Largest milky sea event

Milky seas are a natural phenomenon involving large colonies of bioluminescent bacteria that, under certain conditions, can illuminate vast areas of ocean surface at night with an eerie white glow. The largest documented example was logged by satellite imaging at near 9.5° S, 111° E near Java, Indonesia, between 25 July and 9 August 2019, with the milky sea covering in excess of 100,000 square kilometres (38,600 square miles) - around the same size as Iceland. The event was described in a paper published in Scientific Reports on 29 July 2021.

Milky seas are particularly prevalent in remote waters of the north-west Indian Ocean and in the seas between the Indian and Pacific oceans (aka the Maritime Continent). Optimal conditions appear to be cool, nutrient-rich waters with a high density of microalgae. In order to become visible as a milky sea, bacteria concentration has to reach ~10^8 cells per millilitre.

A similarly large-scale milky sea event occurred off Somalia in Jan-Feb 2019, but the Java example was larger, more intense and lasted for much longer.

The marine scientists estimate that such a substantial milky sea must have comprised somewhere in the region of 6 x 10^22 to 6 x 10^23 luminous bacteria. An 500-km2 (193-sq-mi) area at the centre of the bioluminescent display glowed so brightly that clouds between the satellite and the ocean appeared dark in contrast in infrared scans.

The study into satellite-recorded milky sea events that took place globally between 2012 and 2021) was a collaboration between Colorado State University, Monterey Bay Aquarium Research Institute, the University of Wisconsin-Madison, the National Oceanic and Atmospheric Administration (all USA) and Seoul National University (South Korea).