Zooplankton 261 



60 r 

 40 



- 20 

 o 



" 

 o 



sS 40 r 



Length 



1971 



I ■ I ■ II 11 



Brood Size 

 1971 



ii 



o 20 



^ O 



1972 



■ ■■■■III,. 



1972 



I..l|l|.. 



40r 



20 



1973 



2.0 



^J4l 



1973 



■llll.. 



32 



2 4 6 8 10 

 Brood Size 



24 28 



Length, mm 



FIGURE 6-6. Size (length) frequency distributions of the 

 overwintering generation of D. middendorffiana while 

 gravid with the first brood. Frequency distributions are 

 also shown for each year. Means indicated by arrows. 



while animals grown at 3 times the natural food supply had 3.2 embryos 

 per brood. Above this food concentration, the carapaces and appendages 

 of the Daphnia became covered with peritrich protozoans which interfered 

 with movements. As a result, at 5 times the control concentrations of food, 

 the brood size decreased to 1.6. Thus, food concentration did affect the 

 brood size. 



It was less easy to interpret data on D. pulex from a trough pond 

 (Near Ditch). In 1972, a dry summer, the pond volume decreased greatly 

 with a resulting increase in numbers of animals per liter. Daphnia which 

 had hatched from the overwintering embryos were 1.4 to 2.8 mm long at 

 maturity and had small broods (less than 11). In contrast, the animals 

 were sparse the next summer but were larger (2.4 to 2.8 mm) and had 

 larger broods (9 to 18). Despite the large differences in length and brood 

 size, the constants for the equation describing the relationship between 

 length and brood size remained the same. 



Biomass 



The carbon content of various life stages and sizes of the important 

 species was measured so that numbers could be converted to carbon. Early 

 in the study the carbon content for individuals or groups was measured 

 with a dichromate oxidation (Strickland and Parsons 1968). Later, the 

 animals were combusted and the CO2 measured with an IR gas analyzer. 



