Algal galaxy

Source: Modified Copernicus Sentinel-2 data (2018) / Google Earth Engine

The Baltic Sea regularly turns a vibrant blue-green in summer. This is caused by the bloom of cyanobacteria, in this case off the eastern coast of Götland, Sweden. These microscopic organisms concentrate in locations where the vertical and horizontal water movements generate the best nutrient and light conditions for algal growth, and these hotspots are then drawn out by the water circulation. Note the straight, dark lines in the image: the bloom has been stirred up by powerful ship’s propellors.

Satellite data can track the growth and spread of harmful algae blooms in order to alert and mitigate against damaging impacts for tourism and fishing industries.

Recent UZH-led research (Abolfazl et al, 2024) on freshwater algae in Lake Geneva showed that a certain sequence of meteorological conditions triggered an algal bloom in 2021: heavy rainfall promoting excessive organic matter and nutrients loading, followed by wind-induced coastal upwelling, and a prolonged period of warm, calm weather.