80 



PLANKTON DISTRIBUTION IN REFERENCE TO THE EPILIMxNION, THERMOCLINE, AND 



Hl'POLIMNION. 



Seventy-four and six-tenths per cent, of the plankton inhabited the 

 epilimniou. The per cent, of the eleven forms studied quantitatively are : 

 Rotifera 87.4, Lyng'bya 80.7, Ceratium 88.6, Microcystis 76.5, Anahaena 84,8, 

 Tribonema 64.3, NavpUi 60..5. Diaptnmus 71.2. Frnqilnrin 76.7. Cyclops .38.8, 

 Cladocera 31.6. 



Seventy-three and three-tenths per cent, of the phytojiliinkton and 83.3 

 per cent, of the zooplankton inhabited the epilimnion. 



The thermocline contained 21.8 per cent, of the plankton of the lake. 

 The following synopsis gives the per cent, of each of the ele^•en forms : 

 Diaptomus 23.8, FrafiUaria 19.5, Vyclops .34.4, Cladocera 40.7. Tribonciua 

 26.6, Nauplii 18.7, Microcystis 20.3, Anahacna 14.4. Ccratiiuii 10.3, Lynghija 

 IS, liotifera 10.8. Of the zooplankton 12.3 per cent., and of the pliytdphuik- 

 ton 22.<i per cent, lived in the thermocline. 



The hypolimnion contained 3..5 ])er cent, of the iilanktoii, 4..". per cent, of 

 tlie zooplankton, and 3.4 per cent, of the phytoplankton. The i)er cent, of 

 each form in the hypolimnion was : Cladocera 27.7, Ci/cIops 26.7. Frauilaria 

 3..5, Nauplii 2<K8. Diapto)niis .">, Anahacna .8, Tribonema 9.1, Microcystis 3.2, 

 Lyngbya 1.1, Rotifera 1.7, Ceratium 1.1. 



At 23 m. tliere were more forms per liter than at 20 m. This was prob- 

 ably due to the presence of some dead organisms that by tlie loss of activity 

 had sunk to the bottom. The end of tlie liose at 23 m. was very close to the 

 i>ottom. 



THE EFFECTS OF PHYSICAL FACTORS UPON DISTRIBUTION. 



The large per cent, of plankton in the epilimnion was due to the pres- 

 ence of sunlight and plenty of food. In the upper half (I)est lighted jiart) 

 of this stratum, there was sufficient carbon dioxitle to permit rapid photo- 

 synthesis. Apstein ("96) found light t() be the most important factor in ex- 

 plaining the presence of fifty-six times as much plankton from 0-2 m. as in 

 the remainder of the water. That direct sunlight has a repelling (>ffect upon 

 some plaidcton, was demonstrated by the fact that r>:> i)er cent, inhabited the 

 tirst two meters, while only 12 per cent. lived at the surface. /. c. in the sur- 

 face meter. Other factors enter into 1h(> explanation of th(> jirolilic life in 

 the epilimnion. Many organisms were too licavy to sink into the cold 

 heavy waters of the thermocline. Juday in his work on the Wisconsin 



