ZOOPLAXKTON OF CENTRAL PACIFIC 



381 



2 



8 40 

 O 



o 



24 



9 16 



(A) 



1 — I — 1 — r 



1 — I — I — r 



!/ 



II I II NT 



I I lo — o NIGHT SAMPLES 



I I J0--—0DAY SAMPLES 



I /°\ I 1° o TWILIGHT SAMPLES 



>•■;<> 



-I— - 



o- ,^ 



J \ UJ LL 



■>, 



I 



I 



— I:- 



Mil \ L 



NEC 



2° 0° 2° 4° 6° 

 -LATITUDE— N 



FiGiRE 16. — Variations with the current system in (A) 

 average volumes of night, day, and twilight hauls and 

 in (B) the ratio of night to day zooplankton volumes. 



of moderate to deep thermocline, has a high ratio. 

 We must conclude, therefore, that neither thermo- 

 chne depth in itself nor the nig;ht/day ratio appears 

 to he related to the general level of zooplankton 

 abundance. Both high and low ratios are found 

 in areas of poor zooplankton catch. We must 

 leave this problem for the present without an 

 explanation. 



SHORT-TERM VARIATIONS 



Two cruises of the Hiiijh M. Smith (cruise 11 

 and 15) crossing the ecjuatorial currents on 150° 

 and 140° W. longitude provide information on 

 temporal changes in zooplankton volume and 

 distribution as related to changes in the physical 

 environment. 



On cruise 11 in August-October 1951, the north- 

 bound leg (stations 28-50) was worked immedi- 

 ately after the southbound leg (stations 1-28). 

 During the time interval (approximately 6 days) 

 between crossings of tlic Erjuator, the wind (SE.) 

 decreased from ai)out 20 knots to about 12 knots. 

 As indicated by tlie change in positions of the 80° F. 

 isoth(>i-m (fig. 17), the zone of mixing nt the 



Equator, i. e., the zone of cool, newly upwelled 

 water, shifted to the south and narrowed in width 

 during the 6-day interval. On the first leg the 

 zooplankton maximum occurred at 1° X. latitude; 

 on the second leg it occurred at 0° with a second 

 peak of almost equal abundance at 2° S. latitude. 

 These changes would seem to be evidence that 

 during this 6-day period there was a shift in 

 zooplankton distribution correlated with changes 

 in zonal How. 



In the region of the Countercurrent during 

 cruise 11, there was little change in winds within 

 the interval (about 32 days) between sections, but 

 there was a marked increase in rate of flow, as 

 indicated by the broadening of the Counter- 

 current and steepening of the thermocline. These 

 changes in the current were accompanied by a 

 significant change in the zooplankton distribution 

 (fig. 17). At the time of the first crossing there 

 was little variation among stations within the 

 Countercurrent; at the second crossing, following 

 an increase in the current velocity from 45 to 80 

 cm. /sec, there was a marked gradient in zooplank- 

 ton concentration with the larger volumes being 

 taken in the area of shallow thermocline at the 

 northern boundary of the Countercurrent. 



Additional information on time changes in the 

 environment and the distribution of zooplankton 

 along a particular meridian was obtained during 

 May 195.3 on Hugh M. Smith cruise 15 when 4 

 consecutive hydrographic and plankton sections 

 were completed along 140° W. longitude with 

 sampling from 9° X. to 7° S. latitude. The time 



Fici-RE 17. — Soutli and northbound sections of Hugh .U. 

 Smith crui.se 11 in August -October Hlol, showing associ- 

 ated changes in zooplankton distribution (adjusted 

 volumes) and temperature along 150° W. longitude. 

 ITemperature sections adapted from .Austin 1954a. ] 



