384 J.E. Hobbieetal. 



may well be exerting a control on the rate of carbon flow within the entire 

 sediment ecosystem that is far greater than their biomass or predation 

 rates would indicate. 



SUMMARY 



Bacteria 



Bacteria are abundant in the water (10^ mP^) and in the sediment 

 (10^ ml ') of the ponds. In a square meter of pond (20 cm deep, 5 cm of 

 sediment) there are 2 to 4 mg C in the plankton bacteria and an average of 

 1000 to 3000 mg C in the sediment bacteria. The numbers peak as soon as 

 the ponds melt; in the case of the plankton this is likely caused by the 

 influx of soil bacteria from the surrounding tundra, while in the case of the 

 sediment bacteria this peak may be growth caused by the overwinter 

 release of nutrients from dead microbes and other organic matter. A rapid 

 decrease in numbers and then slow buildup takes place, with another peak 

 in late summer. 



The small molecules of the dissolved organic carbon are turned over 

 every 2 days in the water and every 5 hours in the sediment. If the total 

 amount of this DOC fraction is 120 ng C liter ', the total turnover is 3.6 g 

 C m"^ yr \ of which only 20% is planktonic. In the plankton, the 

 heterotrophic activity was positively correlated with the primary 

 productivity rate so it is likely that much of the DOC used by the 

 planktonic bacteria comes directly or indirectly from the algae. The 

 sediment uptake was too high to come from algal loss so most likely comes 

 from decomposition of particulate matter. 



Bacteria in the plankton were grazed by the zooplankton at a daily 

 rate of 15% while the sediment bacteria were grazed at a daily rate of 

 about 1.5% (1% by microbenthos, 0.5% by macrobenthos). Total biomass 

 removed was around 1 g C m~^ yr ' at a maximum. The respiration of 

 bacteria in the plankton was too low to measure but in the sediment the 

 bacteria respiration was 9.4 g C m^ yr' (all other biota was 4.3 g C). 

 The increase in biomass of the sediment bacteria was 9 g C m~^ yr~'. 

 Methane production accounted for another 1.1 g C m ^ The sum of these 

 changes and losses was a bacterial production rate of22.5gCm"^yr'. 



Bacteria die in great quantities during the winter freeze and this may 

 be the most important control on the biomass. Animals of the sediment 

 are unimportant controls of biomass but grazing by the zooplankton may 

 control the number of planktonic forms. 



Bacterial productivity is controlled by the rate of supply of small 

 molecular weight molecules. In the plankton, most of this usable material 

 comes from algal excretion while in the sediment it comes mostly from 



