The Amazon Basin

   The River Amazon is by far the largest river in the world. About 20 % of all the freshwater flowing into the oceans of the world enter via the River Amazon, which has an average flow of 200,000 cubic meters per second at its mouth. This is more than ten times the flow of the Mississippi. Even given the heavy rainfall that can occur within the Amazon basin (average rainfall varies between 1500 and 3000 mm per year over the majority of the catchments) such a flow still requires a huge catchments. The Amazon drains an area of more than 7 million square kilometers and includes within its catchments Brazil, Venezuela, Colombia, Ecuador, Peru and Bolivia. The Amazon can be considered as a collecting channel for a series of large rivers.
   The largest of these tributaries is the Rio Negro. Other major tributaries have their origin in the high Andes and these give the river its essential character because they contribute much of the sediment and nutrients present in the lower Amazon river. The depth of the River Amazon down stream of its confluence with the Rio Negro averages 25-30 mt. River Solimões (Amazon) in the vicinity of Tefe, Brazil.
   For much of its course through the Amazon basin it flows through low lying land and thus is able to meander across a wide floodplain forming a great mosaic of lakes, ponds and channels: the template from which the aquatic diversity of the region is formed.
   The Amazon is essentially a giant river valley bordered to the north and south by the Guiana and Brazilian shields respectively, which comprise hard Precambrian rock. The total area of the Amazon Basin is 7.5 x 106 km (795 km2) of which about 80% is rain forest. During the Paleozoic the Basin was a huge marine inlet into which many tributaries flowed.
   Until the Andes began to form at the end of the Miocene this inlet opened into the Pacific ocean and thus some elements of the Amazonian fauna are related to marine fish from the Pacific rather than Atlantic ocean. During the Quaternary water levels within the Amazon basin changed with the sea level. When the sea level was high, huge lakes formed in the valley into which large amounts of sediment formed.
   During periods of low sea water level the rivers cut through the sediments forming river valleys. The modern sediments of the Amazon valley were formed during this period. These sedimentary deposits are up to 300 m thick and are called the Barrier formation. Because these sediments have been heavily leached they hold low levels of nutrients and easily soluble cat ions such as calcium, and thus water flowing from the sedimentary deposits in the basin always has a low conductivity.
   The first European to visit the Amazon was Vicente Yanez Pinzon, who sailed up the river in 1500. The river Amazon was first ascended from its source in the Andes to its mouth by the party led by Don Francisco de Orellana in 1541-42. This journey was forced on the Spaniards who had traveled ahead of the main body to seek provisions. However, they traveled down-stream to a point where they did not have the resources to make their way back up the Andes to Peru. After reaching the mouth they made the journey back to Spain via Trinidad. Orellana later returned to the Amazon, but the expedition was a disaster and he drowned when his ship capsized in the mouth of the Amazon. The first scientist to travel the entire length of the river was Ch. M. de La Condamine.
   Intensive biological study of the region commenced in the 19th century. First, the German explorer Alexander von Humboldt, accompanied by the French botanist Aimé Bonpland, mapped the connection between the Amazon and Orinoco systems through the Casiquiare River. The British natural historians Henry Walter Bates and Alfred Wallace followed in these studies mid century. Bates spent the years from 1848 to 1859 along the Amazon, collecting thousands of species of animals. Much of his time was spent in Tefe, then called Ega. He wrote a book, The Naturalist on the River Amazons, which is one of the best accounts of the ecology ever written. Wallace was less successful than Bates and lost all his samples in a shipwreck. However, he moved on to Asia where he had the idea of evolution which he later co-presented with Charles Darwin.
   At the beginning of the 20th century Theodore Roosevelt took a great interest in the region and led an expedition to explore what became the Roosevelt river. Between 1910 and 1924 expeditions sponsored by Harvard University undertook much work. Today Brazilian scientists undertake research supported by visitors from many countries. The major research institutions are Museum Paraense Emílio Goeldi in Belém and INPA in Manaus.
   While there has been considerable habitat degradation over this century the Amazon still remains the greatest green wilderness in the world and attracts many biologists and ecologists to its riches.

   The two great Brazilian cities of the Amazon are Belém at the mouth and Manaus upstream at the confluence of the Rio Negro with the river Amazon. Belém is a long established town that was given city status in 1655. It is a trading city that grew rapidly during the rubber boom years and although rubber is no longer important it still is a trading centre for Amazonian products such as Brazil nuts and wood. It is also a centre for Amazonian studies and the Museum Paraense Emílio Goeldi should be visited by all those interested in the Amazonian flora and fauna.
   Manaus is also a port for ocean-going ships although it is 1,450 km from the Atlantic Ocean. It is the capital of the state of Amazonas and an important manufacturing centre. The city has recently undergone a rapid expansion due partially to economic assistance from the Brazilian federal government, which gave the city valuable tax concessions. From 1890 to 1920, the rubber boom brought great wealth to the city and many fine buildings were erected, including the famous opera house. The city then went into decline until the 1960s when the present boom commenced. The city has grown rapidly and the once graceful old city has now been almost destroyed or abandoned. Modern life in the city now follows the normal suburban pattern of trips to shopping arcades on the outskirts rather than visits to the old town centre. The rapid growth of this city has drawn people from the forest and has resulted in a partial de-population of some Amazonian areas which in some cases now have populations lower than when the Spaniards first arrived. Along the edge of the Rio Negro the city has fine sandy beaches during the low water season. Manaus has a large airport which is the main entry point for Amazonian tourists. Anyone interested in wildlife should visit INPA which has a small zoo set in a small forested park.
   The third most important Brazilian Amazonian town is Santarém, which is situated on the right bank of the Tapajós River, near its confluence with the Amazon River. It is another rapidly growing city but manages to retain more of the traditional feel of an Amazonian town.
   The most important Peruvian city on the Amazon is Iquitos. This port is about 3,700 km from the Atlantic Ocean. The city grew rapidly during the rubber boom but then went into rapid recline from which it is yet to fully recover, although the population has recently increased rapidly.
   The native peoples of the Amazon have almost all joined the dominant western societies. Over the last 100 years, there has been great cultural destruction and Amazonian Indian numbers in many areas have declined dramatically. This was probably due to disease. The inter-breeding of European settlers, their Negro slaves and local Indians has created a hybrid culture. Many of the hunting and agricultural skills of the original Indian tribes are still retained by these people. The staple food is manioc and the main source of protein is fish. Fish are caught by a wide variety of traditional methods including trapping, harpooning, bow and arrow, throw netting, line and hook and poisoning. Today the most common methods are gill and seine netting. Monofilament gill nets are highly effective and have undoubtedly resulted in a decline in fish stocks in some waters. Commercial fishing activity has recently intensified because of demand from cities such as Manaus and Belém. This has resulted in a rapid decline in the most sought after species, particularly pirarucu and tambaqui. The establishment of the Mamiraua Ecological Reserve is an attempt to address this problem by allowing local subsistence fishermen to continue to hunt in floodplain lakes while excluding the commercial boats.


Seasonal Flooding
   Because of seasonal variation in rainfall in their Andean headwaters, the great rivers of the Amazon Basin show considerable variation in their rates of flow. When flow rates are low the rivers remain within their permanent banks, but the main Amazon basin is low lying, so
during periods of high flows the rivers burst their banks and flood thousands of square miles of forest. This seasonal alternation of wet and dry season is one of the most important factors determining fish distribution, behavior and diversity within the floodplain. It results in marked changes in habitat availability and suitability. Changes in water depth between maximum high and minimum low water levels in some regions can be as great as 15 meters ! As a typical example, within one year, the difference between the lowest level, normally in October or November, and highest level in about July for the Mamirauá Reserve between the Rios Japura and Solimões is about 10 to 11 meters. The graph shows changes in water depth on the edge of this reserve in the port of Tefe. These data show that the difference between the maximum and minimum water levels over a 5 year period is greater than 14 meters.
   Variation within the annual flood cycle alters both the amount and duration of habitat availability. Forest dwelling fish must adapt to ever-changing habitat availability. Using a long time series (1902-1986) of water depth measurements taken at the Solimões-Negro confluence at Manaus by the Brazilian Port authority PORTOBRAS we can see how variable the aquatic world is. The table shows the frequency at which different water depths were experienced in the Mamirauá reserve over the twentieth century. These data show that the drying up of shallow floodplain lakes, which have a bed at +3 meters, is an almost consistently annual phenomenon. Floodplain channels which largely have a bed at 0 meters dry out on average about every 4-5 years (about 22% of low water seasons). Throughout the floodplain most lakes have beds at -1 to -2 meters and would be expected to be dry once every 10 to 50 years. Thus every century the fish of the floodplains will experience at least two dry seasons when almost all their habitat will be lost. However, there are always small areas where they may shelter. For example, some channels in the Mamirauá reserve such as the upper Cano do Lago Mamirauá have stretches with a bed depth of about -15 meters such special refuges will never dry out.
Fishes and Amazonian habitats
   It is easy to consider the Amazonian fishes as one large group, and assume that they lead similar lifestyles and have similar requirements. In fact, the range of different habitats supporting fishes is nearly as diverse as the fishes themselves. The main bodies of water are grouped loosely into black, white and clear waters, which will be dealt with in more detail in later sections. However, waters mix and mingle, creating areas where the composition is neither one thing nor the other, and the seasonal floods bring both chemical and other environmental changes.
   Some of the fishes are hardy, and can be found in more or less any body of water they have been able to reach. Others are more specific in their requirements, and are restricted to a particular area or habitat; many of these are hindered from achieving a wider distribution by 'chemical barriers' - changes in water chemistry that they are unable to swim through to reach other waters. The widespread fishes are usually familiar in the aquarium trade and public aquaria (except where they are excluded because they are particularly large or aggressive, and even then the most unlikely fishes have fans somewhere). Others are unlikely ever to become familiar to most people - a tank filled with dead leaves containing microscopic fish is unlikely to provide much entertainment. However, the most fascinating fishes with remarkable adaptations are often those found in obscure habitats, that cannot be appreciated by the casual observer.
   Amazonian waters have traditionally been classified as white, black or clear. Whitewater rivers, such as the Rio Solimões, which is the name given to the river Amazon above its confluence with the Rio Negro, carry a high sediment load derived from their headwaters in the Andes. Black and clear water rivers have catchments confined within the Amazonian forest and carry little inorganic sediment. White waters are actually light creamy-brown in color.


   The Rio Negro is the largest river in the Amazon system to have its entire headwater system within the low-lying forest. This results in the main river receiving acid, peaty water with little dissolved nutrients. The color of shallow water flowing over white sand is pale, clear, yellow. However, deep water appears dark brown, almost black. As the river carries little sediment, the beaches are sandy although the bed of the river comprises soft mud.
Black Waters
- Black and clear water rivers have catchments confined within the Amazonian forest and carry little inorganic sediment. The water is the color of weak tea, but is rather transparent because of the low sediment load. The brown coloration comes from the break-down of plant debris within the forest .
Clearwater rivers
   Rivers which flow with water derived from catchments within the Amazon forest are classified as either black or clear waters. The soil within the catchmentss, the vegetation and the ground water level are the three main factors determining if a river will hold clear or black water. Clear water tributaries such as the Xingú and Tapajós have higher mineral content and lower levels of humic and fulvic acids than the Rio Negro and other black water rivers.
   Because of their higher mineral content and increased clarity clear water rivers can support phytoplankton and large blooms are sometimes seen in the vicinity of the union with the river Amazon. Some forest streams also flow with clear water. These normally receive water from catchments which do not contain much regularly inundated low land and swamps. Some streams flow with clear water during the rainy season and black water during the dry season. During the dry season they are fed with water that has laid in contact with dead leaves and other rotting vegetation for sufficient time to take up large amounts of humic acid.
Chemical comparison
Comparison of plank tonic organisms
   As can be seen in the table, black and white waters differ greatly in their ionic composition. Black waters have ionic concentrations not much greater than that of rainwater. They are however, much more acidic and this results in black waters having an aluminum concentration greater than that of the more neutral white waters. The most striking differences are in the concentrations of sodium, magnesium, calcium and potassium, these are very low in black waters. This has considerable ecological implications. Some animals groups, such as snails, need much calcium to build their shells and so are not abundant in black waters. The lack of dissolved ions in black waters results in a low conductivity, similar to that of rainwater.
   Quite how black waters have become so acidic is still rather unclear. The acids are derived from the break down of plant material on the forest floor. Layers of peat and leaves interspersed between layers of sand as shown in the figure are common in the floodplains of streams and these produce a highly acidic solution and slowly drains into the streams. I think that decomposition under these conditions might produce acetic acid (vinegar) which can produce the observed acidity.
   To keep black water fish in an aquarium it is important to use rainwater, as almost all tap water will hold too many dissolved ions. The water should be passed through a peat filter or allowed to equilibrate with peat in a tank prior to use to produce the correct acidity. If this yellowed water is used with a white sand substrate and some bog wood and a few dead leaves you will have the perfect black water stream habitat. It will show the colors of tetras to perfection.
   Black and white waters also differ in their plank tonic fauna and flora. The two tables compares the number of plank tonic animals caught in black and white water localities only a few meters apart. In fact, the black water was not even an extreme example as can be found in the Rio Negro system. However, it can be seen that the black water held far greater numbers of rotifers but fewer crustaceans and mites. These crustaceans are important foods for larval fish. The zones where the two waters mix are particularly attractive to ostracods and young fish. Anywhere in the world where you see these mixing zones there tend to be high numbers of animals and this is certainly the case in the Amazon. The high abundance of animals is shown clearly in the second table which compares the numbers of animals present in 10 liters of water in each habitat sampled.
Amazonian Fish website