The Uaupes, a tributary of the Rio Negro, is a blackwater river. The ecology of blackwater rivers differs from temperate aquatic systems. In temperate climates, the base of the food chain is usually microflora, produced by photosynthetic and chemosynthetic action on nutrients in the water. Early researchers were faced with the apparent paradox that many Amazonian rivers, while containing both low levels of nutrients and low in situ production of primary phytoplankton, nonetheless support flourishing fish populations.
Food supplies in blackwater rivers come from external sources. River margins provide food for fish – vegetable matter such as leaves, fruits, flowers, seeds, and microflora, and numerous animal forms such as insects, insect larvae, arachnids, crustaceans, and worms. This food enters the river as floating and decomposed matter and mud.
Water levels fluctuate with seasonal rain. When flooding peaks , waters overspill their banks merging aquatic and terrestrial zones, and allowing fish into the flooded forests to feed. Some of the fish populations of these rivers have evolved to exploit increased numbers of foods available during floods and to store these energy reserves as fatty deposits.
Uaupes soils, podsols and latosols, are eroded from the ancient Guyana Shield through which the Uaupes River flows. These soil types are typical of blackwater areas.
Podsols result from intense weathering of soils in a strongly acid medium. Leaching removes soluble minerals leaving the soil high in aluminum and iron compounds. The unweathered surface material is a white sand, devoid of important nutrients such as nitrogen and phosphorous.
Latosols are weathered soils rich in iron. When exposed to repeated drying and moistening, the iron segregates, forming nodules and concretions which cement to a rock-like hardness known as laterite.
These soils’ principal source of nutrients is derived from the accumulation of leaf fall and other debris from the forest. Upon clearing, nutrients are quickly eroded. Thus, to remove the forest cover is to remove the nutrients of both the terrestrial and aquatic systems. Without the leaf litter from the forest, the soils become either white sands or brick-like laterites, both inhospitable to agriculture. Finally, were the forest margin to be denuded for agriculture, these soils would quickly erode, altering the river channel and depositing silt in the river.
Agriculture is a short-lived phenomenon in deforested blackwater regions because nutrient supply is quickly exhausted. Greater protein yields per hectare from blackwater areas are obtained by maintaining the forest as a grazing ground for fish.
One can clearly see how the soil layer of fertile soil in the Amazon is unstable and how little it needs to be degraded. Tribes living in this area cultivate to a very limited extent, therefore their impact on the environment is small. Much worse results are caused by a lumbering timber economy, which by cutting trees in huge areas causes soil sterilization by removing a thin layer of humus. In the place of the cut forest, nature is reborn very slowly.
A separate threat to this region is the legal and illegal exploration of raw materials. When extracting, for example, gold, mercury or cyanide is used. The waste after the recovery of gold goes very often directly to smaller and larger rivers, causing mass poisoning of fish and other aquatic animals.
This creates a huge threat to the people living in this area. It is very common to use violence against the Indians, and even murders of some tribesmen to intimidate others.
