English: Bright jewel-toned swirls colored the dark waters of the southern Caspian Sea in late December 2023. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite acquired a true-color image of the stunning scene on December 27.

The dynamic colors are likely a combination of sediment and blooms of phytoplankton, which are microscopic plant-like organisms that float freely in these waters year-round. When given enough nutrients, the right water temperature, and adequate sunlight, they can reproduce explosively to create massive blooms that are easily viewed from space.

Phytoplankton blooms are often harmless and provide an important food source for marine life. Some blooms, however, can be harmful. Sometimes they can deplete the water’s oxygen and a few species produce toxins that can be harmful to both aquatic creatures and humans. There are many different species of phytoplankton and scientists identify them by taking water samples and examining them under a microscope. Different species also may carry different pigments, so large blooms may have slightly different colors based on the predominant species. This may be at least part of the reason that the swirls within the Caspian Sea look different in this image.

Suspended sediment also can discolor water, typically looking muddy when it floats near the surface then turning green as it sinks. Based on color, it is likely that most tint along the western shore is primarily sediment while phytoplankton predominates in the southeast. Garabogazköl, the shallow and saline bay east of the Caspian Sea, is known to be high in sediment and maintains a cloudy look year-round.