FISHERY BULLETIN: VOL. 76, NO. 2 



Average Vertical Structure 



Comparing Ring-D biomass partitioned accord- 

 ing to depth, the upper 200 m in the ring con- 

 tained, on the average, less biomass during both 

 sampling periods than did the Sargasso Sea (Table 

 2, top). This was true both day and night during 

 the August and November cruises. In contrast, 

 ring biomass between 200 and 800 m was higher 

 both day and night (Figures 1, 2). The range of 

 200-800 m biomass values in the ring and in the 

 Sargasso Sea does not even overlap. The combina- 

 tion of lower average surface biomass and higher 

 average subsurface biomass in the ring is highly 

 significant (Sign test, P<0.01, computed using 

 sums of 0-200 m and 200-800 m cm3/l,000 m^ de- 

 rived from Figures 1 and 2). The regional weighted 

 averages of percent 0-800 m biomass present in 

 the upper 200 m in August were Sl'/f , 347c , and 

 2T7c in the Sargasso Sea, slope water, and ring, 

 respectively. In November these averages were 

 457c, 327c, and 257^ (Table 4). Although very dif- 

 ferent sampling systems and tow strategies were 

 employed, data from Atlantis II cruise 71 corrobo- 

 rate the direction of difference of these observa- 

 tions in that the percentages of 0-800 m biomass 

 found at night in the upper 300 m were 64% and 

 52% for the Sargasso Sea and ring, respectively 

 (Table 5). In addition, the 300-800 m biomass was 

 1.73 times larger in this latter ring than in the 

 surrounding Sargasso Sea. 



Diel Migration 



Complicating these general observations and 

 contributing to sample variability are day/night 

 differences in biomass distributions (Table 4). In 



Table 5.- 



Area 



-Ring and Sargasso Sea zooplankton biomass- 

 Atlantis II cruise 71 (mg/m^). 



0-300 m 



0-800 m 



0-300 

 0-800 



100 



■Ring 



Sargasso Sea 



52% 



64% 



all day/night sample pairs the fraction of 0-800 m 

 biomass present in the 0-200 m interval is larger 

 in the night sample (Sign test, P <0.01). This re- 

 sults from either diel migration or day/night dif- 

 ferences in avoidance within the comparatively 

 well-illuminated surface layers. Avoidance does 

 not appear to be an important factor because at 

 some stations the day 0-800 m biomass exceeds the 

 night 0-800 m biomass. This is true in all Sargasso 

 Sea 0-800 m sample pairs and at one slope water 

 station (Figures 1, 2). Furthermore, some species 

 of zooplankton taxa already enumerated, e.g., 

 euphausiids and pteropods, exhibit strong diel 

 migration patterns in all three areas. 



Since we believe diel migration to be the ap- 

 propriate explanation, the data further suggest 

 that while essentially the same percentage of 

 0-800 m biomass was migrating into the surface 

 layers of the Sargasso Sea (24-30% during both 

 sampling periods), there was a reduced percentage 

 migrating in the ring in November ( 2 1% in August 

 versus 9% in November — Table 4). Although a 

 smaller proportion of the biomass may have been 

 migrating in the ring relative to the Sargasso Sea, 

 there was a significantly greater (Mann- Whitney 

 t/-test, P<0.05) day/night biomass ratio in the 



Table 4. — Percent of 0-800 m slope water, ring, and Sargasso Sea zooplankton biomass in the upper 200 m 



(800 m tows only). D = Day; N = Night. 



'On this tow series, MOC 18 and 19, salps were extremely dominant. These tows are excluded from averages. 



328 



