Composition and Distribution of Macrobenthic Invertebrate Fauna 



129 



Biomass of bivalves, as with density, was greatest 

 (252 g/m 2 ) in shallow water and decreased sharply with 

 increasing water depth (Table 13; Fig. 119). On the 

 continental rise their biomass was only 0.4 g/m . 



Bivalves were present in a high percentage of the 

 samples from all depth classes. There was, however, a 

 slightly higher rate (81 to 100%) of occurrence in deep 

 water (greater than 500 m) than in shallow water, where 

 the occurrence rate was 75 to 88% (Table 15). Other 

 characteristics of the bivalve fauna in deep water are 

 lower average density, lower maximum density, smaller 

 average biomass, and smaller maximum biomass. 



Relation to Sediments 



Several clear trends were detected in the correlation 

 between the quantity of bivalves and the type of bottom 

 sediments they inhabited (Table 16; Fig. 120). Density 

 was highest (330/m 2 ) in sediments composed of silt- 

 clay and decreased as particle size increased, except in 

 shell bottoms where the density of bivalves was moder- 

 ately high (180/m 2 ). Gravels contained the lowest den- 

 sity (39/m 2 ). 



An entirely different trend was observed for bivalve 

 biomass (Table 18; Fig. 120). The largest quantities 

 (1 17 to 165 g/m 2 ) were found in sediments of medium 

 grain size. Smallest quantities (5 to 18 g/m 2 ) occurred 

 in till and silt-clay. It should be noted that in four of the 

 six sediment types, the biomass of bivalves accounted 

 for over 42% of the total benthic biomass (Table 19). In 



shelly sediments they formed the exceptionally large 

 proportion of 74% of the total fauna. 



Bivalves were present in a high proportion of the 

 samples in all sediment types. They occurred in all 

 samples from shelly bottoms and in a particularly high 

 percentage (91 to 92%) of the samples from fine-grained 

 sediments (Table 20). Only a moderately high propor- 

 tion of the samples from sand and gravel contained live 

 specimens of bivalves. 



Relation to Water Temperature 



Bivalves occurred in significant quantities in all of the 

 temperature range classes (Tables 21, 23; Fig. 121). 

 Although the density and biomass exhibited a general 

 tendency of increasing as the temperature range broad- 

 ened, two major anomalies in this trend were observed 

 in the two highest range intervals. The 16°-19.9°C class 

 had a large biomass (334 g/m 2 ) and a moderate (252 

 individuals/m 2 ) density, indicating the presence of 

 larger individuals than in other areas. Conversely, the 

 20°-23.9°C class had a high density (1,195 individuals/ 

 m 2 ) but a small biomass (84 g/m 2 ). 



Bivalve biomass was unusually large (106 to 334 g/m ) 

 where the temperature range was moderate (8°-19.9°C) 

 and was comparatively smaller in the low and high 

 ranges (6.8 and 84 g/m , respectively). 



Frequency of occurrence of bivalves was high and 

 rather uniform (75 to 93%) among all temperature 

 range classes (Table 25). 



Relation to Sediment Organic Carbon 



Bivalves were found in significant quantities in all or- 

 ganic carbon content classes except the highest 

 (5.00+%). There was a well-defined positive correlation 

 of increasing density with increasing organic carbon 

 content (Table 26; Fig. 123). Density of bivalves rose 

 from moderate levels (64/m 2 ) in the absence of or- 

 ganic carbon to high levels (1,120/m ) where organic 

 carbon content was between 3 and 5%. Bivalvia is the 

 only taxonomic group showing such a well-defined trend 

 in relation to sediment organic carbon. This trend cor- 

 responds to that shown for depth distribution, wherein 

 higher bivalve densities occurred in the shallower es- 

 tuarine and embayment waters that contained the high- 

 est levels of organic carbon. 



Although the highest biomasses of bivalves (227 to 

 801/m 2 ) were found in the higher carbon content 

 classes (3.00-4.99%, and 2.00-2.99%, respectively) the 

 relationship was not as well-defined as that for density 

 (Table 28; Fig. 122). Moderately high biomass (128/ 

 m 2 ) also occurred in low carbon levels (0.01-0.49%). 

 The other carbon content classes contained significantly 

 lower biomasses. 



Frequency of occurrence of bivalves in samples in the 

 various organic carbon content classes was quite high 



