Amounts of suspended solids and dissolved substances in the middle reaches of the Amazon over the course of one year (August, 1969 - July, 1970)

Abstract

Monthly surface samples have been taken from the middle reaches of the Amazon (Solimoes) near Manaus above the mouth of the Rio Negro from August, 1969 through July, 1970, a full year. The content of these samples in suspended solids and the concentrations of several important dissolved substances in the water were investigated. The results were compared with the water levels in the river and local patterns of precipitation, which define the seasons in this region. The results of the analyses showed that the central Amazon River, too, has a relatively low total salt concentration,corresponding to electrical conductivity values from 44.8 to 83.8 us20/cm. This was combined with a buffering capacity of the water. Most of the total salts consisted of bicarbonates. Despite the weak buffering capacitv of the water, pH values stayed within the range 6.5. to 7.5, very close to neutrality, over the whole year. The results also showed that almost all of the factors investigated were subject to rather large fluctuations in concentration during the year. For a few factors, the variations were not related in any recognizable way to the other parameters in the study. For others, however, quite distinct relationships to each other, and to water level and precipitation conditions, were obvious. This was especiallv true for suspended solids, suspended iron and phosphorus, and the total salt content. All these factors had a decided maximum at the start of the rainy season when the river still stood at a very low level, near the annual minimum, but had just begun to rise again. The concentrations of most investigated substances had their maximum during rising water levels. This situation was naturally very beneficial in the provision of nutrients for the numerous lakes of the vârzea, which are directly connected with the river. In addition, the results of this investigation were compared with other data reported previously for Amazonian water, and new explanations for the decreasing concentrations of many substances in the lower reaches of the river were proposed. Finally, several other important relationships which exist between the river and the lakes, lagoons, etc. conncected with it were briefly discussed.