380 J.E. Hobbieetal. 



turbellarians are predators (see Fenchel (1969) for references). In tundra 

 ponds, we conclude that the most important single food item of the 

 micrometazoans is epipelic algae, while the most important food of the 

 protozoans is the bacteria. If we assume that the food intake of the 

 micrometazoans is 20% of their biomass every 24 hours (at 12°C) and that 

 half of their food is algae, then they will consume 7 mg algal C m ^^ hr '. 



Overall, the microbenthic animals (protozoa and metazoans) have a 

 standing stock of about 100 mg C m^ during the summer. The different 

 elements of this fauna form intricate and complex food chains complete 

 with specialized herbivores and predators. The base of these food chains is 

 the bacteria and algae of the sediment.which are consumed at rates of 20 

 mg bacterial C m' (24 hr) ' and 8 mg algal Cm"' (24 hr)"\ If the 

 standing stock of bacteria is 1 500 mg C m " ■^ and that of algae is 700 mg C 

 m ^ these grazer food chains would appear to have a very broad base and 

 to only graze a small amount (1 to 2%) each day. However, both the algae 

 and bacteria have a large standing stock and a proportionately small daily 

 production of about 100 mg bacterial C m"' and 150 mg algal C m"^ The 

 microfauna thus consume 20% of the bacterial production and 5% of the 

 algal production each day. This may well be enough grazing to exert some 

 degree of control. 



Comparisons with Temperate Communities 



One characteristic of arctic freshwater communities is a low diversity 

 and the absence of whole groups of organisms found in temperate 

 freshwaters. This is certainly true of the macrofauna of these tundra ponds 

 that lack Hemiptera and have but a single beetle species. While the 

 microfauna has only been incompletely investigated, the evidence from the 

 relatively detailed study of the ciliates is that the diversity of the ciliate 

 fauna is in no way lower than in similar temperate ponds. Even the species, 

 or at least the morphologically defined species, are exactly the same as in 

 temperate, oligotrophic ponds. 



In addition, there are no apparent differences between the 

 physiological rates of the pond ciliates and those of temperate ones. 

 Tetrahymena pyriformis from the ponds had growth rates and ^lo's for 

 generation times quite comparable, between 5° and 25°C, to temperate 

 forms (Barsdate et al. 1974). At 25°C, the generation time was around 3 

 hours. In the same way, the ingestion rates of two species of ciliates from 

 these arctic ponds had a ^lo similar to that for temperate species in the 5° 

 to 25°C range (Fenchel 1975). These data imply that there is no 

 temperature compensation in the tundra pond species and, indeed, a 

 similar result was found in a marine ciliate isolated from antarctic pack ice 

 (Fenchel and Lee 1972). Its growth optimum was between 5° and 10°C and 



