doceran species constituted practically the entire mac- 

 roplankton, with 3 other copepod and 3 cladoceran spe- 

 cies appearing in small numbers. The copepods were 

 in excess of the cladocerans along the Canadian shore 

 except in the Niagara River where the latter jumped 

 to 100 percent of the catch. The cladocerans were in 

 excess of the copepods on the American shore, Daphnia 

 being present at every station and averaging 61 per- 

 cent of the total plankton. Another cladoceran, Lep - 

 todora, also was much more abundant on the American 

 side; it is usually regarded as a surface form or at least 

 as remaining considerably above the bottom. In this 

 littoral zone, however, it proved to be much more 

 abundant at the bottom, and in two of the hauls made 

 with the Helgoland trawl near Dunkirk, New York, 

 it constituted practically 100 percent of the plankton. 

 The predominance of the copepods on the Canadian 

 shore was due as much to a diminution in numbers 

 and frequent total absence of cladocerans as to any in - 

 crease in the copepods themselves. 



The littoral zone was characterized by the abun- 

 dance of Cyclop s bicuspidatus, C. robust us, and Diap - 

 tomus oregonensis among the copepods, and of Bos - 

 mina longirostri s , Leptodor a kindtii , and Diaphano- 

 soma leuc htenbergianum among the cladocerans. Five 

 copepod and 42 cladoceran species characteristic of 

 the marginal zone, as well as Mysis and Pontoporeia , 

 characteristic of the lacustric zone, were absent. The 

 percentages of Limnocalanus in this zone were always 

 much smaller than in the lacustric zone, and it was 

 frequently entirely absent from the plankton. 



Lacustric Zone 



This zone includes those waters of the lake which 

 are more than 10 meters in depth, and which as shown 

 elsewhere in this report exhibit a well defined thermo- 

 cllne over a large part of the area. Consequently, 

 there is a much greater difference between the surface 

 and bottom plankton than in the other zones. The 

 difference is manifest not only in the total bulk of the 

 plankton but also in its species composition. The 

 water temperature drops suddenly and rapidly in the 

 thermocline and induces an active vertical migration 

 on the part of many of the Crustacea. Water below 

 the 50 -meter line maintains a constant temperature 

 of between 4 to 6° C. throughout the entire summer. 

 Mysi s and Pontoporeia are never found outside of this 

 cool area after the middle of June, and Limnocalanus 

 shows a more and more decided preference for it as 

 the summer advances. 



Again since this zone is located in what may be 

 termed the middle of the lake there is much less 

 contrast between the American and Canadian sides. 

 L imnocalanu s still shows larger percentages near the 

 Canadian shore while Daphnia is somewhat more abun 

 dant near the American shore. On comparing these 

 percentages one is impressed by the abundance of the 

 plankton in the lacustric zone. During July, August, 

 and September, 1929, 31 of the 5-minute hauls with 

 the meter nets at the surface and the bottom totalled 

 more than 500 cc. , 4 of these hauls reached the 

 thousands and one exceeded 2, 000 cc. In addition, 

 17 others totalled between 400 and 500 cc. and there 

 were but two totals under 100 cc. The few small 

 catches were at the surface and were offset in every in- 

 stance by a large catch made simultaneously near the 

 bottom. 



The lacustric zone is characterized by Mysis and 

 Pontoporeia , not found in the other two zones except 

 possibly during the cold months, and by the greater 

 abundance of Limnocalanus , Diaptomus sicilis , and 

 Sida. The larger single percentages of Limnocalanus 

 and Diaptomus in each of the 3 months mentioned 

 above are aU in this zone. In July and August there 

 were 26 tows in which the percentage of Sida reached 

 two figures, and 21 of these were in the lacustric 

 zone, including aU the larger ones. 



This zone is distinguished from the littoral zone 

 by the absence during the entire survey of D. minutu s 

 and D. o regonensis , and during August and September 

 of C. bicuspidatus and C. robustus. It is also obvious 

 that none of the characteristic copepods and cladoc- 

 erans of the marginal zone appear at any time in the 

 lacustric zone. 



SEASONAL DISTRIBUTION OF THE 

 MACROPLANKTON 



In discussing the distribution of the macroplankton 

 it will be more convenient to combine the two zones 

 just characterized rather than to consider them sep- 

 arately. In these two zones are found 7 copepod spe- 

 cies, 6 cladocerans, 2 amphipods, and 1 mysidacean, 

 in all 16 Crustacea with an occasional Hydra , but 

 with no ostracods, insect larvae or nymphs. The dis- 

 tribution of this plankton will be greatly simplified 

 if we eliminate species and discuss genera, mention- 

 ing species only when necessary. 





154 



