Composition and Distribution of Macrobenthic Invertebrate Fauna 



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PERCENT ORGANIC CARBON 



Figure 44 

 Density and biomass of Alcyonaria in relation to sediment organic carbon. 



Frequency of occurrence was moderate under all 

 temperature range conditions. Occurrence rates varied 

 from 14 to 30% (Table 25). They were higher where 

 the temperature range was moderate, and lower in the 

 extreme (lowest and highest) range conditions. 



Relation to Sediment Organic Carbon 



Zoantharians occurred where sediment organic carbon 

 content was from 0.01 to 3%; they were absent where 

 carbon content was or above 3% (Fig. 50). Density 

 values showed a negative trend of decreasing quantity 

 with increasing carbon content (Tables 26, 27; Fig. 50) 

 with mean number of individuals ranging from 3 to 7/m 2 . 



Biomass values exhibited a somewhat similar trend 

 with the exception that highest biomass was not in the 

 lowest carbon content class in which they occurred 

 (0.01-0.49%). Moderately high biomass occurred in 

 the two classes between 0.5 and 1.5% with significantly 

 lower levels above and below these values. 



Frequency of occurrence of samples in the carbon 

 content classes was fairly uniform, ranging from 15 to 

 29%, with no discernible trend as evidenced by density 

 and biomass measures (Table 30). 



Platyhelminthes 



Turbellaria — Tubellarians are free-living members of 

 the phylum Platyhelminthes. They accounted for a very 



small portion of the total New England benthic 

 macrofauna. In terms of biomass and numbers of indi- 

 viduals they accounted for <0.1%, of the total fauna 

 (Table 3). They are small in size and those large enough 

 to be retained on a 1-mm mesh sieving screen were present 

 in very low density. The vast majority of marine tubellarians 

 reported from New England marine waters are less than a 

 few millimeters in length. Specimens in our samples ranged 

 in size from 2 mm to nearly 2 cm in length. 



Members of this class of flatworms are free-living, 

 soft-bodied forms and their shape varies from species to 

 species, commonly ovoid and dorsoventrally flattened. 



Turbellaria occurred in 16 samples (2% of total). 

 Their density averaged 0.4/m , and their biomass aver- 

 aged 0.01 g/m 2 (Table 5). 



Geographic Distribution 



The few Turbellaria that were present in our samples 

 were relatively more common in the Southern New 

 England Shelf area than in any other section (Fig. 51). 

 Average densities as high as 59 individuals/m 2 were 

 detected in the vicinity of Nantucket Shoals. Elsewhere 

 densities averaged 9/m or less. Members of this group 

 were absent from large portions of the Nova Scotia 

 Shelf, Gulf of Maine, Georges Bank, and the entire 

 Southern New England Slope area. Average biomasses 

 in all localities were 0.3 g/m''- or less. 



The density of turbellarians in each geographic area 

 (Tables 6, 7; Fig. 52) averaged less than 1/trr. The 



