Composition and Distribution of Macrobenthic Invertebrate Fauna 



225 



nental slope depths, and low densities (9-30/m 2 ) were 

 encountered at lower slope and upper rise depths. Mol- 

 lusca showed a similar pattern of density distribution 

 with depth; density was greatest (570/m 2 ) in shallow 

 water, moderate (136-205/m'-') at shelf depths, and 

 moderately low in deeper waters. However, mollusks 

 were numerically dominant at the deeper depths. Echi- 

 noderm density was greatest (133/ m 2 ) at mid-shelf 

 depths between 25 m and 49 m, moderate (87-95/m 2 ) 

 in deeper shelf water, and decreased with increasing 

 depth beyond 200 m. Moderately low densities (47/m 2 ) 

 occurred in shallow (0-24 m) water. 



Mollusca was the dominant fauna] component of 

 biomass at nearly all depths. Greatest biomass (258 g/ 

 in'-') occurred in shallow water (0-24 m) with values 

 moderately high (132-21 g/m 2 ) at shelf depths and 

 diminishing rapidly at depths below 200 m. Echino- 

 derm biomass was greatest (167-106 g/m 2 ) in waters of 

 49 m and less, decreased to 34 g/m 2 at outer shelf 

 depths, but dominated at depths beyond 200 m (1.7-19 

 g/m 2 ). Annelida biomass was highest (27 g/m 2 ) in 

 shallow (0-24 m) water and nearly equal (25 g/m 2 ) in 

 water depths between 50 m and 99 m. Values ranged 

 between 15 g/m 2 and 16 g/m 2 at other continental 

 shelf depths but decreased rapidly with increasing depth 

 beyond 200 in. Crustacea, although numerically domi- 

 nant, ranked fourth in biomass at nearly all depths 

 except the shallowest one (0-24 m) where a value of 37 

 g/m 2 placed them third. Biomass of crustaceans ranged 

 from 2 to 16 g/m 2 at continental shelf depths (25-199 

 m), with a rapid decrease from 4 g/m 2 at 200-499 m to 

 approximately 0.1 g/m 2 in slope and rise waters. 



Relation to Bottom Sediments 



Numerical abundance of the four dominant faunal com- 

 ponents in relation to the six major sediment types 

 encountered in the New England region did not ex- 

 hibit any trend as dramatic as that for depth (Tables 16, 

 18; Fig. 17). Annelids seemed to prefer sand (558/m 2 ), 

 gravel (505/m 2 ), and shell (443/m 2 ) bottoms but were 

 moderately abundant in till, sand-silt, and silt-clay bot- 

 toms as well. Mollusks were generally more abundant in 

 silt-clay (354/m 2 ), shell (229/m 2 ), and sand-silt (276/ 

 m 2 ) but were found in somewhat lower abundance in 

 other sediments also. Crustacea were most abundant in 

 sand (1,336/m 2 ), gravel (710/m 2 ), and sand-silt (275/ 

 m 2 ) and were found in diminishing amounts in shell 

 (124/m 2 ), till (59/m 2 ), and silt-clay (34/m 2 ). Sand-silt 

 (104/m 2 ) and sand (95/m 2 ) contained the most echi- 

 noderms, followed by till and silt-clay (67/m 2 and 65/m 2 , 

 respectively), then shell (28/m 2 ) and gravel (23/m 2 ). 



Density rank order in the various sediments listed by 

 decreasing particle size was as follows: gravel: Crusta- 



cea, Annelida, Mollusca, Echinodermata; till: Annelida, 

 Mollusca, Echinodermata, Crustacea; shell: Annelida, 

 Mollusca, Crustacea, Echinodermata; sand: Crustacea, 

 Annelida, Mollusca, Echinodermata; sand-silt: Annelida, 

 Mollusca and Crustacea equal, Echinodermata; silt-clay: 

 Mollusca, Annelida, Echinodermata, Crustacea. 



The distribution of the biomass of the major taxa 

 among the various sediment types was fairly even. The 

 annelids showed the greatest uniformity with the small- 

 est biomass (11 g/m 2 ) in till and largest (26 g/m 2 ) in 

 sand-silt. Biomass ranged from 15 to 16 g/m 2 in the 

 four other types. Mollusks showed some variability, with 

 shell bottoms containing the largest biomass (168 g/ 

 m 2 ) and till the smallest (6 g/m 2 ). Molluscan biomass 

 in gravel was 94 g/m 2 , in sand 121 g/m 2 , in sand-silt 74 

 g/m 2 , and in silt-clay 18 g/m 2 . Crustacean biomass was 

 20 g/m 2 in gravel and 12 g/m 2 in sand; 7 g/m 2 and 6 g/ 

 ur in sand-silt and shell, respectively, and 2 g/m 2 and 

 0.6 g/m 2 in till and silt-clay .respectively. Echinoderm 

 biomass was greatest in sand (88 g/m 2 ), 43 g/m 2 in silt- 

 clay, and 37 g/m 2 in sand-silt. Median amounts oc- 

 curred in till (15 g/m 2 ) and lower amounts in gravel (6 

 g/m 2 ) and shell (3 g/m 2 ). Biomass rank order in bot- 

 tom sediments was as follows: gravel: Mollusca, Crusta- 

 cea, Annelida, Echinodermata; till: Echinodermata, 

 Annelida, Mollusca, Crustacea; shell: Mollusca, Anne- 

 lida, Crustacea, Echinodermata; sand: Mollusca, Echi- 

 nodermata, Annelida, Crustacea; sand-silt: Mollusca, 

 Echinodermata, Annelida, Crustacea; and silt-clay: Echi- 

 nodermata, Mollusca, Annelida, Crustacea. 



Relation to Water Temperature 



Among the four dominant taxa there were no clear-cut 

 trends discernible with regard to the annual range in 

 bottom water temperature (Tables 21, 23; Fig. 18). 

 Where ranges of temperatures were between 8 and 

 19.9°C, Crustacea was the numerically dominant taxon, 

 with densities ranging from 768 to 1 ,475 individuals per 

 m , whereas annelids dominated in areas exhibiting 

 rather stable annual temperature regimes, between 

 and 7.9°C (212-513 individuals per m 2 ), and in areas 

 experiencing the broadest temperature range of 20- 

 23.9°C, where mean densities of 1,698 individuals per 

 m 2 were found. Densities of Mollusca and Echinoder- 

 mata were fairly consistent at moderate levels (84-345/ 

 m 2 for Mollusca and 21-171/m 2 for Echinodermata) 

 throughout the temperature range spectrum. Mollusca, 

 however, did make a strong showing (1,242 individu- 

 als/m 2 ) where the range in annual temperature was 

 broadest. 



Rank order of dominance for the major taxa in the 

 six annual temperature range classes in terms of den- 

 sity was as follows: 0-3. 9°C: Annelida, Crustacea, Echi- 



