Primary Producers 217 



remainder is excreted in an organic form. This organic "XP" fraction then 

 either undergoes extracellular, enzymatic hydrolysis which again releases 

 PO4 or is sorbed onto water colloids which are in chemical equilibrium 

 with XP (Lean 1973b). Whatever the adaptive value to the cells of this 

 uptake-excretion-hydrolysis cycle (Prentki 1976), the very high rate- 

 constant for PO4 uptake in comparison to that of the sediment-water PO4 

 exchange process obscures any relationship between photosynthesis and 

 ambient PO4 concentrations. The identification of the site of interaction 

 between cellular-P and photosynthesis is thus exceedingly difficult. 



One aspect of the PO4 uptake data leads us to speculate on the 

 reasons for the differences in our results between the short-term nutrient 

 bioassays and long term enrichment experiments. Rates of phosphate 

 uptake at ambient PO4 concentrations appear to occur at, or very near, 

 the Vmax for PO4 uptake, despite the demonstrated limitation of 

 phosphorus on carbon fixation. This observation, together with the 

 demonstrated ability of phytoplankton to undergo adaptive changes in the 

 ratio of the phosphate uptake V„ax to biomass (Stross and Pemrick 1973, 

 Chisholm 1974) suggests that algal cells may respond to continuing 

 phosphate limitation by reducing V„ax for phosphate uptake or 

 photosynthesis in a way analogous to light adaptation in photosynthesis. 

 By so doing the cell would maximize P uptake per unit of enzyme for P 

 uptake. This effect has been observed in the Barrow ponds where algae 

 required a period of adaptation to higher phosphate concentrations before 

 an increase in Pmax or photosynthesis was observed. If so, such a cell could 

 not show a photosynthetic response to phosphate addition until the Vmax 

 values had been increased by synthesis of additional enzymes for uptake. 

 Perhaps this is the reason for our negative short-term bioassay results. 

 Conversely, positive short-term bioassay results (Ryther and Guillard 

 1959) would only be seen in samples in which the PO4 concentration had 

 been reduced below levels permitting maximal rates of uptake shortly 

 before the bioassay had begun so that an adaptive reduction in Pn,ax had 

 not yet taken place. Two distinct types of nutrient limitation follow from 

 this hypothesis. The first occurs when the "instantaneous" measured Pmax 

 is at or below the maximum genetically set Pmax under long-term 

 saturating rates of supply of PO4. The second occurs when rates of PO4 

 uptake are at or below the "instantaneous" Pmax. The former would 

 correspond to Schindler's (1971) and O'Brien's (1972) nutrient control of 

 algal biomass, the latter to nutrient control of photosynthesis (at least in 

 terms of the time-scales over which a response can be detected). 



Grazing 



Daphnia, which comprises about 80% of the pond zooplankton 

 biomass, has a high potential filtering capacity and, therefore, may exert a 

 strong control on phytoplankton. Daphnia middendorffiana. the large 



