Almost every year large, late summer algal blooms can be observed in the Baltic Sea: the water then turns green because countless tiny unicellular organisms swim in it. Such mass reproductions of phytoplankton are favored, among other things, by the strong solar radiation in summer, the warmth of surface water and the abundant nutrients in the Baltic Sea. Sediment cores extracted from the seabed indicate that cyanobacteria have been blooming in the Baltic Sea for thousands of years.
In a few years, like 2019, cyanobacteria blooms covered up to 200,000 square kilometers of the Baltic Sea’s surface – just a little less than half the size of Sweden. And the picture of the week shows only a part of this year’s algae bloom in the Baltic Sea: The extent extends far beyond the edge of the picture. The picture was taken on August 15, 2020 between Ölland and Gotland – two islands off the south-east coast of Sweden.
As you can see in the picture, the surface of the Baltic Sea is colored green and the water is cloudy. The ocean currents have created artistic eddies and spots in the dense algae broth. Similar to fingerprints, no two vortices are alike. The black, straight lines show where ships crossed the shimmering green water.
Although the eddies look so artistic, they have a downside: Excessive algae growth not only creates a strong green color and cloudiness of the water surface. Some cyanobacteria also produce substances that are toxic to humans and animals. In addition, dead algae sink to the sea floor and are broken down by other bacteria while consuming oxygen. This decomposition reduces the oxygen content of the deep water – and causes oxygen-poor, dead zones in the deeper water layers of the Baltic Sea.
Such massive reproductions of cyanobacteria have increased in the past, for example through the inundation of nutrient-rich water from rivers and sewage discharges. In addition to the over-fertilization of the Baltic Sea, climate change and the associated hotter, sunnier summers probably also play a role in the increased spread of algal blooms in the Baltic Sea.