Major Findings 



Carnivorous 

 C i 1 i a t e s 



U. 1 mg C 



Bac terivorous 

 Ciliates 

 0.2 mg C 



Zoof lagellates 

 35 mg C 



T 

 0.05 mg C 



T 



Bac teria 

 1500 mg C 



15 mg C 



■'-'0.5 mg C 



Algivorous 



Ciliates 



1 mg C 



0.15 mg C 



Algae 

 700 mg C 



MicromeCazoa 

 35 mg C 



FIGURE 1-6. Carbon flow through the 

 protozoans and micrometazoans of the 

 sediment of a tundra pond. Units are mg 

 C m'^ day~^ for the fluxes and mg C m'^ 

 for the standing crop. 



would have to be extraordinarily selective to meet their energy 

 requirements from bacteria and algae alone. 



The bacteria which break down detritus are (along with the benthic 

 algae) the base of another food chain of protozoans and micrometazoans 

 such as nematodes. The feeding rates and production of these animals have 

 not previously been studied in the field. Here, these animals grazed only 1 

 to 2% of the bacteria and algae per day (Figure 1-6). This seems small yet 

 represents 20% of the bacterial production and 5% of the algal production 

 each day; thus, the small animals may control the bacteria to some degree. 



The protozoans and bacteria interact in other ways as well. It has long 

 been known that decomposition proceeds faster, and bacteria are more 

 active, when grazing animals are present. One hypothesis has been that 

 nutrients were rapidly released by the grazers and that this release allowed 

 higher microbial activity. This hypothesis was tested in an experiment 

 which investigated the rate of cycling of phosphorus-32 in small flasks 

 containing Carex, bacteria, and one species of protozoa (Barsdate et al. 

 1974). When the protozoan was present, the bacterial biomass was lower, 

 the bacteria were more active, and phosphorus was taken up faster by the 

 bacteria (1 .67 vs. 0.25 pg P cell ~ ' hr " '), than when the protozoan was not 

 present. Yet, only a few percent of the phosphorus actually cycled through 

 the protozoans. Thus, direct release of phosphorus by the protozoans did 

 not affect the bacteria and the hypothesis was disproved. It is possible that 

 the bacteria are kept in a phase of rapid growth by the grazing and that it 

 is this rapid grovyth that is responsible for the faster decomposition. 



