File:Swirls of Color in the Adriatic Sea (MODIS 2021-10-22).jpg

English: Swirls of teal, turquoise, and tan colored the waters of the Adriatic Sea in mid-October 2021. The Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Terra satellite acquired a true-color image of the scene on October 20.

The Adriatic Sea is the northernmost arm of the Mediterranean Sea, separating the Italian Peninsula from the Balkans. Italy sits along the western coast of the Adriatic and, in this image, Croatia can be seen in the east. Many islands sit off the mainland of Croatia and the waters are deep; the waters are shallower off of Italy and few islands exist.

The jewel-toned colors are typical of blooms of phytoplankton which often occur in the Adriatic Sea in the autumn, the season when conditions are just right to encourage rapid growth of the microscopic plant-like organisms. While it is possible that some of this color could be associated with such a bloom, it is much more likely that most of this color comes from sediment. Sediment frequently washes off of the coast, is churned up from the shallow bottom in the western Adriatic, or pours into the Sea from sediment-laden rivers. When floating near the surface, sediment generally looks muddy-tan. As it sinks underwater and disperses, the change in reflectivity makes sediment appear green and then blue.

The Adriatic region has survived a stormy October, with severe storm warnings issued in Croatia on October 6-7. Looking at satellite images taken of the region through the month of October, it is evident that the waters off the coast of Italy were clear on October 4. By October 5, storm clouds hovered over Italy, the Adriatic Sea, and Croatia and remained present until October 13, when skies cleared. On that date, the waters just off the Italian coast sported a halo of tans, greens, and turquoise, which almost certainly was sediment washed off the coast and rivers due to the storms. Since that date, the colors have lightened and spread further into the Adriatic. The fanciful swirls seen in the October 20 image are primarily created by dispersing sediment riding the currents near the Italian shore.