424 Birge — The Crustacea of the Plankton. 



lower water, in the absence of more exact investigations on the 

 subject. 



In lake Mendota the lower water is always clear, but the 

 whole region below the thermocline rapidly becomes unfit to 

 support life, so that the life in the lower waters ceases very 

 shortly after the formation of the thermocline. In lakes 

 with a smaller amount of plankton the bottom water may 

 become unfit to support life in late summer, although the 

 plants and animals extend far below the thermocline. In 

 Pine lake on September 5, 1896, Cyclops was by far the most 

 abundant crustacean in the cold water, and numbered 21, 000 

 per cubic meter between 12 and 15 meters, and 3,000 between 

 15 and 18 m. It was practically wholly absent between 18 and 

 24 m., only 8 individuals being taken by the net within that 

 distance, and no other forms of Crustacea were taken. In Okau- 

 chee lake the Crustacea are numerous to a depth of 24 m. in Sep- 

 tember, but between 24 and 27. 5 m. they were very few. In lake 

 G-eneva, Wisconsin, the Crustacea in September extend to the 

 bottom at a depth of more than 42 meters. This lake is ex- 

 tremely poor in plankton. The statistics given by Marsh for 

 Cyclops and Diaptomus ('97, p. 191, 204) may indicate a partial 

 exclusion of the Crustacea from the lower water of Green lake in 

 late summer and autumn. 



While the plants and animals of the upper water are excluded 

 by this means from the lower part of the lake, animal life is by 

 no means entirely wanting. Worms are found in the mud at 

 the bottom, as also is Cyclas, in considerable numbers. There 

 must, therefore, be oxygen enough in the water to support some 

 life. 



Cyclops and Chydorus are the least sensitive of the limnetic 

 Crustacea to these injurious influences. As shown by the tables 

 on page 416, they always predominate in the lower strata of the 

 inhabited water and form almost the entire population of the 

 water below the thermocline. 



It is possible that the exhaustion of the oxygen from the 

 lower strata of the water is the cause of the death of Cyclops 

 and Vaphnia hyalina&t the bottom in spring and early summer. 

 I have, however, no positive evidence on this point and in the 



