RECENT FRESHWATER INVESTIGATIONS 327 



(Pitard, 97, Grarbini, 98a) and particularly the presence of a 

 greater amount near the shore*, it is probable that proper re- 

 gard was not paid to depth and that there really exists no con- 

 siderable difference. Uniformity of horizontal distribution has 

 been shown to be modified by large inflow and by the existence 

 of areas more or less separated from the main body of the lake, 

 by shallows, or in deep bays (Huitfelt-Kaas, Zacharias.) 



Regarding vertical distribution, Hofer (95) is alone in 

 placing 35 m., or in one case 65 in., as the lower limit of the 

 plankton. Other observers have noted no such limit and 

 Ward (96b) found plankton even down to 130 m., although he 

 shows that in comparison with the upper portions of the water 

 the deep stratum, 25 m. to the bottom, contains very little 

 plankton. 



All investigators agree that the upper strata of the water 

 contain proportionately more plankton than any below. Reig- 

 hard (94) found at a depth of 5 m. that half the plankton oc- 

 curred in the upper one and one-half meters of water. Apstein 

 (94) and "Ward (96) show that much more is found in the sur- 

 face 2 m. than in any equal stratum below this. From enu- 

 meration of the Crustacea alone, Birge (97a) demonstrates that 

 in water having a total depth of 18 m. during the summer 45 

 per cent is found in the upper 3 m., 25 to 30 per cent in 

 the 3-6 m. level, 15 to 18 per cent in the 6-9 m. level, leaving 

 only 8 to 12 per cent for the lower half of the water. In the 

 fall and winter, however, the distribution of the Crustacea is 

 nearly uniform. 



France (94) found in Lake Balaton a regular diurnal migra- 

 tion of at least a part of the plankton, governed by light and 

 storm. Zacharias (95) was unable to find any such movement 

 of the plankton in Lake Plbn. It is, however, confirmed for 

 Lake Balaton by Daday (97c) in his investigations on the lim- 

 netic Crustacea. Marsh (97) and Birge (95a) are positive that 

 it does not exist in the lakes which they studied. Pitard 



* Two observers make directly contrary statements in this respect 

 concerning the same lake (Blanc, 95, and Pitard, 97). 



