FISHERY BULLETIN: VOL. 76, NO 2 



data obtained in August 1975 from 5 nine bottle 

 hydrocasts, the number of phytoplankton cells per 

 liter averaged 10,000 in the slope water, 2,500 in 

 the ring, and 2,000 in the Sargasso Sea. Cells 

 smaller than 4-5 /xm were not enumerated and 

 were, therefore, excluded from these computa- 

 tions. Values were integrated from to 200 m — a 

 conservative procedure tending to reduce slope 

 water versus ring or Sargasso Sea differences. The 

 species composition of the ring, while distinct, was 

 more like that of the Sargasso Sea than that of the 

 slope water. Again, considering the 0-200 m depth 

 interval, the number of different phytoplankton 

 species an animal would have encountered in a 

 liter of water would, on the average, have been 6.0 

 (slope water), 9.6 (ring), and 10.4 (Sargasso Sea). 

 Converting the mean cell volume of each species to 

 carbon (Strathmann 1967) and multiplying by the 

 number of individuals present, yielded values of 

 average phytoplankton carbon of 1,400, 200, and 

 140 ng C/1. Thus, to acquire the same ration of 

 food, a herbivore would have had to filter more 

 than five times more water in the ring than in the 

 slope water, and even more in the Sargasso Sea. In 

 addition, the evenness of species' carbon equiva- 

 lence was 0.46, 0.75, and 0.76. That is, the total 

 carbon per liter was more evenly distributed 

 among different species in the ring and the Sar- 

 gasso Sea than in the slope water. (Evenness 

 equals HIH„^^^ (Pielou 1966) where H is the 

 Shannon-Weaver diversity index computed upon 

 species carbon equivalence rather than abun- 

 dance and //max ^ ^ogj, S where S = number of 

 species.) This last result implies that a herbivore 

 capable of selecting by carbon content (i.e., parti- 

 cle size) would have found it less advantageous to 

 concentrate on a particular species in the Sargasso 

 Sea and the ring than in the slope water. 



These properties of the phytoplankton popula- 

 tion, i.e., species composition, carbon concentra- 

 tion, cell concentration, and cell carbon distri- 

 bution, have profound effects on a filter-feeding 

 herbivore's harvesting ability. We believe that 

 early in ring evolution herbivorous slope water 

 species are deleteriously affected and, therefore, 

 may be replaced by Sargasso Sea forms more 

 quickly than deeper living carnivorous or om- 

 nivorous slope water species. If we are correct, ring 

 biomass distribution may deepen in part because a 

 ring's 0-200 m biomass declines more rapidly than 

 does its 200-800 m biomass. 



Identification of some of the taxa in August 1975 

 samples, although limited, support the argument 



that in Ring-D epizooplanktonic herbivores were 

 replaced before epizooplanktonic carnivores or 

 omnivores. The species list of Ring-D thecosoma- 

 tous pteropods, a largely herbivorous group, was 

 quite similar to that of the surrounding Sargasso 

 Sea.^ Grice and Hart (1962) found that chaetog- 

 naths, a purely carnivorous group, were consider- 

 ably more abundant in the Sargasso Sea than they 

 are in slope water. In 6 nine-net fine-mesh tow 

 series (12.5 cm diameter, Clarke-Bumpus nets 

 with 67 ^tm mesh) taken in August, chaetognaths 

 were five to ten times more abundant in the sur- 

 rounding Sargasso Sea than they were in Ring-D. 

 Other epizooplanktonic carnivores, e.g., Stylo- 

 cheiron suhmii and S. abbreviatum, which are 

 routinely found in the Sargasso Sea were not 

 found in Ring-D August MOCNESS tows. 



Organic Flux to Deep Sea 



Rings may contribute a disproportionate frac- 

 tion of the utilizable organic material available to 

 the northern Sargasso deep sea. We feel this is 

 likely both because of their generally higher pro- 

 ductivity and because of their unique zooplankton 

 biomass distribution and the factors that have re- 

 sulted in that distribution. Ring zooplankton 

 biomass below 200 m, in that it exceeds Sargasso 

 Sea biomass and ultimately declines to a similar 

 level, contributes to this augmentation. Differen- 

 tial seasonal mixing processes could also increase 

 downward particulate flux. For example, in 

 November 1975 we observed that winter mixing 

 had proceeded further in Ring-D than in the sur- 

 rounding Sargasso Sea water column. Herbivor- 

 ous ring zooplankton (i.e., Sargasso forms) may 

 have been unable to fully capitalize upon the sud- 

 den opportunity afforded by the increased primary 

 production that accompanied the mixing (Table 7). 

 If so, a larger fraction of this enhanced phyto- 

 plankton production would sink into the aphotic 

 depths. Physical evidence obtained on two cruises 

 undertaken to study rings during the summer has 

 suggested to us that the seasonal thermocline may 

 often be less stable in rings than in the Sargasso 

 Sea. 



Finally, there is a possibility of enhanced con- 

 tribution of organic matter into the deep sea due to 

 a lower overall trophic efficiency within the upper 

 200 m of rings (and slope water). If we divide 



^John Wormuth, unpubl. data; cited with permission. 



332 



