Through detection of phytoplankton, chlorophyll-a concentration provides a measure of biological activity. Phytoplankton are microscopic, free-floating photosynthetic organisms that form the basis for most of the ocean's food chain and play an important role in the global carbon cycle by removing carbon dioxide (CO2) from the atmosphere during photosynthesis. The subtropical gyres north (where Hawaiʻi resides) and south of the equator are regions in which the nutrients required for phytoplankton growth are strongly depleted year-round. These areas are described as oligotrophic ("ocean deserts") and exhibit low surface chlorophyll.
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