In spite of the small number of dates on which samples v;ere 

 taken, it is possible to detect seasonal changes in abundance similar 

 to those observed in the Island Section. The diatoms showed two periods 

 of great abundancej the greens and blue-greens only one. In general, 

 the times of abundance agreed with those in the open lake, although there 

 were some notable differences. The most marked differences between the 

 two sections were in the rather consistently greater abundance of all 

 groups (especially blue-greens), and in the earlier appearance of 

 greens and blue-greens in large numbers at the inshore station. 



It seems probable that three factors have been especially 

 important in bringing about the greater abundance as conpared with the 

 Island Section. One of the factors is shallowness of the water. 

 Station l59 has a depth of only 3.5 meters, arid the 6.U meter contour 

 lies at a distance of about four miles from the shore at this point. 

 In general, the total amount of plankton in lakes of the same region 

 varies directly with the area, and the amoxint per unit volume of water 

 varies inversely with the mean depth,^ If the area near Station 1$9 

 be considered as a somewhat distinct unit with respect to plankton pro- 

 duction, it would be expected to have greater abundance of plankton per 

 unit volume than the deeper water of the Island Section, 



A second factor is the proximity of Station lS9 to the 

 estuary of Ftortage River, For a distance of about nine miles above 

 Port Clinton the river is virtually a lake, and, being shallow, should 

 contain an abundance of plankton. If so, the river would help to colonize 

 Lake Erie near its mouth and keep the plankton counts higher than in the 

 open lake. Samples taken in the river at Port Clinton on three dates 

 in 1930 had counts not far different from those at Station 1$9 on, the 

 same dates. 



These two factors have contributed to the natural richness of 

 the Portage River Section. In addition there has been the stimulating 

 effect of nutritive salts derived from domestic sewage. The general 

 principle that increased raw material for food manufacture results in 

 increased abundance of plankton algae is too well established to require 

 proof here. Portage River receives domestic sewage from a number of 

 communities and there is no doubt that the plankton algae of the estuary, 

 and of the lake nearby, make use of the elementary nutrient materials 

 derived from it. How great an effect pollution has had cannot be 

 determined because nothing is known of the abundance of plankton under 

 natural conditions o 



6/ The inverse relationship between depth and plankton production has 

 been designated as the Law of Huitfeldt - Kaas by Naumann (1932, p. 82). 



181 



