The Detritus-Based Trophic System 419 



occurred in the top 2.5 cm, and over 98*^0 in the top 5 cm of the soil. 

 Sixty-six percent of the total Enchytraeidae occurred in the top 2.5 cm, 

 and 86% in the top 5 cm. The Nematoda showed the greatest tendency to 

 occur at depth. Approximately 85% of the free-living and predaceous 

 and 55% of the plant-parasitic nematodes were found in the top 5 cm. 

 The occurrence of plant-parasitic nematodes below 5 cm seems related to 

 the presence of dicotyledonous plants. Plant-parasitic nematodes were 

 the only animals to occur in significant numbers in the mineral soil 

 beneath the peat. Their small size may allow them to exist within the 

 small pores of mineral soil. 



Depth distribution of Enchytraeidae showed a marked seasonal pat- 

 tern (Figure 11-2); worms were concentrated near the surface at snow- 

 melt, but moved to deeper layers by mid-season. After the first sample of 

 the season, the three dominant enchytraeid species were segregated by 

 depth. Individuals of the smallest species, Cernosvitoviella atrata, were 

 found even below 15 cm in the tundra and, when sampling was con- 

 cluded in late August, had shown no tendency to return to the surface. 

 Over 50% of the populations of Mesenchytraeus sp. and Henlea perpu- 

 silla were found in the top 2.5 cm in all but the late July sampling. 



Concentration of animals in the surface layer was greatest in the 

 basins of low-centered polygons where well over 90% of the animal bio- 

 mass, including 99% of the Collembola, 97% of the mites, and 89% of 

 the Enchytraeidae, occurred in the top 2.5 cm. C. atrata, the deep- 

 dwelling enchytraeid species, was entirely absent. 



Although soil invertebrate densities are conventionally presented as 

 number per unit area, the number per unit volume more accurately 



1 — ' — \ r- 



Henlea perpusilla 



Cernosvifoviella atrata 

 • •^ •- 



J 1 I L. 



10 20 



Jul 



10 20 



Aug 



FIGURE 11-2. The seasonal variation in depth distribu- 

 tion (percentage of the population in the top 2.5 cm) of 

 the three dominant species of Enchytraeidae. 



I 



