Zooplankton abundance

Zooplankton abundance in the central Pacific was investigated on four cruises of the Hugh M. Smith in '1950 and 1951. Quantitative oblique hauls were made to 200 meters ' depth, employing.1-meter nets of 30xxx grit gauze. Composition of the collections was remarkably uniform and averaged by number 57 percent Copepoda, 12 percent Chaetognatha, 6 percent Tunicata, 5 percent Euphausiacea, 4 percent Siphonophora, and 4 percent Foraminifera. All analysis of variance of zooplankton volumes demonstrated significant differences between d'ay and night. hauls, between cruises, and among latitudes but not between longitudes. The greatest abundance, both by number and volume, of zooplankton' occurred in the region of the Equator. The rich zone, extending from about 6 ° N. to 5 ° S., latitude, supported populations three to four times as great as more nOltherly or southerly latitudes. The greatest concentrations were found n01th of the Equator, when related to a "convergence; " when no marked convergence existed the peak of abundance was displaced a few degrees southward. The abundance of zooplankton was correlated with inorganic phosphate, oxygen, tem-perature, ·and thermocline depth. These environmental factors are. influenced by upwelling associated with-the equatorial divergence, which replenishes the supply of nutrients in the euphotic zone and creates favorable conditions for the growth of plant and animal life. While the data presented do not give a measure of the rate of produc-tion, they do provide a useful index to the relative productivity of.different areas of the central Pacific