Decomposers, Bacteria, and Microbenthos 381 



it did not survive above 17°C. In contrast, the pond ciliates from the Arctic 

 survive to at least 25°C. 



The protozoans are versatile animals broadly adapted to a variety of 

 environmental conditions. For example, by the middle of June when only a 

 few centimeters of sediment have thawed and temperatures are at 2°C, the 

 protozoa fauna is fully developed. Some forms are known to have cysts but 

 it is not known how Paramecium can survive as it does not form cysts 

 (Kudo 1966). In any case, overwinter survival of the protozoans is high. 

 The low temperatures throughout the summer do slow their activities, but 

 even at the average summer temperatures at Barrow the protozoans will 

 have generation times between 15 and 200 hours (Fenchel 1968). Over the 

 summer, the smallest forms will have at most about 80 generations while 

 the largest forms will have at most only 6. By extrapolation of this 

 relationship, the larger animals will have a generation time of over a year; 

 this is the case for macrobenthic animals. 



The small metazoans of the microfauna, i.e., rotifers, tardigrades, 

 turbellarians, and gastrotrichs, are similar to the protozoans in that 

 numbers and species resemble those in temperate ponds. These metazoans 

 also have mechanisms for winter survival such as resting eggs or anabiotic 

 stages. 



Slightly larger organisms, which may have several generations a year 

 in temperate ponds (Gerlach and Schrage 1972, Muus 1967) have only one 

 generation in the tundra ponds. In the case of nematodes, some adults 

 apparently survive the winter and reproduce the following summer. 

 Harpacticoids likely overwinter as juveniles, as a mature or near-mature 

 form was seen in mid-June. Because only one developmental stage at a 

 time is present, these arctic animals exploit much less of their total range 

 of foods than temperate populations do. Unfortunately, we have not 

 identified many of the larger microfauna so can not tell to what extent this 

 limitation affects the diversity and community structure. 



The larger animals of the macrofauna are strongly affected by the 

 stresses of the 9-month freeze and the short summers. Their diversity is 

 drastically reduced and they often have special adaptations in their life 

 cycles or physiology. Some groups are completely eliminated and the 

 absence of competition and predation (e.g., no fish or amphibians) alters 

 the community structure. Thus, in both the flora and the fauna, the 

 smallest organisms are least adapted and affected by the arctic 

 environment while the largest organisms are the most adapted and show 

 the greatest community disruption. 



Control of Microfaunal Populations 



The number of protozoans in the ponds appears to be a function of 

 food availability, especially the bacteria and the benthic microalgae. Some 



