Decomposers, Bacteria, and Microbenthos 379 



carnivorous 

 ciliotes 

 0.1 mg 



T 



bacterivorous 

 ciliotes 

 0.2 mg 



0.05 mg 



T 



zooflagellotes 

 35 mg 



~~r~ 



I5mg 



olgi vorous 

 ciliotes 

 I mg 



O.ISmg 



bacteria 



microptiytes 



FIGURE 8-16. Carbon flow through the most 

 important groups of the protozoans based on 

 samples from JO July 1973 and a temperature of 

 12°C. Units are mg C m~^ for the standing stock 

 (inside rectangles) and mg C m~^ day' for the 

 rates (arrows). (After Fenchel 1975.) 



assumes a temperature of 12°C at the sediment surface (during the period 

 10 to 14 July, the temperature varied from 6 to 14°C). The rates would 

 change with temperature, of course, but the general picture will be 

 unchanged. 



The values given in Figure 8-16 are probably underestimates both of 

 the biomass and of the rates. We feel, however, that better estimates would 

 not change the relative rates or biomasses. The zooflagellates consume 

 about half their body weight (expressed here as carbon) of bacteria every 

 24 hours, whereas the ciliates feeding on bacteria take up only one-quarter 

 of their weight. Yields (grams of protozoa produced per gram of bacteria) 

 of protozoans feeding on bacteria are quite high and values of 0.58 and 

 0.78 have been reported for Tetrahymena and Colpoda, respectively 

 (Curds and Cockburn 1968, Proper and Garver 1966). Thus, the pond data 

 agree with previous estimates of a 24-hour generation time for other 

 ciliates under optimum conditions (12°C) calculated by Fenchel (1968). 

 Ciliates that eat algae consume an amount equal to 15% of their biomass 

 every 24 hours, but this is likely an underestimate because many of the 

 algae are larger than the 100 ^g C assumed here. Also, many of the ciliates 

 grouped as algivores are also predators but the predation rate has not been 

 quantified. Despite this lack of precision, we conclude that ciliates are 

 responsible for only a modest amount of the grazing on algae. Overall, the 

 calculated feeding rates of protozoa on bacteria and algae are probably 

 correct within a factor of 10. 



Estimates of micrometazoan grazing rates are much less certain due 

 to a lack of knowledge of even the qualitative aspects of their feeding 

 biology. It is known that some rotifers, nematodes, chaetonotids, and 

 microcrustaceans feed on bacteria and that some rotifers, nematodes, and 



