in the following year (tables 21 and 22). In July and 

 August 1928 the production averaged about 33 per- 

 cent higher than in 1929, but in September of the 

 former year the volume dropped 53 percent while in 

 the second year it rose 8. 5 percent. As the temper- 

 atures during 1928 averaged consistently higher until 

 September than in 1929, it may be that the breeding 

 season terminated earlier and the numbers declined 

 with the slight drop in mean temperature that month. 

 The mean temperature rose steadily in 1929 both 

 at the surface and bottom throughout the season, 

 reaching its peak in the region of the Deep Hole in 

 September. Slightly lower temperatures were re- 

 corded in the western area on the last cruise to- 

 gether with indications that production was declining 

 in the western part of the Great Plains Basin. 



Evidently the summer macroplankton volume 

 increases with the temperature and begins to de- 

 crease as soon as the temperature peak has been 

 passed. During this interval the relative proportion 

 of the various members of the plankton community 

 fluctuates as the breeding season of none of the domi- 

 nant species extends over the entire summer. 



Faunal Zones 



The lake may be divided Into 3 faunal zones: 

 a marginal zone confined to the alongshore waters 

 to a depth of 1 meter, and including tributary 

 streams; a littoral zone comprising the shoal areas 

 from 1 to 10 meters; and a lacustric zone from 10 

 to 62 meters. The following conditions found in the 

 eastern area in 1928 applied equally well to the lake 

 as a whole in 1929, "More or less overlapping oc- 

 curred in the two latter zones, but although the 

 dominant species were found in both, they were pres- 

 ent in such different proportions that it was possible, 

 particularly during the early part of the season, to 

 determine without difficulty which were endemic 

 and which exotic in any particular zone. Later in 

 the season when the dominant species of the lacus- 

 tric zone expanded laterally into the littoral waters 

 they outnumbered the species of the latter to such 

 an extent that the proportions of lacustric species 

 were often as great in shallow water as in the deeper 

 parts of the lake. " (Fish et al. 1929.) 



Margina l zone . - -This zone is of particular 

 significance for two reasons: first^ because it con- 

 tains some of the most important macroplankton 



production centers in the lake; and second, because 

 it has a rich and distinct fauna more closely related 

 to that of the tributary waters than to the open lake. 

 Breeding grounds almost surround the lake being more 

 numerous and relatively larger on the Canadian shore. 



There are 3 major centers covered with reeds and 

 rushes, with a total area of more than 100 square miles, 

 2 on the Canadian shore and 1 on the American. Each 

 forms an important breeding ground for a different 

 species of the genus Diaptomus , as well as for many 

 other forms; in fact aU the cladocerans and copepods 

 in the lake, with the exception of the cold-water 

 genus Limnocalanus, were found breeding in at least 

 1 of these 3 localities. 



Vast numbers of certain species carried into the 

 lake never reappear beyond the immediate vicinity 

 of the outwash. Such marked distribution limits are 

 due to the rapidity with which freshwater Cladocera 

 and Copepoda die or pass into resting stages when un- 

 favorable environmental conditions are encountered. 

 Contributions of organic matter of this sort from 

 streams and ponds must supplement considerably local 

 production in the marginal area and perhaps may ex- 

 plain the presence of large schools of small fishes in 

 these localities at certain seasons. Although many 

 species of Entomostraca never are able to establish 

 themselves in the lake, some of the bodies of water 

 in the drainage area were found to contain very rich 

 faunas of important Lake Erie species. As might be 

 expected, the marginal zone is characterized by 

 much greater Inequalities in distribution than the 

 littoral or lacustric zones. Qualitatively the vari- 

 ation was not very great but the abundance of 1 or 2 

 species and the small numbers of all others at each 

 station was particularly striking. Rarely did the 

 same species dominate at even adjacent stations. 



Littoral zone . --Except for the "arctic relict" 

 species which when present always occur here in 

 smaller numbers, the fauna of the littoral zone is 

 very similar to that of the lacustric zone. The major 

 distinction is that both cladocerans and copepods 

 which later extend over the entire lake appear first 

 in the littoral zone. The zone is best defined by the 

 absence or presence of marginal and lacustric species 

 rather than by any truly characteristic species of its 

 own, although several species are found at all times 

 in greatest abundance here. During the period of 

 rising temperatures great differences were observed 



187 



