Composition and Distribution of Macrobenthic Invertebrate Fauna 



185 



ranked third. Both mean density (range to 2/ 

 m 2 ) and mean hiomass (range 0.24 to 0.33 g/m 2 ) 

 were significantly lower in sand, sand-silt, and silt- 

 clay, each of which offered very limited attach- 

 ment potential. 



Samples in till substrates yielded the highest oc- 

 currence frequency of specimens (41%), shell and 

 gravel were about even, but considerably lower in over- 

 .tll frequency than till (17 and 14%, respectively; Table 

 20). Only 2 to 3% of samples in the other sediment 

 types provided brachiopod specimens. 



Relation to Water Temperature 



Brachiopods were quite restricted in their rela- 

 tion to the annual range in water temperature 

 and showed a very strong tendency of decreasing 

 abundance with increasing temperature range 

 (Tables 21, 23; Fig. 200). None were found where 

 the annual temperature range exceeded 11.9°C. 

 The\' were most plentiful (average density of 9/nr 

 and biomass of 1.93 g/m 2 ) in the narrowest (0- 

 3.9°C) temperature range and declined rapidly and 

 steadily as the temperature range broadened. 



The frequency of occurrence of brachiopods in 

 samples in the temperature range groupings was 

 low (10 to 2%) and followed the trend established 

 in the abundance measures (Table 25). 



Relation to Sediment Organic Carbon 

 Brachiopods preferred low levels of sediment or- 

 ganic carbon (Tables 26, 28; Fig. 201), being ab- 

 sent at levels above 1 .49% carbon content. Nu- 

 merical density was 3 to 5 times (range 16 to 3/ 

 m 2 ) higher where no measurable carbon was found 

 than where small amounts occurred. Biomass was 

 greatest (1.74 g/m 2 ) in the 0.5 to 0.99% carbon 

 content class versus a slighdy lower biomass (1.31 g/ 

 m ) in the 0% grouping. Significantly lower levels 

 occurred in the other carbon content groupings. 



Frequency of occurrence of brachiopods in 

 samples declined rapidly from a high of 40% in 

 the 0% organic carbon level grouping to only 6% 

 in the next (0.01-0.49%) grouping, then more slowly 

 to 2% in the 1.00 to 1.49%. grouping (Table 30). 



Echinodermata 



The phylum Echinodermata is represented, in the New 

 England region, bv members of five classes: Crinoidea, 

 Holothuroidea, Echinoidea, Ophiuroidea, and 

 Asteroidea. All but Crinoidea provide significant con- 

 tributions to the total benthic fauna. In terms of contri- 

 bution to overall density, members of Echinoidea are 

 second in dominance, providing 20% of the total num- 



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GEORGES SOUTHERN GEORGES SOUTHERN 

 BAN" NEW ENGLAND SLOPE NEW ENGLAND 

 SHELF SLOPE 



GEOGRAPHIC AREA 



Figure 197 



Density and biomass of Brachiopoda in each of the six geo- 

 graphic areas. 



ber of organisms in the region, and Holothuroidea 

 rank fourth with a 7% contribution (see Table 3). 

 Ophiuroidea and Asteroidea are subdominants, pro- 

 viding 1.8 and 1.2%, respectively, of the total number 

 of organisms. In terms of biomass, however, none of 

 the four rank among the top dominants, but 

 Ophiuroidea and Echinoidea are subdominants pro- 

 viding 2.9 and 1.9%-, respectively, of the total. Holo- 

 thuroidea and Asteroidea contributions to biomass, 

 0.3 and 0.1% respectively, are significantly below their 

 showing in terms of density (see Table 3). 



