Water chemistry of the Amazon basin: The distribution of chemical elements among freshwaters

In the ecological studies of the Amazon region, more attention has been paid to the aquatic systems than to the terrestrial ones. In particular, emphasis has focused on freshwater nutrients. It appeared from the outset that the waters of the region were chemically and physically very heterogeneous, and that in general, chemically poor waters predominated. The first classification of Amazonian freshwaters (Sioli 1950) was based mainly on optical characteristics, and distinguished blackwater, with a high content of humic compounds, whitewater, with a high content of suspended sediment, and clearwater, which is neither turbid with suspensoids or coloured by humic compounds. However, chemical analyses showed that in spite of the generally low levels of electrolytes, there were considerable differences in water chemistry between individual water bodies of a particular optical type (Sioli 1953, 1957a, 1968a). This was found to be the case particularly with the clearwaters (Sioli 1965a, 1968a). Thus the optical types could not be clearly separated on the basis of ionic composition. Differences in water chemistry were, however, closely linked with the geological, geochemical and petrographic properties of the source regions of the waters (Gibbs 1967; Sioli 1968a; Fittkau 1974). Markedly acid, electrolyte-poor waters arise in the highly weathered tertiary sediments which cover vast areas of the Central Amazonian lowlands (Sioli 1954b; Fittkau 1964, 1967; Schmidt 1972a; Furch 1976; Irion 1976; Stallard 1978; Furch & Junk 1980; Stallard 1980).