Decomposers, Bacteria, and Microbenlhos 371 



fourth instar chironomid larvae or 700 Lepidurus (13-mm) per square 

 meter were added to cores. 



Our results (Table 8-14) show that the respiration of the microbes 

 actually decreased in four of five cores when chironomids were added. To 

 calculate this, the respiration of the animals has to be subtracted; as noted 

 earlier, these respiration data were measured on confined animals and may 

 not be correct. Also, the experiment run at 1 .9°C (Table 8-14) assumes 

 that the animal respiration was effectively zero. 



Lepidurus appears to stimulate respiration of the sediment and this 

 species does not eat detritus and microbes. However, the calculation does 

 depend upon a separate measure of the animals' respiration. It is possible 

 that the chironomids eat enough of the microbes to affect the results but 

 this is doubtful, as the concentrations of animals added approximated the 

 natural levels. In an additional test, Lepidurus also stimulated the release 

 of '^COi' from labeled barley straw which had been boiied to remove the 

 soluble compounds (Figure 8-9). These experiments indicate also that the 

 number of chironomid larvae is important as ten animals were twice as 

 effective as five animals. We have already described how the protozoans 

 increased microbial activity (Microbenthos section). 



The increase in microbial activity may be due to an increase in the 

 flux of oxygen into the sediment. A similar effect was found in Marion 

 Lake where Hargrave (1969) found that respiration decreased when the 

 oxygen fell below 8.6 mg Oi liter"' in cores. In contrast, Pamatmat 

 (1965) and Edwards and RoUey (1965) found that at low temperatures a 

 moderate stirring had no effect on the respiration rates. Here, it is possible 

 the activity of the Lepidurus in the top 0.5 cm of sediment might affect 

 oxygen diffusion; on a microscale the protozoans might have the same 

 effect on diffusion around particles. 



4000- 



■o 3000 

 «> 



w 



'5. 



in 



cr 



3 2000 



Q. 



1000 



Density Experimental Chamber 



FIGURE 8-9. Effect of inver- 

 tebrates on the decomposition 

 of barley as measured by 

 ^*COi release. Experiments 

 were run at 10°C for 24 hours 

 in 500-ml flasks with 60 \xm 

 fragments of barley straw. 



