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, behaviour 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 metres! 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 Solimőes is circa 10 to 11 metres. 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 metres.
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 Solimőes-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 metres, is an almost consistently annual phenomenon. Floodplain channels which largely have a bed at 0 metres 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 metres 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 metres such special refuges will never dry out.
|The frequency at which the water level declines below selected water levels during the annual low water season between 1902 and 1984. The values are given for an arbitrary 0 m datum and the frequencies were calculated using PORTOBRAS data.|
|Water depth (metres)||Frequency|
|-2||Once every 25 years|
|-1||Once every 10 years|
|0||Every 4-5 years|
|1||Every 2-3 years|
|2||Every 2 years|
|3||8 out of 10 years|
|4||9 out of 10 years|
|Biomass averages through time as g per sq.m|
|Date||Apara Parana||Ressaca. Caetono||L. Tefe|
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