As spring 2022 turned the page to summer, the Black Sea turned from dark to vivid. The artists are abundant phytoplankton, which can paint the water with color so brilliant it becomes visible from space.
The phytoplankton bloom is visible in this natural-color image acquired on June 20, 2022. The image blends data from the Visible Infrared Imaging Radiometer Suite (VIIRS) on the NOAA-20 satellite and the VIIRS on the Suomi NPP satellite to eliminate sunglint and the seam lines between satellite passes.
The turquoise swirls indicate the presence of phytoplankton tracing the flow of water currents and eddies. One type of phytoplankton commonly found in the Black Sea is coccolithophores—microscopic plankton that are plated with white calcium carbonate. When aggregated in large numbers, these reflective plates are easily visible from space and make the water appear bright, milky blue.
In most years, the colorful work of coccolithophores tends to show up in satellite images in May and peak in June. Just one month before the VIIRS sensors acquired the image, the Black Sea more closely resembled its name. For example, satellite images on May 20, 2022 show only a faint trace of milky blue water hugging the coastlines, while most of the sea appeared dark blue to black.
But a dark Black Sea does not mean that it was devoid of phytoplankton; on the contrary, diatoms were likely present. This type of phytoplankton is common in these waters during spring and can darken the water more than brightening it. Research focused on the northeast part of the sea suggests that the seasonal changes—from smaller species of diatoms earlier in spring to coccolithophores in late spring and summer—are related to changes in the type and amount of nutrients that are available.
Diatoms rapidly multiply in spring, when surface waters have abundant nitrogen and phosphorous. In late spring and early summer, when warmer temperatures and fewer storms leave the seawater more stratified, less nitrogen gets mixed into the surface waters—a condition in which coccolithophores are known to dominate. Later in the summer, larger species of diatoms usually show up. These phytoplankton take advantage of nutrients supplied by the occasional mixing that occurs as winds shift direction and storms pass by.
The seasonal shift in the dominant species of phytoplankton can have a rippling effect on the structure of the food web in the Black Sea. For example, coccolithophores provide fodder for species like Noctiluca scintillans (sea sparkle), while small diatoms feed pelagic fish and large diatoms feed jellyfish.
NASA Earth Observatory image by Joshua Stevens, using VIIRS data from NASA EOSDIS LANCE, GIBS/Worldview, and the Joint Polar Satellite System (JPSS). Story by Kathryn Hansen.