The Baltic Sea is particularly vulnerable to eutrophication because it is quite shallow and its water changes slowly. In addition, the sea is surrounded by a dense population, which results in a high concentration of nutrients in wastewater and runoff from its catchment area.

Nitrogen and phosphorous cause a growth spurt

Of the various nutrients, both nitrogen and phosphorus, in particular, make the Baltic Sea eutrophic since they are naturally scarce in seawater. When excessive nitrogen and phosphorus flow into the sea, there is an explosion in algal growth.

Besides nutrients, algae also need heat and light to photosynthesize and grow. However, there is always enough of these in the summer months. Thus, during the growing season, it is indeed the availability of nutrients that regulates the growth of algae and aquatic plants.

Algae and aquatic plants are known as primary producers because they form the basis of all marine life. With a large number of basic producers, the entire food web gets a boost and marine animals, such as fish, also increase. While this should be good news, e.g. for fishermen, in reality, eutrophication causes problems for everyone, including the well-being of the sea itself.

Eutrophication affects everything

One of the most visible signs of eutrophication is increased algae growth. When there are many small planktonic algae in the water, the water clarity is reduced, and sunlight does not penetrate as deeply as before. This is detrimental to the lives of many species and such effects are reflected throughout the whole food web.

Such increased marine production also means that there will also be an increase in the number of droppings and dead bodies of organisms being deposited on the seabed. This will lead to an acceleration of decomposition activities on the seafloor. Decomposer microbes consume oxygen in the water, which can result in oxygen depletion. Many areas of the Baltic Sea seafloor are currently plagued by permanent anoxia.

Such large changes affect the entire ecosystem. While some species benefit, others suffer. On the rocky bottom, the filamentous algae become rampant, while the bladder wrack declines. Although more cyprinid fish species may be caught, the number of species, such as flatfish, decreases. 

Watch a video of a habitat in poor condition dominated by filamentous algae!

The nutrient load increased most after the mid-20th century

The growth of nutrient loading in the Baltic Sea, i.e. the nutrient flow from land and air to the sea, kept pace with industrialisation. Pollution increased most strongly between the 1960s and 1980s when marine nutrient concentrations also increased markedly. Since then, this trend has stabilised and, in some cases, nutrient levels have even fallen.

However, nutrient loads, as well as nutrient concentrations, vary greatly from one marine area to another. Therefore, each sea area should be considered separately and as an entity unto itself.

Although loading has decreased, the state of the sea has not improved

Over the last 20 to 30 years, efficient water protection measures have reduced the nutrient load to the Baltic Sea. Although marine eutrophication has stopped, the state of the sea has not yet seen a significant improvement.

Part of the reason for the continued poor condition of the Baltic Sea is due to the accumulation of a nutrient reservoir in the deep water layers, as well as within the sediments of the seafloor. This phenomenon is known as internal loading and is responsible for maintaining marine eutrophication.