The Unique Baltic Sea

The Baltic Sea is a relatively small and shallow sea, but approximately 85 million people live in its exceptionally large catchment area using the sea for various purposes. Only the shallow and narrow Danish straits Skagerrak and Kattegat link the Baltic Sea with the ocean, which means that the water in the Baltic Sea exchanges slowly and harmful substances stay in it for decades. What comes to the recovery of the health of the Baltic Sea ecosystem, eutrophication is the key problem.

A unique feature of the Baltic Sea is its brackish water – a mixture of fresh and saline seawater. Brackish water and cold wintertime create a challenging environment for organisms in the Baltic Sea. Many species of the Baltic live on the extreme limits of their adaptability and the fl ora and fauna of the sea are very sensitive to changes in the environment. The International Maritime Organization (IMO) designated the Baltic Sea as a Particularly Sensitive Sea Area in 2004.

Eutrophication – Common challenge

Eutrophication means enrichment of water bodies with nutrients (nitrogen and phosphorus) and the negative consequences of this process. Eutrophication disturbs the ecological balance of the watercourses, harms fi shing and aff ects negatively the use of water in drinking, household and recreational purposes. The pollution of waters with an excess amount of nutrients stimulates primary production: algae blooms are common in eutrophicated water bodies. The abundance of microscopic planktonic algae decreases the transparency of water. Typical to highly eutrophic watercourses are blooms of blue-green algae, many species of which are toxic.

The symptoms of eutrophication include overgrowth of the shoreline by macrophytes (aquatic plants) as well as increased growth of epiphytes (plants growing upon other plants) and harmful macroscopic algae. Eutrophication has also on negative eff ect on the living conditions of many fi sh species with a high commercial value. Due to increased primary production, more oxygen is consumed to decompose the organic matter. This leads to reduced oxygen content in the water, especially in the near-bottom layers and/or in wintertime.
The lack of oxygen and formation of hydrogen sulfi de (H2S) kills water organisms. Oxygen depletion in the near-bottom layers of water leads to an active release of phosphorus from the bottom sediments to the water and intensifi es the eutrophication process.

The main sources of nutrients to the waters are communal and industrial wastewaters as well as agricultural production. To improve the state of the water bodies, it is essential to cut down the nutrient load, especially the load of phosphorus, as it is most often the limiting nutrient for phytoplankton production in inland waters.

One of the most economically effective and fastest ways to combat eutrophication is to enhance the phosphorus removal from cities’ wastewater treatment plants. HELCOM (Baltic Marine Environment Protection Commission) recommends reaching a yearly average of 0.5 mg/l phosphorus in treated wastewaters. It is essential also to secure sustainable sludge handling as the phosphorus removed from the wastewater stays in the sludge.

Eutrophication brochure
Eutrophication poster

PURE – Projet on Urban Reduction of Eutrophication