Decomposers, Bacteria, and Microbenlhos 345 



TABLE 8-3 Net Growth Constants and Apparent Doubling Times for 

 Bacterial Populations of Tundra Ponds 



Pond 



Year 



Net growth 

 constant K 

 (day ^ 



Apparent doubling 

 time (days) 



Plankton 



Sediment 



J 



A 



1971 

 1973 



0.0514 

 0.0529 



13.5 

 13.1 



Average 



0.0478 



14.8 



melt the following June. The death of these bacteria plus the mechanical 

 damage to cells during freezing and thawing may release organic 

 compounds that then allow the bacterial pulse in the early spring. 



There is a close similarity of this seasonal pattern with earlier arctic 

 studies (Boyd and Boyd 1963, Morgan and Kalff 1972) and with studies in 

 alpine and temperate locations (Tilzer 1972, Francisco 1970, Romanenko 

 1971). 



Heterotrophic Activity 



For the other organisms in the ponds, the best measure of their effect 

 and importance is a production rate or perhaps a feeding rate. For 

 bacteria, however, this is not easy to measure as there are no good ways to 

 measure bacterial growth and they feed on a great number of substrates. 

 Heterotrophic bacteria do take up the simple compounds from solution 

 and the process occurs by means of specific transport systems. They 

 appear to be taking up a number of substrates simultaneously so that the 

 rate of uptake of a single compound (or a group of compounds) will be an 

 indicator of the activity of this general group of microbes. This is an 

 imperfect measure but the data from a variety of lakes, estuaries, and 

 oceans show that heterotrophic activity changes in step with primary 

 productivity over a range of 4 or 5 orders of magnitude. 



Heterotrophic activity was quantified by measuring the uptake 

 kinetics of '^C-labeled organic compounds. The basic method described by 

 Hobbie and Crawford (1969) was to incubate subsamples with four 

 different low concentrations of the substrate. After 1 to 2 hours the 

 samples were killed with H2SO4, the '^C02 collected onto phen- 



