90 



PROBLEMS OF LAKE BIOLOGY 



the surface. The most accurate and effi- 

 cient collecting' apparatus for zooplankton 

 is the plankton trap of Birge and Juday 

 (Juday 1916) which strains a definite quan- 

 tity of water and has such a large opening 

 that probably few plankters escape it. Its 

 one disadvantage is its bulkiness. This is 

 inconvenient but is soon forgotten by the 

 manipulator after a few lakes have been in- 



also to reduce the sampling error. Plank- 

 ton pumps are ideal especially where large 

 quantities of water must be strained, but 

 they are by far the bulkiest and most un- 

 wieldy of collecting devices requiring a good- 

 sized boat and preferably a power drive. 

 For Protozoa, the regular water sampler 

 and centrifuging through the Foerst cen- 

 trifuiie seems the best method. Verv re- 



FiG. 2. Eaft used to obtain limnological samples in the tropics. Sanpaloc Lake, Philippine Islands, 



April, 1932 



vestigated. This trap in the 10 liter size 

 has been used since 1931 by the New York 

 State Biological Survey with verj^ good re- 

 sults. In the Philippines the trap was used 

 on many lakes, even on those which had to 

 be reached by a 10 or 20 kilometer hike 

 into the mountains. Last summer (1938) 

 it was tried in salt water in the bays of 

 Long Island. Here, two 10 liter samples 

 were taken at each depth due to the com- 

 parative scarcity of zooplankton forms and 



cently Clarke and Bean have devised a col- 

 lecting device consisting of a small cylinder 

 with net attached which may be towed at 

 any depth and the actual amount of water 

 strained measured by a propeller and 

 counter. In tests, this device has shown 

 results which compare very well with col- 

 lections made with the plankton trap, and 

 it has the added advantage that it may be 

 towed in multiple with other units all on 

 the same line. It seems a little complicated, 



