Composition and Distribution of Macrobenthic Invertebrate Fauna 



73 



Biomass trends were the same as those described 

 above for numerical density, with the exception of un- 

 usually large quantities between 50 and 100 meters 

 (Tables 13, 14; Fig. 35). In the shallow zone (0-24 m) 

 the average weight was 1.2 g/m 2 . In deeper water, at a 

 depth of 1.000 m and greater, the average quantity 

 decreased to <0.01 g/m 2 . The extraordinarily large quan- 

 tity between 50 and 100 m resulted from relatively few 

 very large colonies. 



The frequency of occurrence of hydroids in samples 

 was highest (21%) in the shallow water zone (0-24 m), 

 dropping to 3% at depths below 1,000 m (Table 15). 



Relation to Bottom Sediments 



Hvdrozoans occurred on all sediment types found in 

 the region; however, they were much more prevalent 

 on gravels than on other types of substrates (Tables 16, 

 18; Fig. 36). Both numerical density and weight dimin- 

 ished with decreased sediment particle size. Average 



density dropped from 20 colonies/m 2 in gravel to 2 

 colonies/m 2 in silt-clay. Wet weight declined from an 

 average of 2.6 g/m 2 in gravel to 0.01 g/m 2 in silt-clay. 

 Intermediate quantities were present on substrates com- 

 posed of medium-sized particles. 



Frequency of occurrence also reflected the general trend 

 of higher densities on coarse substrates and low densities 

 in fine-grained substrates (Table 20). In coarse sediments 

 the percentage of samples containing hydroids ranged 

 from 23 to 33%. The percentage occurrence in sand, an 

 intermediate grade, was 14%. In the fine-grained sedi- 

 ments the frequency of occurrence was only 4 to 6%. 



Hvdrozoans constituted only a small part (1.4%) of 

 the total benthic fauna, even in the sediment type 

 (gravel) in which they were most abundant (Table 19). 



Relation to Water- Temperature 



The density of hydroids increased as the annual range in 



bottom water temperature broadened. Average density 



