STA 5B3 



le MG /M,' 



5B 2 5B 1 



18 MG /M^ 12 





Figure 30. — Chlorophyll a profile and chlorophyll a-depth curves for a triplet of observations that closely cor- 

 responded in position to the first (B) triplet of hydrocasts at front 5. Dashed-line contours in the profile are 

 somewhat speculative, c = cool side; m = middle; w = warm side; • = Van Dom Sampler depths in pro- 

 file; units are mg./m.3 



nets sampled so little water (~70 m.3) per tow 

 that the differences probably are not reliable. 

 The C-B samplers in the thermocline caught 

 increasing amounts from the warm side to 

 the cool. These C-B samplers were towed at 

 three depths at one time on one wire; the one 

 below the thermocline failed in each haul. 



The horizontal surface hauls show increas- 

 ing volume towards the cool side, the ratios 

 (1.0:3.4:8.7) being significantly different. This 

 trend resembles that obtained using C-B 

 samplers in the thermocline and that shown by 

 surface productivity determinations. Each of 

 the horizontal surface hauls immediately fol- 

 lowed a nekton-net haul. 



I presume that micronekton are strong 

 swimmers and therefore are not mechanically 

 aggregated by the front. The zooplankton com- 

 ponent (mostly large euphausiids, about 3 cm. 

 long) of the nekton net hauls was much more 

 abundant in the middle and on the cool side 

 than on the warm side. The large euphausiids 

 were not caught by any nets other than the 

 1.52- by 1.52-m. nekton net, which samples to 

 a maximum depth of 115 m. (Blackburn and 

 associates, 1962). 



Aliquots of each sample were sorted, and 

 the numbers of the predominant groups (e.g.. 



copepods) were counted. The volumes of the 

 most abundant of these were measured by the 

 method of Yentsch and Hebard (1957). Table 3 

 shows the ratios of counts standardized to the 

 full sample and to lO-'m.^ of water strained. 

 These ratios, by groups, can be qualitatively 

 compared with those given, by sample volume, 

 m table 2. Again, the value for the warm side 

 is set at unity. 



Some of the ratios in table 3 are particularly 

 interesting and are summarized in table 4. 

 Besides these are the relatively high euphausiid 

 catches in the middle and on the cool side by 

 the nekton net, as mentioned above. Pteropods, 

 which were very abundant in the middle accord- 

 ing to the results of the A series oblique tows, 

 also were very abundant in the catch on the 

 cool side by the F series oblique tow, but were 

 absent from the hauls in the same (F) series 

 in the middle and on the warm side. Much more 

 sampling is needed to elucidate precisely the 

 effects of the front on animals. 



A factor that cannot be assessed is grazing 

 of the zooplankton by the micronekton. The 

 average grazing rate, for example, might be 

 less in the middle than it is on either side. If it 

 were, it could perhaps explain the relatively 

 greater amounts of copepods and euphausiids in 



37 



