Comparison between white and black waters

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 aluminium 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 colours of tetras to perfection.

Black and white waters also differ in their planktonic fauna and flora. The two tables compares the number of planktonic 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 litres of water in each habitat sampled.

Mean ionic composition, specific conductivity (m S/cm), and pH in Amazon waters. Data from Ribeiro and Darwich (1993).
  Solimoes or Amazon river – whitewater. Rio Negro – blackwater.
Na (mg/l) 2.3 ± 0.8 0.380 ± 0.124
K (mg/1) 0.9 ± 0.2 0.327 ± 0.107
Mg (mg/1) 1.1 ± 0.2 0.114 ± 0.035
Ca (mg/l) 7.2 ± 1.6 0.212 ± 0.066
Cl (mg/l) 3.1 ± 2.1 1.7 ± 0.7
Si (mg/l) 4.0 ± 0.9 2.0 ± 0.5
Sr (m g/l) 37.8 ± 8.8 3.6 ± 1.0
Ba (m g/l) 22.7 ± 5.9 8.1 ± 2.7
Al (m g/l) 44 ± 37 112 ± 29
Fe (m g/l) 109 ± 76 178 ± 58
Mn (m g/l) 5.9 ± 5.1 9.0 ± 2.4
Cu (m g/l) 2.4 ± 0.6 1.8 ± 0.5
Zn (m g/l) 3.2 ± 1.5 4.1 ± 1.8
Conductance 57 ± 8 9 ± 2
pH 6.9 ± 0.4 5.1±0.6
Total P (m g/l) 105 ± 58 25 ± 17
Total C (mg/1) 13.5 ± 3.1 10.5 ± 1.3
HCO3-C (mg/1) 6.7 ± 0.8 1.7 ± 0.5


Planktonic organisms collected in black (Japura) and white (Solimoes) waters
Animal groups present Black water Mixed water White water
Rotifera 284 23 0
Cladocera 5 29 43
Ostracoda 39 97 29
Calanoida 11 51 66
Cyclopoida 22 49 61
Chironomids 0 3 3
Acari (mites) 0 0 2


Planktonic organisms collected in black, white and mixed waters
  Black water Mixed water White water
  open water forest open water forest open water forest
             
Volvocaceae 42   38      
Rotifera 87 5 34      
Cladocera 6   5   8 1
Ostracoda 2 11 3   7  
Calanoida 23 3 10      
Cyclopoida 5 27 19 1 13 1
Mysidacea   1        
Diptera         1  
Acari (mites)     1   1  
Larval fish     1   1  


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Peaty layers in the white sand sub-soil on the bank of a stream near Manaus, central Amazonia. These layers release high acidity water into the catchment when a drought is followed by heavy rain
Peaty layers in the white sand sub-soil on the bank of a stream near Manaus, central Amazonia. These layers release high acidity water into the catchment when a drought is followed by heavy rain

The confluence of the Rio Negro and Rio Solimoes (Amazon) close to Manaus, Brazil. The black water of the Rio Negro is quite distinct from that of the white water River Amazon
The confluence of the Rio Negro and Rio Solimoes (Amazon) close to Manaus, Brazil. The black water of the Rio Negro is quite distinct from that of the white water River Amazon

The confluence of the Negro and Solimoes, where black and white waters meet
The confluence of the Negro and Solimoes, where black and white waters meet