Meridional and seasonal variations in the satellite‐sensed fraction of euphotic zone chlorophyll

The key to estimating ocean primary production from satellite color images is to project accurately the total euphotic zone chlorophyll content from the remotely sensed signal, emitted from merely the near-surface reaches. In situ profiles of chlorophyll fluorescence and solar irradiance (and temperature), collected at different times of the phytoplankton growth season with a towed undulator along a section running from the Azores toward Greenland, are used for simulating satellite measurements. Given by the simulations, the satellite-sensed fraction of the euphotic zone chlorophyll content accounts for only 7.5% on average, and it varies meridionally and seasonally by several times of its mean value. The major variations are attributed to changes in the chlorophyll vertical distribution, which are related to the succession of growth phases during the phytoplankton seasonal cycle: the onset of the spring bloom, the development of the bloom to full spate, and the transition to oligotrophy. To improve satellite-based estimates of integral euphotic zone chlorophyll quantities, it is suggested that use be made of the sea surface temperature field as a predictor of the time-dependent meridional transitions between the different growth phases.

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