Phytoplankton-zooplankton relationships in the Western Pacific Ocean and adjacent seas

Regressions of biomass and daily food requirements of herbivorous zooplankton on daily primary production were calculated, using assumptions based on data collected in various sea areas of the western Pacific Ocean and adjacent seas. A regression coefficient (1.470) of calculated herbivorous biomass on observed daily primary production is significantly higher than unity (P<0.01). This indicates that the herbivorous biomass sustained by unit amount of primary production is large in the more productive high latitudes, and small in the less productive tropical sea areas. This is attributed to relatively larger food requirements per unit biomass of the tropical herbivores as compared with those found in cold waters. Despite distinct areal differences in the herbivorous biomass-primary production ratios, the calculated daily food requirement of herbivores was in direct proportion to the daily primary production, when equilibrium had been established between phytoplankton and zooplankton. Under conditions of limited food supplies, the small body size of the tropical herbivores may be advantageous both in reducing the total energy consumption per individual, and in inducing rapid growth and reproduction. Therefore, the low ratio of biomass to primary production in the tropics could beregarded as a result of possible regulation of tropical herbivores to scarce food conditions rather than as evidence of failure of adaptation to such conditions.