474 Mr Saunders, The- Ammonia Content of the Waters, etc. ‘ 
the pond with the high ammonia content contained swarms Es 
Daphne while the other had none. In my aquaria the two which 
are aerated have no small crustacea living in them, while those | 
which are unaerated and have a higher ammonia content contain | 
plenty. A high ammonia content, coinciding with the presence of | 
quantities of nannoplankton, was also found in a jar, which was 
one of two kindly given to me by Mr Elton, of Christ’s College, 
Both jars were started with collections from a small pond, but in | 
one the water had turned green through the presence of minute 
algae while in the other it had remained clear. The green water 1 
| 
contained 1°520 and the clear water ‘300. In these cases we find 
that the presence of much nannoplankton coincides with a high | 
ammonia content and its absence with one below ‘400. From | 
this we must conclude that the ammonia content of the water is | 
one of the factors controlling the appearance in large quantities 
of these minute algae. 
The rainfall, then, through its influence on the ammonia con- 
tent, will react on the swarms of crustacea, for the swarms are 
dependent on the minute algae and these in their turn on the 
amount of ammonia. Thus, in all probability, one of the factors— | 
there must be others—which cause the appearance and disappear- 
ance of large swarms of Cladocera and Copepods in small ponds is 
the presence or absence of much rain. 
