The Detritus-Based Trophic System 415 



low abundance. These results indicate that nematodes are relatively un- 

 important in the coastal tundra at Barrow compared with other 

 ecosystems. 



The northern Coastal Plain near Barrow is totally lacking in earth- 

 worms (Annelida: Lumbricidae), but is rich in smaller worms of the fam- 

 ily Enchytraeidae. These worms are largely aquatic, living in the soil in- 

 terstitial water, and are less abundant in the drier areas. Mean densities 

 ranged from 1 1 ,000 and 13,000 individuals m"^ on the well-drained poly- 

 gon top and rim to 94,000 worms m"^ in the moist polygonal trough. The 

 Spearmann ranic correlation of enchytraeid abundance and soil moisture 

 across the nine microtopographic units (Table 11-1) was positive and 

 significant (r = +0.75; p < 0.05). 



Biomass of Enchytraeidae was determined from the distribution of 

 body lengths of each species, using the geometric equations of Abraham- 

 son (1973). Biomass is dominated by a large species of the genus Mesen- 

 chytraeus, which had a mean individual biomass of 65 ^g dry wt, and 

 mean population biomass of 1500 mg m"^ in the polygon trough. The 

 biomass of this one species exceeded the sum of nematode, mite, and col- 

 lembolan biomass in this habitat, and the total enchytraeid biomass ex- 

 ceeded the sum of all other animal biomass in eight of the nine micro- 

 topographic units sampled. Mean enchytraeid biomass across all units 

 was 2100 mg dry wt m"'. Thus, Enchytraeidae achieve considerable 

 abundance and biomass in the coastal tundra at Barrow and strongly 

 dominate the soil fauna. 



Mean density of Acari ranged from less than 10,000 individuals m"^ 

 in the basins of low-centered polygons to 83, (XX) m"^ in the mesic mea- 

 dow. The rank correlation with moisture (r = -0.62; 0.10 >/? > 0.05) in- 

 dicated greatest density in drier areas. This trend occurred in all three of 

 the major mite suborders: Prostigmata, Mesostigmata, and Oribatei. 

 The Oribatei were particularly lacking from the basins of low-centered 

 polygons. 



In most ecosystems the Oribatei are the dominant group of Acari; 

 however, the small prostigmatid mites are relatively abundant in tundra 

 ecosystems (Behan 1978). Prostigmata comprised 16 of 37 species and 

 43% of all individuals in our samples (Table 11-2). The average indivi- 

 dual weighed only 1 ptg dry weight (Douce 1976); hence, the numerically 

 dominant Prostigmata contributed relatively little to biomass. The most 

 abundant oribatid species, Liochthonius scalaris Forsslund (= L. 

 sellnicki S. thor) is also very small, with adults weighing only 0.5 yig; 

 however, the mean weight of oribatid mites was 4.0 Mg- Overall, the Ori- 

 batei composed 48<^o of the density but 66% of the biomass of mites. The 

 predaceous Mesostigmata are the largest of the mites, averaging about 6.8 

 Mg, and thus made a much larger contribution to biomass (18%) than to 

 density (8%). Density and, especially, biomass of mites are generally low 



