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Haemig PD  (2013)  The Amazon Barrier.  ECOLOGY.INFO 29

The Amazon Barrier

Note: This online review is updated and revised continuously, as soon as results of new scientific research become available.  It therefore presents state-of-the-art information on the topic it covers.

The torrential rains that fall upon the mountains and forests of northern South America wash mud and other sediment into nearby streams.  These streams then flow into rivers that wind slowly across the continent and eventually deposit their freshwater and sediment into the sea.

The two largest rivers that drain South America, the Amazon and the Orinoco, are the first and third largest rivers in the world in terms of water volume, together accounting for 19.5% of the Earth's total freshwater runoff (Degens et al. 1991).  By a remarkable convergence, these two rivers, as well as some smaller coastal streams, deposit their enormous loads of freshwater and mud into the Atlantic Ocean within a relatively small area along the northeastern coast of South America.

Nowhere else in the world does a marine environment receive so much freshwater and sediment as the coastal waters of this area.  From the Orinoco Delta in Venezuela to Fortaleza, Brazil, mud from the rivers covers the bottom of the continental shelf, and the seawater is characterized by greatly reduced salinity and enhanced turbidity.

These greatly altered conditions in the coastal waters make it impossible for some marine organisms to survive in or disperse through this area.  Thus, the enormous amount of freshwater and silt deposited here by the great rivers is thought to create a barrier in the ocean that separates the nearshore marine biota of the eastern Brazilian Coast from that of the Venezuelan Coast.

Ecologists call this barrier the Amazon Barrier because, although the Amazon is only one of the many rivers flowing into the sea here, it contributes more freshwater and sediment than the others.

The Amazon Barrier, however, is much more than just an impediment to dispersal.  It is also a mechanism that increases and preserves regional biodiversity.  By preventing some Caribbean marine species from expanding their ranges to the coast of eastern Brazil and vice-versa, conspecifics in the two areas become isolated from each other and, over time, diverge and evolve into new species. 

The barrier also makes it possible for the Caribbean and east coast of Brazil to act as refuges for less fit taxa that would normally be replaced or eliminated by superior species from the other area.  For example, Turbinella laevigata, a large (15 centimeter) gastropod found only along the tropical coast of Brazil southeast of the Amazon Barrier, is the remnant of a formerly more widespread lineage that once occurred in many other areas of the western Atlantic, including Florida, but which has died out elsewhere except Brazil (Vokes 1964; Vermeij 1978). 

In this review, we discuss in detail the nature and extent of the Amazon Barrier and give examples of its effects on the distribution of taxa and regional biodiversity.

Characteristics of the Amazon Barrier

To understand the Amazon Barrier, one must first understand the immensity of the largest river that contributes freshwater and mud to it.   The Amazon River discharges into the sea over 4 times as much freshwater as the Congo, the second largest river in the world; and over 14 times as much as the entire Mississippi-Missouri-Ohio River system of North America (Degens et al. 1991).  With more than 1000 tributaries, seven of which are longer than 1600 km, the Amazon drains 4,690,000 square kilometers (Degens et al. 1991), an area equal to one-third of the South American continent (Depetris and Paolini 1991). 

Each year, the Amazon discharges into the Atlantic Ocean 5780 km3 of freshwater (Degens et al. 1991).  This amounts to approximately one-sixth (16.4%) of the Earth's total freshwater runoff (Degens et al. 1991).  In contrast to other rivers of the world, the Amazon's discharge rate of freshwater into the ocean is relatively uniform throughout the year, with only a small difference between its maximum and minimum discharges (Richey et al. 1991).

Also unlike other great rivers, the Amazon has no delta.  The flow of its mighty volume is so strong that only a small amount of the sediment it carries is deposited at its mouth to form temporary alluvial islands covered with mangroves. The rest is swept out to sea.  Most islands in the Amazon estuary are consequently detached parts of the mainland that the river and sea have not yet eroded (Murphy 1936). 

Each day, the Amazon discharges into the Atlantic Ocean 3 to 3.5 million metric tons of fine sediment (Meade et al. 1985).  This amounts to 1.1 to 1.3 billion metric tons of sediment per year, an incredible sum (Meade et al. 1985).  The sediment is largely deposited on the continental shelf, however, about 20% is transported westward by the Guyana Current along the north coast of South America as "roughly 150 million tons in suspension and 100 million tons stored in migrating mud banks (Wells and Coleman 1978; Eisma et al. 1991; Augustinus 2004; Froidefond 2004)."

Allison and Lee (2004) refer to these massive moving mud banks and suspended sediments as a "mud stream."  The mud not only covers the continental shelf here but the beaches as well, creating the world's longest mud shoreline (1600 km), with extensive mangrove wetlands in spite of relatively high wave and tidal activity.  "Unlike most areas, where mangrove wetlands are confined to protected coastal settings (Thom 1982), the wave damping behavior of offshore underconsolidated mud deposits allows colonization by mangroves at the shoreline as well as inland along river mouths to estuarine limits (Allison and Lee 2004)."

These mud banks are hazardous to humans as well as to rocky-bottom marine organisms.  One remembers, for example, how Henri Charrière's comrade Sylvain became stuck and drowned in this mud along a beach in French Guiana, when these convicts' made their epic escape from the penal colony on Devil's Island (Charrière 1970).

Discharge from the Orinoco River also makes a significant contribution to the Amazon Barrier.  Although dwarfed in size by the mighty Amazon, the Orinoco is by itself a truly impressive river.  Its annual discharge of freshwater (1100 km3) is, as mentioned earlier, the third highest in the world, behind only the Amazon and the Congo (Degens et al. 1991). 

Theoretical considerations

There are many taxa that seem to have their dispersal and distributions limited by the Amazon Barrier.  These taxa include certain species of light-loving corals, shallow water reef fishes, rocky-shore gastropods and one marine bird.

Yet, before we look at some of these taxa in detail, we must mention that most species of marine organisms are not limited by the Amazon Barrier and occur both east and west of it.  These species have either dispersed through the barrier, or avoided the barrier by going over or around it.

For example, the West Indian Manatee (Trichechus manatus) is found in the Caribbean and, until its recent persecution by humans, along the east coast of Brazil as far south as Espirito Santo.  The failure of the Amazon Barrier to limit this marine mammal's distribution can be explained by the fact that this species inhabits both freshwater and saltwater, and readily crosses water gaps to islands.  It either dispersed through the barrier or swam around it.

Another example comes from seabirds.  All seabird species in the region, except one (discussed below), occur on both sides of the Amazon Barrier.  The massive amounts of freshwater and sediment deposited by the Amazon and Orinoco rivers have not hindered their dispersal.  They have either flown over the barrier or around it.

Most cryptic inter-tidal mollusks, mangrove-associated animals and shade tolerant-corals are also not hindered by the Amazon Barrier (Vermeij 1978).  Many of these taxa can live in the unique environmental conditions of the barrier and so for them the barrier is not a barrier at all, but an avenue of dispersal.

If a marine species occurs in either the Caribbean or along the east coast of Brazil, but not in both areas, does that fact prove that the Amazon Barrier prevented it from colonizing the other area?  The answer is no.  It is possible that the organism dispersed through, over or around the barrier in sufficient numbers for colonization, but did not establish itself in the second area because some environmental factor in the other area was unfavorable or because some enemy there extirpated it.

To prove that the Amazon Barrier acts as a barrier, one needs to perform a field experiment by introducing a species endemic to one area into the second area.  However, this would be an unethical experiment since it would risk introducing a new species into the other area through human intervention.  Past experience shows that natural communities and human economies can be greatly harmed by such introductions of alien species.  Therefore, scientists must try to study the Amazon barrier without using field experiments.

The freshwater and silt of the Amazon Barrier are found only in the nearshore waters of the coast.  Therefore, one strong argument for the Amazon Barrier functioning as a barrier, is the fact that it is primarily species of nearshore marine organisms bound to the coast that seem to be hindered by it.

Other nearshore marine organisms that are able to survive further from the mainland, for example in the shallow waters of oceanic islands and atolls, are found both in the Caribbean and along the coast of eastern Brazil.  These unaffected species appear to circumvent the Amazon Barrier by "island hopping" around the barrier.  For example, the Manoel Luis Reefs, which are located 180 kilometers north of Sâo Luis, Maranhão State, Brazil, lie far enough offshore that they receive clear, saline seawater from the west flowing Equatorial Current, and seem to function as a major stepping stone between the Caribbean and the eastern Brazilan coast for nearshore marine taxa not tied to the coast (Collette & Rutzler 1977; Moura et al. 1999). 

We will now look in detail at marine taxa whose distributions seem to be limited by the Amazon Barrier.

Brown Pelicans

Along the Atlantic Coast of North America, the Brown Pelican (Pelecanus occidentalis) breeds as far north as North Carolina, USA (35ºN).  However, along the Atlantic Coast of South America it breeds only as far south as Venezuela (9ºN), where a large population (17,500 individuals in 1983) lives just west of the Amazon barrier (Guzman & Schreiber 1987).  Wandering Brown Pelicans (typically immatures) are sometimes seen east and south of this point all the way to Rio de Janeiro. However no breeding colonies of Brown Pelicans are known from this area, despite the fact that the entire coast of Brazil south and east of the Amazon barrier seems to be ideal habitat for this species (Mitchell 1957; Sick 1993).  Either too few pelicans reach the south tropical coast of Brazil to establish breeding colonies that persist, or the settlement preferences of these birds are such that they decide not to stay there and breed.  Let us look more closely now at the case of this pelican.

The Brown Pelican eats primarily fish.  It forages in nearshore marine waters, seldom straying far from the coast. In fact, during censuses of seabirds in waters off the west coast of Central America and Mexico, it was found that Brown Pelicans stayed closer to the coast than all other seabird species (Jehl 1974).  Specifically, Brown Pelicans were never seen more than one mile (1.6 kilometers) from the coast, while Masked Boobies (Sula dactylatra) and Red-footed Boobies (Sula sula) were "fairly common more than 10 miles [16 kilometers] from shore," Brown Boobies (Sula leucogaster) sometimes seen "more than 20 miles [32 kilometers] from the coast," Magnificent Frigatebirds (Fregata magnificens) up to 5 miles [8 kilometers] from shore, and Red-billed Tropicbirds (Phaethon aethereus) up to 10 miles [16 kilometers] from the beach (Jehl 1974).

Murphy (1936) hypothesized that the muddy water of the Amazon Barrier discouraged the Brown Pelican from extending its breeding distribution southeastward to the "blue-water paradise along the southern tropical coast of Brazil."  He suggested that the turbid waters of the barrier either did not support "schooling fish in numbers sufficient to support a population of pelicans," or else the water was "so nearly opaque that the pelicans are unable to see and capture their prey."

Murphy further noted that Masked Boobies, Red-footed Boobies, Brown Boobies and Magnificent Frigatebirds were rare along the coast of the Guianas and Amapá, in the heart of the Amazon barrier.  Yet, unlike the Brown Pelican, these four species had all somehow crossed or circumvented the Amazon barrier to bred in the Caribbean as well as along the eastern coast of tropical Brazil.  Murphy reasoned that these boobies and frigatebirds were not limited by the Amazon barrier because they were not as bound to the coast as the Brown Pelican.  They could circumvent the Amazon barrier because they had "less pronounced inclinations against leaving the immediate neighborhood of the coast."  The data cited above from seabird censuses off the coast of Central America and Mexico support Murphy's contention that these other seabirds are not as bound to the coast as the Brown Pelican.

For many individual Brown Pelicans, the Amazon River may also function as a maze that confuses them and misleads them so that they never find the south coast of Brazil.  Helmut Sick (1993) reports that wandering Brown Pelicans are occasionally seen quite a distance up the Amazon river and some of its tributaries, such as the Rio Tapajós and Rio Branco.  Imagine how wandering Brown Pelicans must perceive their environment as they fly south along the coast from Venezuela.  Since they prefer to remain close to the the coast, they will follow it until it reaches the estuary of the Amazon.  Because the estuary is many kilometers wide at this point, the pelicans simply do what comes natural and, instead of crossing the wide gap of water in the estuary, fly up the north side of the Amazon River as if it was the coast.  Once the pelicans enter the vast maze of the Amazon watershed, they may become lost or curious and wander extensively before making it back to the Atlantic. 

Finish reading this review on Page 2.

Photograph at top of page: Brown Pelicans by Linda M. Bell (USA)


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