Composition and Distribution of Macrobenthic Invertebrate Fauna 



87 



to 1.5/m 2 ) occurred at depths greater than 

 500 m. 



The relation between average biomass and 

 water depth was very similar to that for nu- 

 merical density, but the range in values was 

 much more limited. Largest average biom- 

 ass was 1 g/m 2 in the 25-49 m depth class. 

 At depths greater than 500 m the biomass 

 was very small, averaging 0.12 g/m 2 or less 

 (Tables 13, 14; Fig. 59). 



Frequency of occurrence of nemertines 

 in samples from the eight depth classes 

 ranged from 18 to 51% (Table 15). Higher 

 rates of occurrence were most prevalent in 

 the shallow-water classes, and low occur- 

 rence was typical of deepwater classes. 



Relation to Sediment*, 



Although nemertines were present in .ill 

 types of sediments, they were common (13 

 to 28 individuals/m 2 ) in only two types: shell 

 and sand (Tables 16, 17; Fig. 60). Densities 

 of about 3 to 5 individuals/m 2 occurred in 

 gravel, sand-silt, and silt-clay. Lower density 

 (0.9/ in 2 ) was found in till. Biomass of nem- 

 ertines in shell bottoms was moderately large 

 in absolute terms (6 g/m 2 ), but in relative 

 terms it was exceptionally large. In all other 

 sediments types the average biomass was 

 0.83 g/m L or less. An unusually small quan- 

 tity (0.06 g/m 2 ) was present in till substrates 

 (Tables 18, 19; Fig. 60). 



The percentage of samples within each 

 sediment type in which nemertines occurred 

 was in close agreement with the quantity 

 present. Samples from shell bottoms had 

 the highest incidence of nemertines (50%), 

 till the lowest (14%); their incidence in 

 other sediment types was intermediate (28 

 to 47%) (Table 20). 



Relation to Water Temperature 

 The average density of nemertines ranged 

 from 2.4 to 25.3 individuals/m 2 throughout 

 the entire temperature range of the study 

 area (Tables 21, 22; Fig. 61). The highest 

 density occurred in areas with an interme- 

 diate annual temperature range of 12° to 

 15.9°C. In areas where the range was either 

 greater or less than this, the densities were 

 substantially lower. Where the range was 

 greater (16° to 23.9°C) density values de- 

 creased to 2.9/m 2 . Where the range nar- 

 rowed from 11.9° to 0°C, the values de- 

 creased from 9.0 to 2.4/m 2 . 



