File:Phytoplankton bloom off Chatham Island, South Pacfic Ocean (MODIS 2017-11-06).jpg

English: On October 30, 2017, the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Aqua satellite passed over Chatham Island and captured a true-color image of swirls of turquoise in the surrounding South Pacific Ocean.

The swirling colors mark a bloom of phytoplankton (single-celled, plant-like organisms) which live in the chilly waters in small numbers year-round. When the lengthening light of spring, nutrients, and water temperature combine to create conditions favorable for growth, phytoplankton begin to reproduce explosively, resulting in colorful stains in the ocean. The colors come from chlorophyll and other pigments that the phytoplankton use to capture light for photosynthesis. The milky turquoise parts of the bloom may include phytoplankton called coccolithophores, which have a chalk-like plating that is very reflective.

The Chatham Rise, an underwater plateau that stretches from the South Island of New Zealand (not in the image) to beyond the Chatham Islands archipelago, separates two areas of deeper water to the north and south. Tides and other currents flowing over this submarine topography enhance vertical mixing in the water column. That mixing and the location of the rise along the subtropical front (where cold currents from the Antarctic meet warmer, subtropical waters) foster large blooms of phytoplankton in the area, especially in the Southern Hemisphere spring and summer.

Like plants on land, phytoplankton are the foundation of the marine food web, using sunlight to convert atmospheric carbon dioxide and water into the carbohydrate glucose. All other marine life—from squid, to fish, to whales—depend directly or indirectly on phytoplankton, which makes tracking the size and frequency of phytoplankton blooms important to biologists monitoring the health of our oceans.