434 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1961 



These lie in the almost flat grassy plains to the west of the Paraguay 

 River, which are in fact an extension northward of the pampas. Their 

 latitude is near the southern Tropic and the climate is therefore sub- 

 tropical rather than typically tropical; it has a seasonal change of 

 mean temperature of 8° C. (27° to 19° C.) . But in the hotter weather 

 the characters of the environment are typical of those in true tropical 

 swamps. The climate is moderately humid with an annual mean 

 rainfall of 55 inches. The rain falls at intervals of about a fortnight, 

 more frequently in the hot season. Between the storms the tempera- 

 ture gradually rises until the next storm comes. 



The swamps occupy depressions in the plains only a few feet below 

 the general level. They drain very slowly toward the river so that 

 their water is for all practical purposes stagnant though its level 

 varies in dry and wet periods by 2 feet or more. In the deepest parts 

 of the swamp, which hardly ever dry, the water is at most 5 or 6 feet 

 deep. The substratum is a black mud full of marsh gas (methane), 

 consolidating in its deeper layers and passing gradually downward 

 into a stiff and impervious clay. 



The shallower parts of the swamp (pi. 1, fig. 1) near its edges 

 are occasionally dry, and the water is covered by a floating blanket of 

 aerial plants of many species, among which the swamp-lettuce {Pistia) 

 and the swamp-hyacinth {Eichhornia) are dominant. Between these 

 plants the blanket is completed by the smaller fronds of the water- 

 ferns Salvinia and Azolla (pi. 1, fig. 2). There may also be open 

 pools where the blanket is missing. The more central parts of the 

 swamp are filled with large clumps of a flowering plant [Thalia) 

 reaching 10 feet or so above the water and of the bulrush {TypJia). 

 Between these clumps the water is clear without vegetation and highly 

 colored (pi. 2, fig. 1) . 



Investigation of the conditions in the waters of these swamps shows 

 first that the content of nutrient salts is high. Phosphates, for in- 

 stance, are present in concentrations of 2 to 4 mg. per liter, whereas 

 in temperate waters concentrations around 0.1 mg. per liter are usual. 

 Many other conditions such as the pH (6.2-6.8) and the bicarbonate 

 content are suitable for the growth of phytoplankton, which we should 

 therefore at first sight expect to be plentiful. In fact, in all parts 

 of the swamp the water contains only a sparse plankton, both animal 

 and plant, and in the central part there is almost none. 



One probable explanation of this anomaly lies in the heavy shading 

 of the water by the vegetation above its surface and the shallow 

 penetration of the light into the higlily colored water even if it is not 

 shaded. In other similar tropical waters it has been found that the 

 amount of light in the water may be below the compensation point 

 for plants within a few inches of the surface. This is so in spite of 

 the strength of the tropical sunlight. 



