382 J.E. Hobbieetal. 



of the food chains have already been described. One piece of evidence is 

 that the populations of ciliates and zooflagellates have peaks close to the 

 two peaks in bacterial numbers (Figures 8-1, 8-10, 8-1 1). Also, the higher 

 numbers of ciliates along the shore than in the center of the pond 

 correspond to the higher rate of production of bacteria in this zone where 

 the fresh leaf detritus is abundant. Other evidence for the close link 

 between population size and food availability comes from the changes in 

 the food preference of the ciliate community over the summer (Figure 8- 

 13). Bacteria feeders dominate early and late in the summer in response to 

 the two peaks of bacteria. Algal feeders make up more than 50% of the 

 ciliates in late June and early July, a period coinciding with the maximum 

 epipelic algal productivity. 



The significance of predation in controlling the microfauna is difficult 

 to assess. Undoubtedly most predation on this group is by other 

 microfauna; the best data are about the ciliates. In the ponds at Barrow, 

 the carnivorous ciliates mostly preyed on other ciliates, although ciliates 

 are also known to feed on zooflagellates and even small metazoans. As we 

 might expect with predators, the carnivorous forms constitute only a few 

 percent of the total ciliate fauna in early summer, but they are most 

 abundant in late July when they make up 15 to 20% of the total (Figure 8- 

 13). In mid-July their biomass is about 0.1 mg C m "'. If their food needs 

 are similar to those of the bacterivorous ciliates, then they will daily ingest 

 about 30% of their body weight or about 3% of the total fauna. They could, 

 therefore, be responsible for a part of the decline in ciliate numbers in the 

 middle of the summer. 



Metazoans also consume microfauna but we could not quantify this 

 predation at all. Certainly rotifers, nematodes, and turbellarians do feed 

 on protozoa and other micrometazoans. There are only slightly more data 

 on the feeding of macrofauna. In total, the chironomids and oligochaetes 

 consuiAe about 1 00 mg Cm May ' ' . The amount of food that is made up 

 of detritus, of algae, and of bacteria is not known. Assuming that the 

 detritus is not eaten and that the bacteria, algae, and microfauna are eaten 

 in the same percentage as their biomass (15:7:1), then some 2% of the 

 microfauna would be consumed each day. If the macrofauna are able to 

 select their food at all, then this percentage may be even higher. 



Feedback of Grazing on Bacterial and Algal Production 



It is now well known that microbial activity is higher when grazers are 

 present than when they are absent. For example, Fenchel (1970) and 

 Hargrave (1970) showed that detritivores increased the productivity of 

 sediment algae and bacteria. Fenchel (1977) also found that the 

 decomposition of plant material by bacteria was much faster when a 



