Abstract
The limnological conditions in Lago do Castanho, a middle-sized várzea lake of central
Amazonia, are determined largely by the Rio Solimões (Amazon River). The lake is connected to the river by means of a canal throughout the entire year, so that the river's fluctuations in water level always affect the lake directly. These changes in the water level,
which amount to an amplitude of about 10 m annually on the average in that region indicate that the lake basin is filled each year with river water and then almost entirely emptied again. This, thus, causes sharp seasonal variations in the chemical-physical and biological conditions of the water.
Other factors which exhibit clear seasonal fluctuations in the lake are first and foremost the general stratification and oxygen conditions and the total salt content, including
all of its most important constituents, but also the turbidity of the water and its seston
content.
The height of the water level proved to be especially important for the stratification
and circulation conditions in the lake in view of the considerable density differences of
the water from temperature unit to temperature unit at the high temperatures which prevailed. It was determined that circulation brought about by normal nightly cooling - seasonal temperature differences practically don't occur at all - can penetrate only up to certain depths in the water mass. The critical depth, or boundary, is usually about 5 m in Lago do Castanho.
As a consequence of this situation, the lower water layers stagnate extensively as soon
as the water level has risen commensurately, so that in this region a persisting absence of
oxygen and formation of hydrogen sulfide occur. At the same time, increasing concentrations of free carbon dioxide, phosphate, and dissolved iron were found in the anaerobic
deeper zone, which exists from about March to September. Within the scope of this investigation it could not be established whether in normal years full circulation is not actually achieved occasionally during the high-water phase. The problem is being debated, in
the process of which it has been pointed out that all investigation results suggest that such
an event supposing it occurs at all during this season of the year, at least does so very seldom.
The salt content of the river water decreases gradually during the entire time the river
water remains in the lake basin. Rain falling on the lake's surface and drainage water from
the immediately surrounding landscape are cited as the cause for this, since no true streams
flow into Lago do Castanho.
Other factors are subject to greater fluctuations of a daily than a seasonal nature. However, only the water to a depth of about 5 m will be affected by these fluctuations. Included among these factors are primarily the water temperature and the stratification conditions in the upper depths of the water, which are products of the existing circulation conditions. The oxygen-free deeper zone which is observable at high-water is not always separated clearly from the often completely mixed upper zone by strong density discontinuities. A distinct arrangement of the water mass into epi-, meta-, and hypolimnion isn't found
over longer periods of time, but is usually observed only during the day. At high-water during the night, and often during the day as well, the temperature curve is not klinograde,
but usually of a heterograde stair-step nature. In contrast, at times of corresponding low
water, full circulation with homothermy occurs almost every night and also many times during the day.
Factors which are related to the process of photosynthesis likewise show greater diurnal than seasonal variation. To these belong pH, free carbon dioxide, and oxygen saturation.
Fluctuations of relatively short duration and lacking any seasonal tendencies were obsetved as well in the vertical distribution of nitrite and nitrate.
The suspended solids contained in the white-water which flows into the lake basin at
times of rising water level are sedimented out relatively quickly. As a consequence of this,
the transparency of the water is increased in the lake, but never reaches values which are
much greater than about 2 m. The causes of this are assumed to be phytoplankton development and also organic detritus of autochthonous and allochthonous origin.
If the maximum water depths reached in the lake during the course of the year fall below about 4 m, resuspension of the upper region of the sediment begins due to wave action. Simultaneously, the transparency is decreased very sharply, during the phase in which
lake sediment is being resuspended, a considerable increase in the concentrations of total-iron and total-phosphorus can be observed as well. The causes and consequences of this
process are being debated.
Further into the study, the limnological conditions found in Lago do Castanho were
compared with those of other standing Amazonian waters. So far as the information available to us at this time shows, Lago do Castanho is a typical representative of those middle-sized várzea lakes in central Amazonia that are connected year-round with the main river.
In conclusion some similarities to be found in tropical lakes of dilferent regions were pointed out. Finally, the practical use of várzea lakes - particularly the possibility of damming
up such lakes - is discussed in some aspects basing on the presented limnological conditions.
