Image of the chlorophyll-a concentration in the Caspian Sea on 26 July 2015. This image was captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument aboard the Aqua satellite. Chlorophyll-a is a light harvesting pigment found in all photosynthetic plants. The concentration of chlorophyll-a is used as an index of phytoplankton biomass. Phytoplankton fix carbon through photosynthesis, taking in dissolved carbon dioxide in the sea water and producing oxygen, enabling phytoplankton to grow. Changes in the amount of phytoplankton indicate the change in productivity of the ocean. The presence of phytoplankton often is a sign of good health, but harmful algal blooms contain phytoplankton that is toxic to marine life. Chlorophyll can be used to trace oceanographic currents, jets and plumes.