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Photosynthetic production of organic matter by microscopic oceanic phytoplankton fuels ocean ecosystems and contributes roughly half of the Earth's net primary production. For 13 years, the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) mission provided the first consistent, synoptic observations of global ocean ecosystems. Changes in the surface chlorophyll concentration, the primary biological property retrieved from SeaWiFS, have traditionally been used as a metric for phytoplankton abundance and its distribution largely reflects patterns in vertical nutrient transport. On regional to global scales, chlorophyll concentrations covary with sea surface temperature (SST) because SST changes reflect light and nutrient conditions. However, the oceanmay be too complex to be well characterized using a single index such as the chlorophyll concentration. A semi-analytical bio-optical algorithm is used to help interpret regional to global SeaWiFS chlorophyll observations from using three independent, well-validated ocean color data products; the chlorophyll a concentration, absorption by CDM and particulate backscattering. First, we show that observed long-term, global-scale trends in standard chlorophyll retrievals are likely compromised by coincident changes in CDM. Second, we partition the chlorophyll signal into a component due to phytoplankton biomass changes and a component caused by physiological adjustments in intracellular chlorophyll concentrations to changes in mixed layer light levels. We show that biomass changes dominate chlorophyll signals for the high latitude seas and where persistent vertical upwelling is known to occur, while physiological processes dominate chlorophyll variability over much of the tropical and subtropical oceans. The SeaWiFS data set demonstrates complexity in the interpretation of changes in regional to global phytoplankton distributions and illustrates limitations for the assessment of phytoplankton dynamics using chlorophyll retrievals alone.

A hydrographic survey of the central Arabian Sea was conducted in May 1986 and stations were correlated with Nimbus-7/Coastal Zone Color Scanner (CZCS) overpasses. As calibration of the CZCS degraded severely during its lifetime, pigment concentrations from samples at four sites in the Arabian Sea were used to refine the calibration of the CZCS. This vicarious calibration technique provided a final estimate of the sensor's sensitivity loss for sensor Gain 2. Calibration using normalized water-leaving radiance for the 520 nm and 550 nm bands showed considerable final sensor degradation. Future missions must incorporate calibration validation programs in order to maintain derived product accuracy requirements. Profiles of pigment concentration, salinity, temperature, and light attenuation are used to describe the bio-optical characteristics of the Arabian Sea during the intermonsoon period.