Hazardous substances are defined by their persistence in the ecosystem, their toxicity to animals and their tendency to bioaccumulate in the food chains. Due to long water residence times in the Baltic Sea, large quantities of hazardous substances have accumulated on the bottom of the sea. These substances wash into the sea from i.e. waste water plants, farming, marine traffic and industry. Some of the hazardous substances have come as emission from air. They may have originated far from the Baltic Sea area.
Many substances have accumulated on the bottom of the Baltic Sea but there is a lot of spatial variation in their occurrence. There are also artificial radionucleids, most of which were deposited in 1986 after the Chernobyl nuclear power plant accident. Additionally, chemical munitions were dumped in the Baltic Sea after the two world wars. They still release harmful chemicals into the surrounding environment.
The amounts of some substances have declined both in the bottom sediments and in the sea’s organisms. This is due to bans and stricter regulation during the past decades. However, new chemicals enter the sea continuously from many different sources. Certain pesticides, anti-fouling paints used on ships, and flame retardants are examples of substances causing problems in the ecosystem. Pharmaceuticals, microplastics and chemicals from cosmetics flush to the sea as part of the communal waste water.
These substances are harmful because of their negative effects on marine life. Small animals at the bottom of the food chain absorb these substances from their food and the environment. If the body of the organism is not capable of metabolizing the substances fast enough, the predator eating the animal will ingest these substances as well. As a result, the amount of harmful substances grows on every trophic level.
Especially the top predators, such as seals, white-tailed eagles, and large predatory fish have suffered from hazardous substances. Substances may affect the animals’ mating behaviour, damage the shells of birds’ eggs, or impair the development of offspring. The substances may also weaken the immune system of the animals, thus making them more vulnerable to diseases. Humans are likewise at risk of being exposed to hazardous substances due to bioaccumulation. This is why it is recommended to eat only limited amounts of fish from the Baltic Sea.