354 J. E. Hobbie et al. 

 Controls of Bacterial Biomass 



The sediments of the tundra ponds are highly organic and, indeed, the 

 tundra is covered with plants and their remains. Why then are not the 

 bacteria, and other microbes, even more abundant than they are? In fact, 

 the abundance of bacteria in the water and the sediments is entirely typical 

 for temperate ponds in general. Unfortunately, we know little about 

 production of bacteria in any environment so comparisons are extremely 

 difficult. 



The biomass of bacteria in these ponds is likely controlled by the 

 complete freeze each year and predation by animals. As noted, bacteria 

 can become inactive and remain inactive for long periods of time. Thus, 

 the actual amount of biomass can be independent of the rate of 

 production. Despite this ability to survive, the bacteria of the sediments 

 were reduced in number over the winter (Figure 8-1). This was caused 

 presumably by freezing and by mechanical destruction by ice crystals. 



Three types of animals graze on bacteria: the specialized bacterivores, 

 the generalized detritus feeders, and the filter feeders. In the plankton, the 

 filter feeding zooplankton, especially Daphma, filter large amounts of 

 water each day; in fact, they filter the entire volume of the pond every two 

 days. Recent experiments (Peterson et al. 1978) have shown that D. 

 middendorffiana is about 30% efficient when it feeds on bacteria compared 

 with its feeding rate on larger particles. Thus, about 15% of the bacteria 

 could be removed each day. This is clearly enough predation to hold the 

 numbers fairly constant in the plankton and thus it appears that 

 zooplankton grazing controls the bacteria. 



The specialized bacterivores of the sediment are mostly flagellate and 

 ciliate protozoans. According to Fenchel (1975), the protozoans in the 

 ponds remove up to 20 mg C m^^ of bacterial carbon each day (this is 

 about 1% of the total amount present). This overall rate of predation is low 

 and does not appear to constitute any sort of control on the bacterial 

 biomass. However, the potential for control is there as the protozoans can 

 grow rapidly in response to increased numbers of bacteria. There is also a 

 good possibility that our scale of sampling is wrong for the heterogeneous 

 sediment and that micropockets of high numbers of protozoans are 

 present. If this were true, then grazing by protozoans would control the 

 bacterial numbers because of their selectivity and ability to move to 

 localized pockets of bacterial abundance. 



The chironomids, oligochaetes, and other detrital feeders of the 

 sediment appear to be somewhat selective but it is not known how well 

 they can select for bacteria. Assuming a 6-fold selectivity factor, the 

 calculations then give 1.0 g C m"^ yr \ or a rate about half that of the 

 protozoans. This additional sediment grazing does not change the 

 conclusions about the lack of control by sediment grazers. 



