Algal blooms

Higher water temperatures and more rain are likely to increase future algal blooms in the Baltic Sea. Consequently there is an increased risk of larger areas with anoxic basins.

Algae and cyanobacteria are often called phytoplankton, and are referred to as blooms when they grow in large quantities. Warm and calm weather are prime conditions for cyanobacteria, and in a relatively short time they can spread over huge areas.

However, if the wind starts to blow the water gets mixed and cyanobacteria will no longer be visible on the surface, but the growth continues at a deeper level.  Eventually the cyanobacteria sink to the bottom and consume oxygen when they decompose. This leads to larger areas of hypoxia - a lack of oxygen near the sea floor - in the Baltic Sea.

Anoxic basins kill bottom-living organisms, except organisms that have the ability to move. The lack of oxygen also releases phosphorus from bottom sediments which rises to the surface. Excess phosphorus in surface waters can be absorbed by cyanobacteria again and this process creates a continuous cycle.

This entire chain of events is clearly harmful for the ecosystem, as organisms have a continuously decreasing area to live in.

Increasing algal blooms in the future

In a changed climate with rising temperatures, the number of days that are favourable for cyanobacteria will increase. As water is heated the ice will melt earlier, releasing nutrients earlier in the season. Scientists estimate that precipitation will increase causing the Baltic Sea to be less saline but also causing an increase in the input of nutrients. Cyanobacteria thrive in fresh water and if more phosphorus is added they will thrive even more.

Higher water temperatures in combination with more rainfall leads to density stratification, with a layer of warm fresh water floating on top of denser salt water. Cyanobacteria will in other words be able to mass-reproduce in peace and quiet without the process of water being mixed.

Results show that climate change directly or indirectly would benefit the cyanobacteria’s mass reproduction in the Baltic Sea. This is likely to increase the extent of anoxic areas.

The increased amount of cyanobacteria in the water also affects the amount of light reaching the seabed. If the amount of light decreases, it will affect the species composition of benthic macro algae, plants and in the end fish and animals.