QUANTIFYING SEASONAL PHYTOPLANKTON OSCILLATIONS IN THE GLOBAL OFFSHORE OCEAN

Monthly medians of phytoplankton pigment concentration in the Global D ata Set of the Coastal Zone Color Scanner and direct chlorophyll measu rements at the Japanese Antarctic Station SYOWA were used to study the global scale pattern of seasonal phytoplankton oscillations between 5 5 degrees N and 69 degrees S. Using periodic regression, sine and cosi ne amplitudes for the annual and the semiannual harmonic components we re estimated and the amplitudes expressed as polynomial functions of g eographical latitude. At latitudes >50 degrees, seasonal oscillations run in approximately opposite phases in the Northern and Southern Hemi spheres, with hemispheric summer maxima and winter minima. The same ph ase reversal is clearly seen between 10 degrees-40 degrees N and 10 de grees-40 degrees S, but the maxima are observed in hemispheric winter, and the minima in summer. Between 10 degrees N and 10 degrees S, with 2 maxima of solar radiation, 2 pigment maxima per year are observed, and the same 2 maxima presumably occur near 50 degrees N and 50 degree s S, where the phase of the oscillation abruptly changes. In spite of the intentionally simplistic character of the approach, the derived pa ttern retains and quantifies many features of seasonal phytoplankton c hanges at different latitudes known from the literature.