The Amazon Barrier
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.
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
Nowhere else in the world does a marine environment receive so
much freshwater and sediment as the coastal waters of this area. From the
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.
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.
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.
Amazon Barrier, however, is much more than just an impediment to dispersal.
is also a mechanism that increases and preserves regional biodiversity. By preventing
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,
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
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
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).
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).
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
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.
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
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
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
Atlantic Coast of North America, the Brown Pelican (Pelecanus
occidentalis) breeds as far north as North Carolina,
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).
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
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
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.
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.
this review on Page 2.
Photograph at top of page: Brown Pelicans by
Linda M. Bell (USA)