Global trends in ocean phytoplankton: a new assessment using revised ocean colour data

Area of Science:

• Oceanography
• Remote Sensing
• Biogeochemistry

Background:

• NASA's global ocean color record, utilizing satellite sensors like SeaWiFS, MODIS-Aqua, and VIIRS, tracks ocean chlorophyll trends.
• Previous versions indicated a negative global chlorophyll trend, but revisions address sensor drift and algorithmic improvements.

Purpose of the Study:

• To analyze updated global ocean chlorophyll trends using an 18-year satellite data record (1998-2015).
• To assess the impact of revised data processing and sensor inclusion on trend detection.
• To investigate regional chlorophyll trends and phytoplankton composition shifts.

Main Methods:

• Utilized an 18-year time series (1998-2015) from three satellite sensors: SeaWiFS, MODIS-Aqua, and VIIRS.
• Applied radiometric drift correction and a new chlorophyll algorithm.
• Assimilated revised satellite data into a global ocean biogeochemical model.

Main Results:

• The revised 18-year record shows no significant global trend in annual median chlorophyll.
• Ocean biogeochemical model assimilation confirms the absence of a global trend, indicating robustness against processing changes.
• Significant regional declines in chlorophyll were observed in the North and Equatorial Indian Oceans.

Conclusions:

• Detecting global ocean chlorophyll trends is sensitive to satellite data processing and sensor calibration.
• Ocean biogeochemical model assimilation provides a more robust method for trend detection.
• A notable shift in phytoplankton composition, from larger diatoms/chlorophytes to smaller cyanobacteria, is occurring in the Indian Ocean basins.