286 



R. G. Stross et al. 



1000 



_ 800 



o 



a. 



u 



T3 

 O 



600 



qI 400 - 



Q. 

 O 



Q 



200 - 



50 100 150 



Heterocope Production, ^gC liter'' 



Excluded from regression 



FIGURE 6-16. Linear regressions of the 

 annual rate of Daphnia production on an- 

 nual rate of Heterocope production for 

 each of five or six ponds. Each year was 

 treated separately, a procedure made neces- 

 sary by the annual variability in pond 

 volume. The concentration as a result of 

 the 1972 drought year is particularly 

 striking. 



generated, but more slowly than the growth of bacteria and algae; the 

 result is that cropping increases the food quality. This hypothesis is 

 discussed in the next section. 



An alternative explanation is that the production of algae, or 

 bacterial food, is increased by phosphate excreted by zooplankton. Almost 

 overwhelming evidence now describes algal growth rate in a nutrient- 

 limited condition to be proportional to the rate of active uptake of the 

 limiting resource. Uptake rate and growth rate describe a hyperbola with 

 respect to concentration (Caperon 1968, Dugdale 1967, Eppley and 

 Thomas 1969, Rhee 1973, among others). Judging from half-saturation 

 constants for phosphate and nitrogen (Eppley et al. 1969, Carpenter and 

 Guillard 1971, Rhee 1973, Chisholm 1974), ambient phosphate 

 concentrations in the pond (Chapter 4, Prentki 1976) and bioassay 

 experiments (Stross 1975) show that phosphate is limiting. Growth rates 

 are likely to be increased in proportion to an increase in ambient 



