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Abstract

Changes in the major cation content (Na, K, Mg and Ca) of the Lago Camaleão, one of the várzea
lakes on an island in the Rio Solimões/Amazonas near Manaus were followed during one year. Comparative changes were also followed in the Rio Solimões. Changes in the electrolyte content of the lake water,
presented as total maior cation content, are extremely great during the period of investigation. Nov. 1980
to Nov. 1981, and show a pronounced annual periodicity. During the low water period (Oct. 1981) the
ion concentration reached its maximum and was more than 15 times higher than in the river at the same
time. With rising water level, in flow of comparatively electrolyte poor water from the river and simultaneously increasing rain fall dilute the water in the lake. In the following months there is a relatively sudden reduction of the electrolyte content of the lake water. By the time of high water the levels have
fallen almost to those of the river. With falling water level concentrations of major cations at first increase very slowly followed by an abrupt change within one month, from approx. 20 mg major cations/l
to approx. 230 mg/l. Investigation of different lake sections showed that the electrolyte content is not
uniform throughout the lake. Concentrations rise from the lake entrance to the inner part of the lake,
so that the lake sections distant from the river are up to fifteen times as rich in electrolytes as the river
itself.
The changes in total ion content are accompanied by marked changes in the chemical composition of the lake water during the year, i. e. in the relative proportions of each of the four elements (Na,
K, Mg and Ca). Greatest changes are shown by the element K. The deeper water of the lake during the
high water period is richer in K probably due to the release of elements from the decomposition of K-rich terrestrial and aquatic macrophytes. The uptake of large quantities of nutrients from the system
for the growth of macrophytic vegetation, the accumulation of nutrients in macrophytes and sediment,
and the release of these nutrients during decomposition and resuspension are among the most important
factors contributing to the electrolyte budget in the Lago Camaleão.