Composition and Distribution of Macrobenthic Invertebrate Fauna 



169 



from one type to another (Tables 16, 17; Fig. 175). 

 Density was exceptionally high (1,238 tndividuals/m 2 ) 

 in sand and contributed over 56% of the total number 

 of animals in this type of sediment. Density was moder- 

 ately high in gravel; intermediate in sand-silt, till, and 

 shell; and relatively low (23/irr) in silt-clay. 



Biomass trends were the same as those for density (Table 

 18; Fig. 175). Sand sediments contained the largest bio- 

 mass, an average of 7.7 g/m 2 . Intermediate quantities (0.5 

 to 3.4 g/m 2 ) were present in gravel, till, shell, and sand- 

 silt. The lowest biomass (0.18 g/m 2 ) was found in silt-clay. 



The occurrence of amphipods in different types of 

 sediments ranged from very high to moderate. It was 

 high to very high (93 to 100%) in sand, shell, and gravel 

 types; intermediate in till and sand-silt; and moderate 

 (51%) in silt-clay (Table 20). 



Relation to Water Temperature 



Amphipods occupied the entire spectrum of tempera- 

 ture range variations occurring in the study area (Table 

 21; Fig. 176). Highest average density (1,372 individu- 

 als/m 2 ) was encountered where the temperature was 

 intermediate (8°-11.9°C). Densities were moderately 

 high (598-809/m 2 ) in areas where the range in tem- 

 perature was broad, but were substantially lower (58- 

 31 1/irr) where the temperature range was nil or small. 



Amphipod biomass in relation to temperature range 

 paralleled the same trends as those revealed by numeri- 

 cal density (Table 23; Fig. 176). Greatest biomass (8.1 

 g/m L ) occurred in an intermediate temperature range 

 class (8°-11.9°C). Moderate values occurred in adja- 

 cent broader and narrower range classes. The smallest 

 (0.41 g/m ) biomass was found where the temperature 

 range was less than 4°C. 



The frequency of amphipods in our samples was 

 moderate to high in all temperature range classes (Table 

 25). The highest rates of occurrence (92 to 97%) were 

 encountered where the temperature range was moder- 

 ate. Somewhat lower incidence rates occurred where 

 the ranges in temperature were slightly narrower and 

 slightly broader. Lowest occurrence rates prevailed in 

 those areas where the temperature range was lowest 

 (less than 4°C) and highest (more than 20°C). 



Relation to Sediment Organic Carbon 



Amphipods occurred in all sediments containing or- 

 ganic carbon (Table 26; Fig. 177). Two abundance 

 peaks were clearly evident. Densities were very high 

 (between 1,000 and 1,256 individuals/in-') in both low 

 (0.01-0.49%) and high (3.00-4.99%,) concentrations 

 of organic carbon and were much lower (24 to 164/m 2 ) 

 where organic carbon content was between these ex- 



