EDDY-INDUCED OSCILLATIONS OF THE PYCNOCLINE AFFECT THE FLORISTIC COMPOSITION AND DEPTH DISTRIBUTION OF PHYTOPLANKTON IN THE SUBTROPICAL PACIFIC

Changes in the phytoplankton population caused by a wind-induced mesos cale eddy, first located off the Hawaiian chain on 29 August 1989, wer e examined from 6 to 12 September 1989. These cyclonic eddies produce upwelling of nutrient-rich water into the photic zone which may induce changes in the distribution of phytoplankton. The eddy affected the d epth, magnitude and composition of the chlorophyll maximum. The eddy p roduced an upward shift in the distribution of phytoplankton, shoaling the chlorophyll maximum to 30 m. Pigment concentrations at the chloro phyll maximum were enriched up to 2.2 times inside the eddy. Outside t he eddy, the upper mixed layer (UML, less-than-or-equal-to 50 m) was d ominated by cyanobacteria whereas the deep chlorophyll maximum (DCM) w as dominated by prochlorophytes, chlorophytes and chrysophytes. Inside the eddy, the UML showed an increase in deep water taxa but little ch ange in cyanobacteria. The observed population changes probably result ed from increased growth rates and vertical transport of cells. Based on projections of the track of the eddy, we inferred that its trail cr eated an area of enhanced chlorophyll that was not quickly consumed, a nd concluded that coupling between phytoplankton and grazers was weak. The presence of such cyclonic eddies may explain some of the variabil ity in chlorophyll concentration found in the oligotrophic subtropical Pacific.