Eutrophication presents a challenge for the ecosystems of seas and lakes. Since the 1940s, the input of nutrients such as nitrogen and phosphorus into the Baltic Sea has risen significantly, causing major ecological changes. Some species benefit from the increased nutrients, while others find it harder to compete and decline or disappear completely.

Every year, over 30,000 tonnes of phosphorus and almost a million tonnes of nitrogen find their way into the Baltic Sea. More than 85 million people live within its drainage basin and more than 55 million live near its coasts or alongside the watercourses that flow into the sea. All these people and their activities – such as industry, traffic and farming – result in contaminants being transferred to the sea via watercourses and atmospheric precipitation.

Too many nutrients lead to increased growth from species such as phytoplankton. The amount of organic material increases, triggering a number of physical, chemical and biological changes in plant and animal communities, as well as changes to processes in and on seabed sediments. We can see this in the form of algal blooms at bathing spots.

A surplus of organic material in the system prevents the excess from breaking down. It gathers in the seabed areas which are affected by a lack of oxygen, and creatures that live at the bottom of the sea find it hard to survive. With its slow water turnover, the Baltic Sea region is particularly sensitive to eutrophication.

Algal blooms are a natural phenomenon and are part of algae’s normal life cycle. Different types of algae bloom at different times of the year, depending on factors such as light, temperature, trophic relationships and competition with other species.

Toxic algal blooms are caused by cyanobacteria, which require large amounts of phosphorus in order to grow. It was previously thought that the problem of toxic algal blooms could be solved by eliminating emissions of phosphorus, but we now know that the amount of phosphorus in the system is linked to the amount of nitrogen. Large quantities of nitrogen lead to increased production of other algae, which in turn leads to more deoxygenated seabeds. Phosphorus, which can be used in the growth of cyanobacteria, is released in deoxygenated seabeds. The reduction of nitrogen and phosphorus loads must go hand in hand, and so emissions of both nitrogen and phosphorus must be reduced.

Large amounts of nitrogen and phosphorus come naturally from rivers and watercourses, and end up in the sea. Around a quarter of the total nitrogen and phosphorus input via rivers along the Baltic Sea and the North Sea are from natural sources. However, the rest – almost three quarters – comes from human activity such as discharges from sewage treatment works and industry, as well as leaching from agricultural and forestry land. Mankind’s biggest contribution to the nutrient content of Swedish watercourses comes from farming. Wastewater treatment in populated areas has improved dramatically in recent decades, as have industrial discharges. Today, it is pulp and paper mills that account for the majority of industrial discharges of nutrients to Swedish coastal waters.

These pages include factual information from - a site run by Stockholm University, Umeå University, the University of Gothenburg and the Swedish University of Agricultural Sciences (SLU) within the collaborative framework of the Swedish Institute for the Marine Environment, and in cooperation with the web agency Azote.

Baltic Sea Science Center