TROPICAL CLIIVIATES AND BIOLOGY — CARTER 435 



Measurement of the dissolved oxygen content of the water shows 

 an even more striking contrast with the conditions in similar waters 

 in temperate countries. In the tropical swamps the oxygen is every- 

 where far from saturation even within an inch of the surface. It is 

 in fact astonishing that in these and similar tropical waters one can 

 often take a sample as close to the surface as is practicable — within 

 at the most an inch — and find in it no measurable quantity of dis- 

 solved oxygen. In the central parts of the Chaco swamps even the 

 surface water hardly ever in hot weather contained a measurable 

 quantity of oxygen, and certainly less than 5 percent saturation. 

 In the outer region under the floating blanket the water was also 

 almost always without measurable oxygen. Pools free of the float- 

 ing blanket sometimes contained at midday 2 to 3 cc. of oxygen per 

 liter (about 50 percent saturation) at the surface, but the lower water, 

 even here, was often without oxygen continuously for many days in 

 hot weather. 



How is this lack of oxygen m the swamp waters brought about? 

 I believe that it is the result of several conditions which are all pres- 

 ent in these waters and not normally present in otherwise similar 

 temperate waters. Oxygen can be introduced into a body of water 

 by diffusion from the air, and produced in it by photosynthesis. It 

 will be removed by the respiration of plants and animals and by the 

 chemical and biological oxidations of decay. In the tropical swamps 

 little oxygen is produced by photosynthesis owing to the weak light- 

 ing of the water, and decay, rapid at the high temperature, will ac- 

 tively remove any oxygen that gets into the water. Oxygen can reach 

 the water only by diffusion from the air above it. 



Entry of oxygen from the air must always take place, but in liquids 

 diffusion, though rapid over a distance of a small fraction of a mil- 

 limeter, is negligibly slow over greater distances. A thin oxygenated 

 film at the surface will always be produced, but practically no oxygen 

 can reach the lower layers of the water by unaided diffusion. It can 

 reach the lower layers only if it is carried down by vortical disturb- 

 ance, which may be due either to wind and current — and in the flowing 

 water of rivers and streams all layers of the water are usually well 

 oxygenated — or to convection due to the surface being sufficiently 

 cooled at night to cause overturn of the layers of the water. These 

 waters are stagnant, and the tliick vegetation above them prevents 

 any disturbance in them by wind. Thus, overturn is the only means 

 by which oxygen could reach the lower water. But m these tropical 

 waters, exposed to hot sunlight during the day, there is set up at mid- 

 day a very steep gradient of temperature from the surface downward 

 (often 8° to 10° C. in a column of water 12 or 18 inches high), and 

 in most nights no overturn occurs, so that the water is permanently 



