Macrobenlhos 319 



The bacteria and fungi of the sediments are another source of food in 

 the sediment. The carbon contained in each is about twice the algal carbon 

 so it is difficult to believe that microbial biomass could be the sole food of 

 the chironomids. There have been no data in other studies to suggest 

 selective feeding for a relatively large, generalized detritivore such as these 

 larvae although it is likely that smaller animals such as nematodes do 

 select microbes. The only other study in which bacterial biomass was 

 measured at the same time as feeding rates and energy requirements was 

 that of Cammen (1978) for a marine worm, Nereis. The worm population 

 needed to assimilate 8% of all the organic carbon in the sediment it 

 ingested (per year) but the living portion of the organic carbon was only 

 3%. Thus, Cammen's results agree with ours. However, other studies 

 where microbial biomass was not measured seemed to conclude that only 

 living cells are used (Fenchel 1970, George 1964, Newell 1965, Hargrave 

 1970). It is also possible that microbes produce a great deal of extra- 

 cellular polysaccharide and that it is this, rather than the microbial 

 biomass, that is important to detritivores (Hobbie and Lee in press). 



We conclude that the chironomid larvae must be consuming detritus 

 as well as algae, fungi, and bacteria. This conclusion cannot be proven and 

 the whole question has not been solved in any aquatic system. 



Production of Chironomidae 



Bierle estimated total production of the midge fauna in Ponds J, B, 

 and D with larval and adult population data from 1972 (Table 7-3). The 

 production was the sum of the increase in larval standing crop over the 

 season, the biomass of emerging adults, the estimated biomass lost 

 through mortality, and approximations of weight losses as exuviae at 

 molts and as mucus secretions (drawn from the literature). Adult biomass 

 was estimated as the number of emerging midges times the mean weight of 

 a mixed assemblage. The data were adjusted for size by dividing adults 

 into large (Chironomus, Derotanypus, and Procladius) and small 

 (remaining species) size classes and using separate conversion factors. 



TABLE 7-3 Production of Chironomidae in 1972 in Three Ponds 



