Primary Producers 187 



A Total Biomass 



B Sum of 4 groups 



C 3/i. Flagellates 



D 6/i Flagellates 



E Rhodomonos minute 



F Ctiromulina 



0830 



0240 0530 



1720 2030 

 Time 



FIGURE 5-3. Phytoplankton biomass of a pond measured 

 at different times on 24-25 July 1972. 



grazed by zooplankton. Finally, there is the possibility that the very small 

 forms are migrating within the water column or moving back and forth 

 from the sediment. Among the dominant forms given in Figure 5-3, the 

 Rhodomonas and the Chromulina are well known as planktonic forms and 

 their biomass shows little diurnal change. In contrast, the small flagellates 

 (3 and 6 Mm) are rarely important in lake plankton and do show great 

 diurnal changes. We think it most likely that the small forms move back 

 and forth from the algal-rich sediment. As will be presented later, the 

 algae of the sediments are about 60 times more abundant (per milliliter) 

 than the planktonic algae so there is certainly a large reservoir of algae. 

 These small flagellates could move by themselves or could be mixed into 

 the water column by wind currents. This mixing by wind currents has been 

 shown to be very important for bacteria in these same ponds. Hobbie and 

 Rublee (1975) found a 3-fold increase in bacterial numbers over several 

 hours, so it is reasonable that small flagellates would follow the same 

 pattern. 



The seasonal pattern of early Chrysophyte and later Cryptophyte 

 dominance is typical of these ponds in general but different patterns were 

 seen in a few ponds. Several transects of 16 ponds each were sampled 

 during the study; one pond with diatom dominance and one with a large 

 biomass of the blue-green alga Microcystis were found. We do not know 

 why these differences exist. However, in Pond E a change in dominance 

 occurred (Figure 5-4) after crude oil was added in midsummer 1970. Here 



