severely further phytoplankton productivity. However, this is not the case. 

 It appears that supplies of phosphorus during this time period are sufficient 

 to balance phytoplankton assimilation. Thus the importance of internal cycling 

 of phosphorus (Golterman 1973, Rigler 1973) cannot be ignored. The sum of 

 rates of detritus remineralization, phytoplankton and zooplankton excretion, 

 and diffusion input from the hypolimnion is approximately equal to the rate of 

 assimilation by phytoplankton. This analysis (fig. 7) suggests that decomposer 

 input is about one-fourth the excretion input from algae and zooplankton during 

 summer stratification and that phosphorus input from the lower waters is 

 important only before and after stratification. 



To examine more closely relationships among various processes in this 

 conceptual phosphorus cycle, I constructed a phosphorus flow diagram (fig. 8) 

 from model output averaged over the period July-September and compared the 

 results to available information. Sizes of the five phosphorus compartments 

 are representative of Lake Ontario for this time period (figs. 3-5). Rate of 

 conversion of available phosphorus to particulate phosphorus (Stadelmann and 

 Fraser 1974) and fluxes of phosphorus across the thermocline (Stadelmann and 

 Fraser 1974, Burns and Pashley 1974) are also representative. Zooplankton 

 grazing rates represent 59 to 53 percent of the animals' body phosphorus per 

 day for omnivores and herbivores, respectively. This is consistent with 

 general dry weight rations for small crustacean zooplankton (Parsons and 

 Takahaski 1973). Zooplankton excretion rates represent 14 and 11 percent of 

 body phosphorus per day for omnivores and herbivores, respectively. These 

 rates agree with excretion rates summarized by Ganf and Blazka (1974) if one 

 assumes for these animals a nitrogen to phosphorus body weight ratio of 

 approximately 11 (cf. Parsons and Takahaski 1973). Quantitative information 

 on other processes is not available and therefore, in those cases, model 

 information alone will be used. 



Figure 8 shows that it would take less than 1 day in the summer epilimnion 

 for phytoplankton to deplete the available phosphorus pool if there were no 

 recycling and that external sources and hypo limnetic sources alone could not 

 meet this algal phosphorus demand. In fact, this analysis indicates that 86 

 percent of the assimilated phosphorus is recycled within the epilimnion. 

 Stadelmann and Fraser (1974) estimate this recycling to be 87 to 93 percent 

 based on mass balances for the upper 20 m at a single station during the same 

 time period. 



Rigler (1973) suggested that excretion by zooplankton and direct release 

 from ultraplankton were equally important. Results of the present analysis 

 suggest that, for this five-compartment conceptualization, zooplankton excretion 

 and direct release by phytoplankton are approximately equal and are the most 

 important processes supplying phosphorus to the available pool. The rate of 

 remineralization of detrital phosphorus is somewhat slower. This rate represents 

 regeneration of approximately 1.7 percent of the detrital phosphorus per day, 

 which is within the range measured by DePinto and Verhoff (1977). 



The role of zooplankton in the phosphorus cycle must be emphasized. While 

 the zooplankton has an obvious role in applying pressure to reduce algal con- 

 centrations (fig. 6), it also appears to play a dual role in recycling phosphorus, 

 Not only does the zooplankton input directly to the available nutrient pools 

 through excretion, but it also serves as a supplier of detrital material (feces), 



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