Decomposers, Bacteria, and Microbenthos 353 



mg algal C so it cannot support the chironomids unless detritus carbon is 

 consumed also. However, some selectivity has been found in the feeding of 

 chironomids (Kajak and Warda 1968) so they could do better than this for 

 microbial grazing. Even a 6-fold selectivity, however, would give only 

 about 1 g C m ^ yr"'. This value has been used in Table 8-6, but it is the 

 weakest section of our argument. 



Zooplankton grazing of total planktonic POC has been estimated at 

 approximately 1 g C m^ yr ' (Chapter 6). Particulate organic carbon 

 includes detritus, algae, and bacteria; since bacterial biomass is usually 

 significantly smaller than the algal and detrital components, zooplankton 

 predation of planktonic bacteria is insignificant relative to predation in the 

 sediments. However the absolute amount of zooplankton predation is 

 important for the planktonic bacteria, as Peterson et al. (1978) have shown 

 that D. middendorffiana will filter bacteria but at an efficiency only one- 

 third that for algae. In the ponds, all the bacteria are, therefore, removed 

 every 6 days or so. Predation by protozoans and micrometazoans in the 

 water column is insignificant compared to total predation since the 

 biomass of these organisms in the plankton is small. 



Evolved gas from the sediments was collected ten times during 1973 

 for chemical analysis. From these collections, methane production from 

 anaerobic metabolism was determined to be 1 1 mg CH4-C m "^ day " \ If 

 we assume that the 1971 production was the same as the 1973 production, 

 then methane production was 1.1 g CH4-C m ^^ yr"'. 



The sum of these values for respiration, biomass change, predation, 

 and methane production yielded 22.5 g C m~^ yr ' gross bacterial 

 production for Pond B in 1971. 



Apportionment of this total to the two input parameters, total uptake 

 of DOC and hydrolysis of detritus, is difficult since neither of these inputs 

 was measured. The first input, total DOC uptake, was estimated from 

 kinetic data to be about 3.6 g C m"^ yr~\ There may be two sources of 

 this labile DOC. The first is carbohydrates leached from macrophytes and 

 animal and algal secretion, estimated to be 9.5 g C m"^ yr ~' (Sediment 

 Respiration section). All of this is not available to the bacteria, however, 

 since some DOC is lost during meltwater runoff. A second source is labile 

 DOC formed as a result of hydrolytic decomposition of detritus. No 

 measurement or estimate was made of this source; by subtraction it may 

 be around 1 9 g C m ' ^ yr " ' . 



The value for gross bacterial production in Pond B in 1971 (22.5 g C 

 m^ yr ') is not necessarily representative of other ponds, or even the 

 same pond in different years, since populations, macrophyte input, 

 temperature, and other factors may vary. Further, this carbon budget is 

 based on bacterial production near the center of the pond, which may be 

 significantly lower than bacterial production in the littoral areas and 

 macrophyte beds where considerable decomposition of the first three age- 

 classes of detritus occurs (Decomposition section). 



