Diel and Interannual Variation of Size Distribution of Oceanic Zooplanktonic Biomass

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Abstract
A negative temperature anomaly with an increase in primary production affected the size distribution of zooplanktonic biomass in the central gyre of the North Pacific Ocean, a very large, old, oligotrophic, and highly diverse ecosystem with little seasonal change. Previous reports showed no effect of this anomaly on the species structure of the macrozooplanktonic community. Composite zooplankton samples from the euphotic zone for five cruises corresponding to the normal or steady state (two summers, two winters, and one spring) and one corresponding to the perturbation or anomaly (summer 1969) were size fractionated by filtration through a column with 4, 2, 1, 0.5, 0.3, and 0.183 mm mesh, after manual removal of organisms larger than °8 mm. Total biomass was higher during summer than during winter. Also, the size distributions of biomass were more similar between summers or winters of different years than between different seasons. Vertical migration of zooplankton causes the important scale of variability; the size—differential migration (larger size classes migrate more than smaller ones) has a seasonal pattern: during winter, biomass of all sizes classes increases nocturnally in the euphotic zone; during summer, the nocturnal increase of biomass is due almost exclusively to the largest size classes. The effect of the summer climatic anomaly were: (1) increase of total biomasss; (2) elimination of diel variability in size distribution typical of summer steady state through enhanced migration of small animals, the resulting distribution being similar to the winter steady state with migration of all size categories; (3) increase in the dominance of the 0.5—1.0 mm class. There is some evidence of a modification of age structure rather than an increase of population of small—sized species. We interpret the changes as the summation of the wave of perturbation moving up the biomass spectrum as a result of its input at the smallest size classes of the spectrum, the primary producers, plus an effect on migratory behavior of zooplankton.

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