272 F. L. Bunnell et al. 



decomposer organisms (Table 8-1). Volumes in moss range from 0.35 

 mm' (gdw moss)"' on rims of low-centered polygons to 0.74 mm' (gdw 

 moss) ' in polygon troughs. Volumes in the soil range from 0.43 mm' 

 (gdw soil)"' on the rims of low-centered polygons to 1.02 mm' (gdw 

 soil)"' in the basins. Differences in bulk density modify the ranking of 

 algal biomass in soils. Although basins still contain the greatest amounts 

 (0.40 g m"^ in the upper 1 to 2 cm of soil), wet meadow soils contain the 

 least amount of algal cells, only 0.13 g m"^ and amounts in trough soils 

 are also low (Table 8-1). 



Differences with Depth 



The same broad relationships that influence the relative distribution 

 of microflora groups across microtopographic units influence the distri- 

 bution with depth. In marked contrast to temperate regions the presence 

 of permafrost in tundra soils accentuates the rate of environmental 

 change with depth (Chapters 2 and 7). 



Except in the better-aerated microtopographic units such as rims, 

 the eukaryotic organisms, yeasts and fungi, decline rapidly with depth 

 (Figure 8-4), a phenomenon also common in other arctic sites (Hanssen 

 and Goksc^yr 1975, Widden 1977). In soils of wet meadows seasonal 

 averages of yeast dilution counts in 1971 decline from 1.6x 10* colonies 

 (gdw soil)"' in the upper 2 cm to 9.7 x 10" colonies (gdw soil)"' in samples 

 collected from 2 to 7 cm depth. Because of increasing bulk density with 

 depth, total biomass declines less rapidly, from 0.011 g m"^ to 0.002 g 



Fungal density declines consistently with depth and the observed in- 

 crease in fungal biomass below 6 cm in polygon troughs (Figure 8-4) is 

 due to a sharp increase in bulk density from an organic to mineral hori- 

 zon (Laursen 1975). The decrease in fungal densities parallels the in- 

 crease in bulk density and the decrease in concentration of oxygen. 



In wet meadows, bacteria constitute about 40 to 55% of the decom- 

 poser biomass in the upper 2 cm, and increase their contribution with in- 

 creasing depth (Figure 8-4). Failure of bacteria to decrease with depth 

 indicates their facultative anaerobic nature. The decline of plateable 

 numbers with depth concomitant with an increase in direct counts 

 changes the seasonal average ratio of plate to direct counts from 1 :539 at 

 to 2 cm to 1:4866 at 2 to 7 cm and 1:33750 at 7 to 12 cm depth (Table 

 8-2). Direct microscopic measures invariably overestimate bacterial num- 

 bers, but ATP measurements indicate that most of the bacterial biomass 

 at depths of 2 to 12 cm is viable. The wider plate to direct count ratio 

 at depth may represent an increase in the strictly anaerobic bacterial 

 flora. The shift from fungal to bacterial dominance with depth produces 



