ORTNER ET AL.: SARGASSO SEA ZOOPLANKTON BIOMASS DISTRIBUTION 



0-800 m biomass present in the upper 200 m were 

 42 and 49Vf which corresponds closely to our val- 

 ues (Table 3). Both of our results are virtually 

 identical with those obtained by Menzel and 

 Ryther ( 1961). From their table 1 we can calculate 

 the percentages of 0-500 m biomass and 0-1,000 m 

 biomass present above 200 m. Averaging the re- 

 sults, we find 449c of the 750-m biomass was pres- 

 ent during the day above 200 m. lashnov (1961) 

 presented data for the Sargasso Sea in which 90% 

 of the 0-1,000 m plankton was present above 200 

 m, but he used a relatively fine mesh net (0.180 

 mm). Unfortunately, Deevey and Brooks (1971) 

 characterized 500-m depth intervals with horizon- 

 tal tows at the midpoint of each interval to 2,000 

 m, while Grice and Hart (1962) sampled only the 

 upper 100-200 m. 



Several authors suggest that a vertical biomass 

 structure similar to our slope water and Sargasso 

 Sea observations is to be expected in temperate or 

 subtropical oceanic environments relatively free 

 of advective inputs. Vinogradov (1968: figure 47 

 and stations 3206 and 3829 in table 18) gave 

 examples of oceanic regions with such a distribu- 

 tion. Zenkevich and Birstein (1956) agreed that 

 zooplankton biomass in the North Pacific rather 

 steadily decreases from the surface downwards, 

 although the most marked reduction they discuss 

 might be below our lowest standard sampling 

 depth. The one very deep tow series we obtained in 

 a ring, however, gave no indication of such a re- 

 duction (Figure 2, MOC 31). 



Zooplankton biomass profiles obtained by 

 Murano et al. (1976) in the northwest Pacific 

 above the Sagami Trough exhibit the expected 

 decrease with depth. Reanalyzed in our manner, 

 the data of Marlowe and Miller ( 1975) for Station 

 P in the North Pacific support the above generali- 

 zation; the percentage of their 0-500 m biomass 

 found at night in the upper 200 m was 579^ . If one 

 extrapolates their 500-m values as approximately 

 applicable to the 500-800 m interval — a conserva- 

 tive approach for this argument — the resulting 

 percentage becomes 49*7^ (N). This is not unlike 

 our average slope water percentage of 5 1% ( N ) and 

 quite distinct from the average ring percentage of 

 33% (N) (Table 4). Station P is very different from 

 Ring-D in respect to its vertical biomass distribu- 

 tion. 



In slope water, the intermediate biomass peak 

 in the upper 200 m approximately coincides with 

 the depth of a nitrite maximum of the type discuss- 

 ed by Vaccaro and Ryther ( 1960). Our results and 



those of Marlowe and Miller (1975) appear to dif- 

 fer: they felt that the shallow nitrite peak of Sta- 

 tion P was avoided by zooplankton. Since the 

 levels of nitrite we have observed at the maximum 

 are only slightly lower than those reported by 

 Marlowe and Miller (0.2-0.5 fxg A-N-NOg/l versus 

 0.64 /Ltg A-N-NO2/I), our findings cast doubt on 

 their speculation that nitrite toxicity might have 

 been involved in the maintenance of the biomass 

 minima they observed. 



Explanations for Ring Biomass Structure 



Given the relatively high zooplankton biomass 

 of the slope water, it is clear why cold core rings 

 have a higher average zooplankton biomass than 

 the Sargasso Sea. Further, their higher average 

 primary productivity appears responsible for this 

 differential persisting 10-12 mo after ring forma- 

 tion. Our data suggest the decline in ring biomass 

 takes place rather slowly; the oldest rings sampled 

 (10-12 mo) had ring/Sargasso biomass ratios only 

 20% smaller than the same ratios in the newest 

 rings sampled (3.0 and 3.5 mo, Table 3). Although 

 physically and chemically intermediate between 

 slope water and Sargasso Sea, rings appear to be 

 unique in their vertical distribution of biomass. 



We offer two logically distinct explanations for 

 the small fraction of the 0-800 m biomass found 

 within the upper 200 m of a ring. They are not 

 mutually exclusive and the relative importance of 

 these explanations is species dependent. The sim- 

 pler argument stresses the importance of a physi- 

 cal factor — temperature. If a slope water animal 

 were physiologically restricted to a particular 

 temperature range, its habitat would descend as 

 the ring decayed and isotherms sank. To the ex- 

 tent that the zooplankton population in the slope 

 water exhibited this behavior, ring biomass dis- 

 tributions would deepen. This could apply only to a 

 species which in its home range — the slope wa- 

 ter — remains beneath the seasonal thermocline 

 (i.e., moderately deep-living and exhibiting li- 

 mited diel migration). Such a species would most 

 likely have to be either carnivorous or omnivor- 

 ous. Wiebe and Boyd ( 1978) have documented such 

 a phenomenon for the slope water euphausid 

 species, Nematoscelis megalops. 



A more complex explanation stresses the impor- 

 tance of a biological factor — food resources. The 

 kinds of changes that accompany ring decay must 

 have a substantial effect upon zooplankton- 

 phytoplankton interactions. Using unpublished 



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