Almost all life in the ocean depends on tiny photosynthetic organisms called phytoplankton. These microscopic plant-like organisms collect carbon dioxide from the atmosphere and release oxygen. Phytoplankton are considered the Earth’s lungs and produce about half of our oxygen.
One way scientists can track phytoplankton in the ocean is by measuring concentrations of chlorophyll-the compound that allows plants and phytoplankton to absorb energy from sunlight.
As the seasons change, chlorophyll concentrations change as well. Chlorophyll is especially abundant in the spring and early summer, when plentiful light and nutrients support vast blooms of phytoplankton.
“The color of the ocean provides us with important information about phytoplankton, the health of the ocean and the global climate,” explained Ivona Cetinic, an oceanographer at Morgan State University.
Regional differences in chlorophyll concentrations are due to factors such as the shape of the seafloor (bathymetry), nutrient availability and ocean currents. Near coastlines, phytoplankton thrive on plentiful nutrients washed into the ocean from land and from upwelling of cold, nutrient-dense water from the deep ocean.
Some of the highest chlorophyll concentrations can be found in cold polar waters, where nutrients accumulate during the dark winter months. When spring sunlight returns, phytoplankton thrive. Close to the equator, a streak of high chlorophyll concentrations can be seen where the ocean’s warm upper waters mix with the cooler, nutrient-rich waters from the depths. This process, termed equatorial upwelling, creates ideal conditions for phytoplankton to thrive throughout the year.
https://earthobservatory.nasa.gov/images/150656/breathing-life-into-the-ocean