Diatoms, dominated by species with temperature optima in the lower range 

 of Lake Ontario temperatures (Stoermer and Ladewski 1978), increase their pro- 

 ductivity sooner than the other taxa. Consequently, diatoms assimilate available 

 silicon and become limited by that nutrient before other groups deplete the 

 available phosphorus pool and become phosphorus limited. Late in summer, 

 phosphorus becomes limiting for diatoms as well. Phytoplankton production is 

 limited by nutrients (silicon and phosphorus) from this time until the end of 

 September. This has also been suggested previously, based on mass balance 

 considerations (Stadelmann and Fraser 1974) and on algal assays (Sridharan and 

 Lee 1977). 



During the same period (late summer), grazing stress by zooplankton becomes 

 most intense (fig. 6). This timing of simulated grazing pressure reflects the 

 general seasonal pattern of crustacean zooplankton biomass (fig. 5). All of 

 the dominant species in Lake Ontario produced major biomass peaks during July 

 or August in 1972 (McNaught et al . 1975). Also, for a previous year in Lake 

 Ontario, Glooschenko et al. (1972) measured and compared the relative abundances 

 of chlorophyll a and pheopigments and suggested that a high correlation between 

 average percent pheopigment (relative to total chlorophyll a) and zooplankton 

 abundance was probably a result of zooplankton grazing. On a lakewide average 

 basis, they found highest values for the percent of total chlorophyll <x as 

 pheopigments to occur in August-October. This further substantiates the simula- 

 tion results in figure 6. 



In late September the thermocline deepens (fig. 2) and nutrient-rich, 

 hypolimnetic water is mixed with epilimnetic water. Because of this increase 

 in nutrient concentrations and the simultaneous increase in mixing depth, algae 

 again become limited by light. In early November the lake overturns and becomes 

 vertically homogeneous and phytoplankton concentrations begin to approach winter 

 values. Of course, this is a simplification of the three-dimensional > fects 

 discussed by Simons (1976); however in a one-dimensional model, all advtctive 

 and dispersive processes are parameterized as vertical mixing. 



Although it is clear from the above analysis that nutrient limitation does 

 not solely control phytoplankton dynamics, the role of nutrients, especially 

 phosphorus and silicon, is certainly critical during the period of stratification. 

 Phytoplankton dominant during summer months in Lake Ontario are limited primarily 

 by phosphorus, as demonstrated in simulations discussed above and as shown by 

 recent experimental work (Sridharan and Lee 1977). Therefore, to understand 

 better the control of phytoplankton dynamics in Lake Ontario, one must investigate 

 processes influencing the cycling of phosphorus. 



Figure 7 illustrates the seasonal changes in simulated concentration of 

 available phosphorus and rate of gross primary production in the epilimnion. 

 As discussed above, after spring, phytoplankton become limited by nutrients and 

 thus the production rate decreases sharply. It is interesting to note, however, 

 that although production decreased considerably, it did not approach low winter 

 values and, in fact, after the initial drop, it increased gradually. This 

 sustained production proceeds at the same time that available phosphorus con- 

 centrations, both actual and simulated (fig. 3, 7a), are extremely low. One 

 might expect that, with phosphorus concentrations this low (<lpg P/L) and 

 sustained phytoplankton production, phosphorus assimilation by algae would 

 rapidly drive the concentration of phosphorus to virtually zero and thus limit 



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