These satellite images, acquired from Envisat's MERIS_FR sensor, show a vast algal bloom that could pose a risk to marine life in the Baltic Sea. The blue-green bloom, which stretches from Finland to parts of Germany and Poland, is the largest scientists have seen in the Baltic in recent years. They believe a prolonged stretch of warm sea temperatures and a lack of wind, coupled with fertiliser from regional agriculture washing into the Baltic, have caused the bloom.

Similar blooms have spread over the Baltic each summer for decades as excess nutrients trigger a rapid growth of phytoplankton and algae. This rapid growth consumes oxygen in the water, threatening marine life and destabilising ecosystems across the region. The arrival of strong winds and waves, along with cooler ocean temperatures, could break up the bloom. Although algal blooms are a normal and essential phenomenon, they can be harmful to humans and animals when they produce toxic substances, occur too often or last too long, depleting the concentration of oxygen in the water. Due to the toxicity of some phytoplankton and marine algae species, it is important to monitor blooms so that fishermen, fish farmers and public health officials know about such events as soon as possible.

True colour image (from UCL) of an algal bloom spreading in the Baltic Sea on 27 July 2008 derived from MERIS (left), and Chlorophyll-a fluorescence image of phytoplankton retrieved using the BEAM FLH algorithm (right).

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