amazon, much of this biome has been converted to agriculture and even to urban areas; however, the constant flooding ensures that large areas of varzea remain out of reach of development for now. This unique and threatened area supports habitats not only for a plethora of endangered species, such as manatee and giant otter, but it also has provided for many indigenous peoples who have become largely displaced.
Mammals are abundant and have very high diversity in this biome. Overlapping species from land, the canopy, the river, and the ocean all meet here and trade influences across habitats. In the treetops are the brown four-eyed opossum, Brazilian porcupine, Guianan squirrel, pale-throated three-toed sloth, and white-faced tree rat. Such monkeys as silvery marmoset, Midas tamarin, and red-handed howler are relatively common here.
The occurrence of mangroves and proximity to the river and ocean also make good habitat for crab-eating fox and crab-eating raccoon. During the dry season, the red brocket deer, jaguar, little-spotted cat, and southern naked-tailed armadillo make use of the understory. The short-tailed opossum (Monodelphis maraxina) is endemic to this biome (found nowhere else on Earth), and is considered endangered.
There are over 350 known species of birds on Marajó Island, and the entire biome contains an estimated 500 species. Colorful species with equally colorful names include black-throated mango (a hummingbird), blue-tailed emerald, blue-shinned sapphire, and blue-headed parrot. The proximity to the Atlantic Ocean means that some shorebirds are also frequent, if seasonal, residents, especially along the coastline and mangrove systems. These include American oystercatcher, least sandpiper, whimbrel, and ruddy turnstone. Swamps, flooded forests, and grasslands are common aggregation points for other waterbirds including numerous ducks, as well as snowy egret, little blue heron, cocoi heron, scarlet ibis, and wood stork.
Very large fish occupy this biome; they, too, often roam between the forest and the river system. Characteristic species here include tambaqui (Colossoma macropomum) and pirarucus (Arapaima gigas). The moriche palm (Mauritia flexuosa) is among the dominant species of tree along the swamps and river’s edge. Among the amphibians is the Suriname toad, while reptiles include the tropical flat snake and Brazilian false water cobra.
Fragility and Threats
The varzea is an ever-changing biome where the only constant is the annual flooding, which carries in nutrients from as far away as the Andes Mountains. Flooded forests are unique from nearby terra firma rainforests in many ways—but most notably in that seasonal inundations dramatically change tree recruitment, turnover, and seed dispersal on the forest floor when waters carry away seeds, leaf litter, and so forth. Tree diversity is therefore limited to those species that can survive frequent and prolonged inundations, and which have typically developed relationships with both terrestrial and aquatic fauna to ensure their dispersal and survival.
The effects of climate change on the Marajó Varzea Forests biome could thus be critical. The system is robust, but balanced in very calibrated ways. Sea-level rise, for example, could put just enough stress on less-salt-tolerant vegetation here to give way to radical changes in the plant community. Similarly, incremental temperature increases could threaten the natural timetable for seed germination that has co-evolved with the capacity and seasonality of the unwitting efforts of various animals to aid in dispersal. Changes in rainfall could reduce freshwater availability or exacerbate flooding.
Bibliography
Campbell, D. G. and H. D. Hammond, eds. Floristic Inventory of Tropical Countries. Bronx, NY: New York Botanical Garden, 1989.
Faro, Barbara Luzia Santos de Oliveira, et.al. "Composition and Community Structure of Mangroves Distributed on the East Coast of Marajo Island Brazil." Research Square, 5 Jan. 2022, doi.org/10.21203/rs.3.rs-1196739/v1. Accessed 15 Nov. 2024.
Goulding, Michael. The Fishes and the Forest: Explorations in Amazonian Natural History. Berkeley: University of California Press, 1981.
Pires, Joao M. and Ghillean T. Prance. “The Vegetation Types of the Brazilian Amazon.” In Ghillean T. Prance and Thomas E. Lovejoy, eds., Key Environments: Amazonia. New York: Pergamon, 1985.
Smith, Nigel J. H. Amazon Sweet Sea: Land, Life, and Water at the River’s Mouth. Austin: University of Texas Press, 2002.
“Tides of Life: The Marajó Várzea Ecosystem.” Latin America and Caribbean Geographic, 13 Oct. 2024, lacgeo.com/marajo-varzea-ecoregion. Accessed 15 Nov. 2024.