Plankton is a general term for marine or aquatic organisms that can only swim very weakly, if at all. In addition to creatures like jellyfish, plankton includes hundreds of species of tiny, sometimes microscopic, organisms that can be classified into zooplankton (which eat other plankton) and phytoplankton (which photosynthesize like plants). Phytoplankton form the center of marine food webs, making their population changes critical to entire marine ecosystems.

Understanding phytoplankton distribution, climate impacts, and population dynamics is essential for marine biology, oceanography, and fisheries management. These can be studied from orbit with detailed observations of ocean color. NASA's 2025 PACE mission studies phytoplankton from space using daily data from its Ocean Color Instrument (OCI), which detects more 100 light wavelengths across the visual, infrared, and ultraviolet spectra. Researchers can use machine learning to match these wavelengths to spectral libraries, identifying specific phytoplankton species by their unique light absorption patterns.

While PACE provides unprecedented detail on phytoplankton communities, NASA's MODIS instruments have mapped global marine chlorophyll — the main pigment used for photosynthesis, and a useful proxy for phytoplankton activity — since 2002 using blue-green ocean ratios. MODIS offers broader productivity estimates and valuable long-term data to detect trends, complementing PACE's detailed species breakdown. 

The Visible Infrared Imaging Radiometer Suite (VIIRS) also collects detailed visible and infrared imagery of the world's ocean and land surfaces. VIIRS was designed for compatibility with existing data records from MODIS, ensuring the continuity of this long-running observational record after MODIS is retired in 2026 or 2027.

These phytoplankton studies reveal marine ecosystem dynamics and climate impacts. NASA supports accessibility through tutorials, data recipes, and webinars alongside datasets from PACE, MODIS, VIIRS, and other missions.