or filamentous fungi. The bacteria in this layer are taxonomically 

 similar to the surface bacteria. 



The buried organic layer is not continuous, but where present there 

 is a greater biomass than was observed in the mineral layer. Much of this 

 population consists of strictly anaerobic bacteria on the poorly drained 

 areas. There is little sulfate in tundra soil so sulfate reduction is not 

 significant in^ situ but methane production which results from organic 

 matter decomposition may be a significant drain on the energy and carbon 

 in the tundra. 



Annual production rates for the tundra ponds and the lakes studied 

 are very low compared to most temperate-zone water. Many of these small 

 ponds receive seasonal influxes of organic matter (clippings, f ecus , 

 soluble organic), especially during the spring runoff. During the summer, 

 thermal erosion of organic-rich permafrost also contributes to lake and 

 pond filling. Approximately 25% of the total seasonal lake production 

 occurs beneath the ice before and after the spring thaw and after freeze- 

 up. Production appears limited by a short growing season, low solar 

 intensity during the growing season and nutrient deficiencies, especially 

 for phosphorus . 



The most conspicuous feature of the consumer compartment of the eco- 

 system is the three- to five-year population cycles of large amplitude 

 in the brown lemming (Lemnus trimucroratus) , the dominant herbivore. 

 Numbers of lemmings vary from about 1 to 250 per ha. Insect herbivores 

 are uncommon and other vertebrate herbivores which may be important at 

 other tundra sites are here uncommon (other species of microtine rodent; 

 caribou; geese) or absent (ground squirrels). In the absence of other 

 significant herbivores, variation in lemmings density results directly in 

 variation in the grazing process. When lemmings are scarce, vegetation 

 accumulates as standing dead in the plant canopy. As the lemming popula- 

 tion builds in the winter and spring preceeding the high, much of the 

 vegetation is cut. Only a relatively small amount is consumed; most is 

 converted directly to litter. The litter may be redistributed during 

 spring melt-off, being removed from raised areas and concentrated in 

 lower areas, ponds, and polygon troughs. This also produces a pulse of 

 organic matter entering the saprovore-based food chain. 



When lemmings are abundant, snowy owls and Pomarine jaegers breed in 

 large numbers, and least weasels increase markedly in abundance. In 

 other years these predators are scarce or absent. Ermine and both Arctic 

 and red foxes are present, especially in lemming-high years, but are 

 never abundant. 



Plant litter, animal carcasses and feces form the input into saprovore- 

 based food chain. The most important saprovores are collembola, orbatid 

 mites, and diptera larvae, especially Tipulidae and Chironomidae. Tipu- 

 lidae reach densities of 400/m^ and biomass of greater than 0.5 g dry 

 weight/m . 



The soil saprovores are consumed by an array of carnivorous arthropods 

 (mites, beetles, hymenopterans , spiders, some diptera larvae). Both the 



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