386 J. E. Hobbieetal. 



that is added and decomposed in the pond each year, a large amount of 

 roots of Carex and grass, perhaps as much as 33 g C m ^ is also 

 produced. Little is known of its decomposition, but again only about 10% 

 could be accumulating each year so most of this must be decomposed. 



After 8 weeks of decomposition, both green Carex leaves and 

 lemming feces lost almost all of their P and K while N, Mn, and Zn 

 remained in the same proportion to the total weight of the material. The 

 elements tied to the structural material, Mg, Ca, and Fe, increased. 



Another way of looking at microbial activity and decomposition is by 

 measuring the respiration of an entire system. This was investigated in 

 cores using 4 to 6 hour incubations. Most of the activity occurred in the 

 top 4 cm (77%) and a great deal (2.5 times higher than in the pond center) 

 in the macrophyte beds (presumably due to root respiration). Over a single 

 day, the peak of respiration lagged the peak of sediment temperature by a 

 few hours. Total annual respiration over the entire pond was 15 to 25 g C 

 m"^ over 3 years. Additional CO 2 could have been lost by transfer from 

 the sediment into (and through) the macrophyte roots. However, the rate 

 of CO 2 loss from the sediments measured in these core experiments was 

 only about 50% of the rate measured by the CO 2 evasion technique. 



The respiration rate of the sediments was directly related to 

 temperature, with an average Qio of 1.6. Other factors affected the 

 respiration too. For example, added phosphorus increased respiration by 

 70% even in these short-term incubations. Acetate additions also increased 

 respiration. When chironomids were added to the cores, the respiration 

 decreased slightly. Lepidurus appeared to stimulate respiration. However, 

 the errors in the respiration estimates for the animals themselves may have 

 affected these results. 



Microbenthos 



The sediments of the Barrow ponds contain a variety of protozoans 

 and metazoans (turbellarians, nematodes, ostracods, and harpacticoid 

 copepods). The species composition, relative abundance, and even 

 absolute numbers of this diverse community are very similar to those in 

 temperate ponds. Most of the organisms in these Alaska ponds are found 

 only in the top 2 cm because of anaerobic conditions deeper in the 

 sediment. 



In one pond, the small zooflagellates were numerically the most 

 important part of the microfauna (10'" to 10'' m~^). Micrometazoa were 

 present at 10^ to 10'' ml ~ ' but the biomass of both these groups was equal 

 (about 1 g wet weight m ~^). Ciliates were also abundant (about 10^'ml ') 

 but their biomass was low (20 to 40 mg wet weight m ~^). These numbers 

 are also typical of detrital and muddy sediments in temperate marine and 

 freshwater systems. There are two peaks of abundance of the ciliates, one 



