

ZOOPLANKTON ABUNDANCE IN THE CENTRAL PACIFIC 



131 



EFFECTS OF THE EQUATORIAL CURRENT SYSTEM 



The general pattern of the Pacific Equatorial 

 Current system has been described by Sverdrup, 

 Johnson, and Fleming (1942, pp. 708-712). 

 Cromwell (1951 and unpublished ms. 3 ), on the 

 basis of recent investigations by this laboratory, 

 suggests certain modifications in the generally 

 accepted ideas pertaining to the details of the cir- 

 culation. The existence of the equatorial diver- 

 gence with resultant upwelling of nutrient-rich 

 water, as demonstrated by the Carnegie section of 

 1929 (Sverdrup, et al. 1942), is substantiated by 

 the researches of this laboratory. The presence 

 of a second major divergence at the northern 

 boundary of the Countereurrent has not been con- 

 firmed, however. Cromwell concludes, on the 

 basis of the information obtained on three cruises 

 (2, 5, and 8) of the Hugh M. Smith, that there is 

 no evidence that any enrichment of the surface 

 layer — as a result of upwelling accompanying a 

 divergence — is occurring at the boundary between 

 the North Equatorial Current and the Counter- 

 current. 



On cruise 2, a sharp temperature discontinuity, 

 together with a local deepening of the thermocline, 

 was found to occur at about 2° N. latitude on 

 the western section and at about 4%° X. latitude 

 on the eastern section (fig. 4). These conditions 

 indicate the presence of a convergence or "front" 

 (Cromwell ms.) in the surface currents. The 

 northern boundary of the South Equatorial Cur- 

 rent was discernible at - 5 1 / £° N. latitude on the 

 western section, but was not well defined on the 

 eastern section. The convergence near 2° N. lati- 

 tude was entirely within the South Equatorial 

 Current and, therefore, was not associated with 

 the boundary of this current and the Countereur- 

 rent as indicated by Sverdrup, et al. (1942, pp. 

 710, 711) and Arrhenius (1950). A well-defined 

 convergence, as demonstrated by a marked tem- 

 perature discontinuity and a deepening thermo- 

 cline, occurred at 4 1 /o° X. latitude on the eastern 

 section of cruise 5 (fig. 5) and again was entirely 

 within the South Equatorial Current. An exam- 

 ination of the data from the oceanographic cruises 

 (tables 6 to 8) indicates that the convergence was 

 more pronounced in certain sections than in others 

 and in some it was not strong enough to exhibit 

 the usual signs and may have been entirely lacking, 



'Circulation in a meridional plane in the central equatorial 

 Pacific. 



thus permitting the inference that it is shifting 

 and transitory in nature. 4 



Newly upwelled water is at first poorly popu- 

 lated as regards both phytoplankton and zoo- 

 plankton (Steemann Nielsen 1937). The latter 

 does not benefit directly from the nutrient-rich 

 water but must await the development of a phyto- 

 plankton population. In a region of fairly rapid 

 currents, the phytoplankton maximum and the 

 more slowly developing zooplankton maximum 

 would be expected to occur at a considerable dis- 

 tance from each other both in time and space. 

 Steemann Nielsen (1937) reports that in Iceland 

 waters, when conditions for phytoplankton change 

 from bad to good, the capacity for rapid reproduc- 

 tion in phytoplankton enables a few specimens to 

 establish a rich population in 10 days; large 

 quantities of zooplankton do not show up until 

 about a month later. We are not aware of similar 

 population-growth data for the tropics, but we 

 can safely assume that the rate of development 

 here would be considerably more rapid. Never- 

 theless, it is surprising that in our sampling we 

 found the zooplankton maximum frequently oc- 

 curring on or very near the site of the upwelling. 

 Since the days of Hensen and the German 

 Plankton Expedition of 1889. tropical seas have 

 I icon considered less productive than those of 

 higher latitudes. Explanations for this are 

 usual!; based on the theory that in the tropics 

 surface heating results in a stable stratification 

 of the sea, thus preventing any vertical mixing 

 by convection which would bring nutrients to the 

 euphotic zone (Delsman L939; Graham 1941; 

 Sverdrup, et al. 1942, p. 942; Arrhenius 1950). 

 Such thermal stratification does apparently exist 

 throughout much of the tropical, subtropical, and 

 temperate Pacific in areas removed from land 

 influence. 



It was learned from the last cruise of the Car- 

 negie in 1929, that these generally stable dynamic 

 conditions are disrupted at the Equator by a strong 

 divergence which is accompanied by upwelling. 

 The latter was considered responsible for the en- 

 richment of the surface layers and the production 

 of much higher concentrations of plankton than 



1 The Equatorial Current system is considerably more complex 

 than indicated here. Details of the circulation at the divergence 

 and the convergence and the causal forces involved, are presently 

 being studied by members of this organization and the Scripps 

 Institution of Oceanography. 



