Composition and Distribution of Macrobenthic Invertebrate Fauna 



213 



(6 to 32 individuals/m ) were encountered on 

 the continental shelf; low densities (0.1 to 2.7/ 

 m ) occurred on the continental slope and 

 continental rise. 



Biomass of ascidians, also, was larger on the 

 continental shelf than in deep water beyond 

 the shelf (Table 13; Fig. 239). Average biomass 

 at water depths less than 200 m ranged be- 

 tween 1.0 and 8.9 g/m 2 , whereas at depths 

 greater than 200 m the biomass averaged less 

 than 0.7 g/m 2 . 



Ascidians were present in the samples from 

 nearly all depth classes at about the same low 

 range of 5 to 24% (Table 15). 



Relation to Sediments 



Ascidians occurred in all six types of bottom sedi- 

 ments; however, their density and biomass varied 

 substantially from one type to another (Table 16; 

 Fig. 240). Densities were relatively high (29 and 

 22/m 2 ) on gravel and sand bottoms, and inter- 

 mediate (11/m ) in sand-silt. Densities were low 

 (5.1 or less/irr) in till, shell, and silt-clay. 



Biomass of ascidians followed the same trend 

 as density, but the differences were less pro- 

 nounced (Table 18; Fig. 240). Average biomass 

 was largest (9.7 to 4.4 g/m 2 ) on gravel and 

 sand, intermediate in sand-silt, and small (<2 

 g/m 2 ) in the remaining sediment types. 



Frequency of occurrence in the samples was 

 moderately low (11 to 32%) in all types of 

 bottom sediments (Table 20). In the majority 

 of sediment categories, the small differences in 

 occurrence rate are correlated directly with as- 

 cidian density and biomass. Shell and till val- 

 ues, which are based on a small number of 

 samples, are incongruous. 



Relation to Water Temperature 

 Although the trends of ascidian density and 

 biomass relative to the annual range in bottom 

 water temperature were not consistent, gener- 

 ally the quantities were larger where the tem- 

 perature range was moderate, and smaller at 

 both temperature extremes (Tables 21, 23; Fig. 

 241). In areas where the temperature range 

 was moderate, from 8° to 19.9°C, the average 

 density of ascidians ranged from 6 to 59/m 2 

 and the biomass ranged from 7 to 14 g/m 2 . 

 Where water temperature ranges were extreme, less 

 than 8°C and more than 20°C, the average density was 

 low (2-4/m 2 ) and the biomass was small (0.5-4.1 g/ 

 m 2 ). Two temperature range classes (8°-lI.9°C and 

 16°-19.9°C) contained significantly greater quantities 

 than did the other classes. 



I — I NUMBER 

 ■ WEIGHT 



Ik 



_Q^ 



UJ o 



NOVA GULF OF GEORGES SOUTHERN GEORGES SOUTHERN 



SCOTIA MAINE BANK NEW ENGLAND SLOPE NEW ENGLAND 



SHELF SLOPE 



GEOGRAPHIC AREA 



Figure 238 

 Density and biomass of Ascidiacea in each of the six geographic 

 areas. 



Frequency of occurrence of ascidians was moderately 

 low in all temperature range classes. Their occurrence 

 rate, however, was somewhat high (17 to 24%.) where 

 the temperature range was moderate, and lower (11 to 

 12%) where the temperature range was very small and 

 very large (Table 25). 



