the Waters of Small Ponds. 473 
much difference between the two which are not aerated. Thus on 
6 March 1914, after the aquaria had been kept for 15 months, we 
find the following quantities of ammonia: 
| Rapid Aeration ‘ sy 225) 
| Slow Aeration. 2 eles 
| Standing with Plunger . ‘25 
2a no - qo eBiey 
Thus it is clear that aeration helps to diminish the quantities of 
ammonia that may be present. 
| A reduction in the ammonia content can also be produced in 
the laboratory by dilution, but the experiments on this point are 
not yet complete. 
Effect of Variations of Ammonia Content on the Fauna. 
It is well known that the smaller green algae derive their 
supplies of nitrogen from ammonia and consequently these algae 
will be affected by variations in the ammonia content. Some of 
these small green algae live a planktonic life in the water, and 
where they occur in sufficient numbers they provide nourishment 
for swarms of crustacea and other small zooplankton. In fact the 
zooplankton only reaches large proportions when there are plenty 
of these minute algae. Daphne and Simocephalus feed on these 
algae, the “nannoplankton ” as it is called, and their presence may 
be taken to indicate the presence of minute algae as well. Some- 
times the algae occur in such vast numbers as to be visible to the 
naked eye, as in the “green water” of stagnant puddles, but as 
often as not it is difficult to detect their presence except by 
Special means. 
During the latter part of February this nannoplankton existed 
“mm such numbers in a pond on Sheeps’ Green as to be visible to 
the naked eye, but directly the rains of March began it dis- 
appeared. At the time I was inclined to attribute the disappear- 
ance to disturbance of the water and increased turbidity, but now 
since I have been able to find a large quantity of nannoplankton 
in turbid horseponds, I do not think that turbidity is accountable 
for the extinction. I am inclined to think that it was due to the 
reduction in the ammonia content, which I have shewn above to 
have been caused by the heavy rainfall, and this is supported 
by further evidence. 
All ponds that have a low ammonia content, less than °4, never 
contain the swarms of Cladocera which are dependent on the 
annoplankton for food. Thus in two ponds in the same field the 
ammonia contents were 1°65 and :2 (the high ammonia content is 
due to this pond being used as a watering-place for cattle), and 
