Quantifying seasonal phytoplankton oscillations in the global offshore ocean

Abstract

Monthly medians of phytoplankton pigment concentration in the Global Data Set of the Coastal Zone Color Scanner and direct chlorophyll measurements at the Japanese Antartic Station SYOWA were used to study the global scale pattern of seasonal phytoplankton oscillations between 55°N and 69°S. Using periodic regression, sine and cosine amplitudes for the annual and the semiannual harmonic components were estimated and the amplitudes expressed as polynomial functions of geographical latitude. At latitudes > 50°, seasonal oscillations run in approximately opposite phases in the Northern and Southern Hemispheres, with hemispheric summer maxima and winter minima. The same phase reversal is clearly seen between 10°-40°N and 10°-40°S, but the maxima are observed in hemispheric winter, and the minima in summer. Between 10°N and 10°S, with 2 maxima of solar radiation, 2 pigment maxima per year are observed, and the same 2 maxima presumably occur near 50°N and 50°S, where the phase of the oscillation abruptly changes. In spite of the intentionally simplistic character of the approach, the derived pattern retains and quantifies many features of seasonal phytoplankton changes at different latitudes known from the literature.