FIG. 6-7 Vertical distribution of three species of copepods in the daytime (stippled) and at night (black) in the oligotrophic Lake 

 Nipissing, Ontario, on a July day, when the thermocline occurred between 12 and 15 m (from Langford 1938). 



ing the year. In larger and deeper lakes, a maxi- 

 mum population usually occurs between April and 

 early June, a minimum in August, a second maxi- 

 mum in late September or October, and the yearly 

 minimum in late winter, February or March. How- 

 ever, not all species follow this schedule ; some spe- 

 cies have a maximum in the spring and not in the 

 autumn, or vice versa : and some species reach great- 

 est abundance during the general summer or winter 

 minimum. 



A species can also exhibit alternate increases and 

 decreases in population at other times ; these, as well 

 as fluctuations in total plankton, are called pulses. 

 At times, especially during the summer when the 

 water is warm, an algal form, most commonly a blue- 

 green species, may become so abundant that it dis- 

 colors the water ; these irruptions are known as 

 blooms. The death and decay of such masses of vege- 

 tation may so deplete the oxygen supply that great 

 mortality of fish and other animals results. In some 

 cases the algae produce chemicals toxic to animals. 



The ways in which environmental factors control 

 seasonal and other changes in population are not all 

 clearly understood, but it is significant that the max- 

 ima in total plankton of deep lakes often come at the 

 times of the two annual overturns, times when food 



and oxygen are abundantly distributed at all depths. 

 But the bimodal curve may also be found in shallow 

 lakes and ponds that do not possess thermoclines. 

 In small lakes, however, there is greater irregularity, 

 and one, two, three, or no maxima may occur at vari- 

 ous times of the year (Pennak 19-k)). Periods of 

 high rainfall, which means increased drainage of nu- 

 trients into a lake, may be a factor of importance 

 in producing maxima : seasonal changes in water tem- 

 perature and oxygen tension certainly are important. 

 There appears to be no relation between the pulses 

 of net phytoplankton and zooplankton suggesting ex- 

 clusive dependency of the latter on the former. 



Division.<i 



The lake bottom can be divided into a littoral 

 zone and a profundal zone. 



The littoral cone extends from the water's edge 

 to the limit of rooted aquatic vegetation. It may be 

 subdivided into the eulittoral zone, between high and 

 low water marks at the water's edge where the beat- 



Lakes 69 



