438 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1961 



0.1 percent), so that sulfur can be a limiting factor only for the 

 plankton- feeding and not for plant-eating fish. 



Wlien Hesse analyzed the mud from the bottom of the lake, aston- 

 ishing results were obtained. The sulfur content of the mud was 

 extremely high at all depths down to 13 meters below its surface, but 

 m all the samples far the greater part of the sulfur — in most samples 

 more than 90 percent — was in organic form. Inorganic sulfur is 

 present but in relatively low concentration, sulfides often below the 

 limit for estimation. 



Clearly, the sulfur is not held in the mud by precipitation as sul- 

 fide, and this is confirmed by the fact that the water over the mud 

 is often well oxygenated and the mud itself not in a highly reducing 

 state. Wliat apparently happens is that the sulfur is absorbed from 

 the water of the lake by the planktonic fauna and flora and carried 

 down to the mud in their dead bodies, so that the water becomes 

 denuded of sulfur. In the mud the organic sulfur compounds in 

 the bodies of the planktonic organisms are preserved without break- 

 down even for the several thousand years required for the deposition 

 of 10 to 15 meters of mud. Plants growing in the water, if their roots 

 penetrate the mud, are able to absorb sulfur from this store, and this 

 is apparently the only means by which the sulfur can be carried back 

 from the mud to the water. 



Soils around the lake were also found to have a low sulfur content 

 except where they were covered with forest, bemg leached by water 

 draining toward the lake. The same shortage of sulfur is found in 

 many soils in other parts of Uganda, and Simpson and Butters [8] 

 have found experimentally that addition of lake mud to such soil 

 improves the growth of plants growing on the soil. The organic 

 sulfur compounds in the mud break down if the mud is dried, boiled, 

 or autoclaved. After being so treated it has been found to improve 

 the fertility of fish ponds. 



I now turn from work on fresh waters to a quite different branch 

 of tropical biology, investigation of the control of seasonal rhythms 

 of reproduction and migration m tropical animals. This again raises 

 problems different from those met in temperate regions. In many 

 tropical environments, especially in regions at considerable distances 

 from the Equator, there may be, as we have seen, fairly large seasonal 

 changes in the environment, and there is then no difficulty in showing 

 that seasonal changes in the behavior of the fauna are controlled by 

 the environmental changes, as they usually are in temperate coun- 

 tries. In the Paraguayan Chaco, for instance, many of the amphi- 

 bians and fishes breed after the first heavy rains of the summer season 

 and it is easy to show that the stimulus for breeding is in at least some 



