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How can fertilizer runoff cause eutrophication in the Baltic Sea?

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Map showing the physical features, major cities, and national boundaries around the Baltic Sea, North Sea, and English Channel regions of northern and western Europe. The map highlights coastal countries including the United Kingdom, Norway, Denmark, Germany, the Netherlands, Belgium, France, Sweden, Finland, Estonia, Latvia, Lithuania, Poland, and Russia, with sea depths indicated and key islands, peninsulas, and underwater trenches labeled. — Baltic Sea The Baltic Sea, the North Sea, and the English Channel.

Fertilizer runoff contributes to eutrophication in the Baltic Sea by introducing excess nitrogen and phosphorus into the ecosystem. Because the Baltic is a semienclosed sea with slow circulation and stratified water columns, it is particularly vulnerable to this nutrient loading from coastal and river runoff.123

When these nutrients enter the water, they trigger rapid growth of algae and other microscopic organisms, creating water blooms. These blooms make the water murky and block the light penetration necessary for underwater life. As this excessive organic matter dies and decomposes, the process consumes vast amounts of dissolved oxygen.41

This leads to hypoxia—a state of oxygen depletion in deep waters—which can cause massive fish kills and degrade the overall health of the marine environment. Addressing this requires managing land-based pollution sources, a challenge that has been a focus of international cooperation, such as the Helsinki Convention, since the 1970s.123