The Baltic Sea has historically been subjected to massive amounts of excess nutrients, especially nitrogen and phosphorus. They originate from farming, industry and habitation in the sea’s large catchment area. Nutrients come for example from fertilized fields and waste water. Nitrogen also comes as deposition from the atmosphere.
Large amounts of nutrients in the water increase primary production, i.e. intensify algal growth. Dead algae sink to the bottom, where decomposition consumes oxygen. This is one of the reasons for poor oxygen conditions on the sea floor. As the bottom turns hypoxic, sediments can no longer retain old stored nutrients. These nutrients were previously unavailable to algae and plants, but due to hypoxia, they start to leak from the sediments. This so called internal loading feeds the cycle of eutrophication by accelerating primary production. This increases the amount of dead organic matter and makes hypoxia worse. Benthic animals cannot survive in these conditons, and large areas on the sea floor are currently completely devoid of life.
Cyanobacteria, also known as blue-and-green algae, are able to utilize nitrogen directly from the atmosphere. Their growth is limited by the availability of phosphorus. As the phosphorus-rich deep water mixes with surface water, it makes phosphorus available to cyanobacteria. On calm summer days cyanobacteria bask in the sun, because wind does not mix them deeper into the water. Simultaneous access to sunlight and phosphorus causes massive cyanobacterial blooms during the summer months.
Not all primary producers benefit from eutrophication. Bladder wrack, a type of kelp that grows in the Baltic Sea, has in places been overridden by more fast-growing species of algae. This is especially problematic, as bladder wrack is one of the Baltic Sea’s key species. It is an important habitat for many invertebrates and young fish, as well as a source of food for several animals. Bladder wrack’s decline is thus a threat to the Baltic Sea’s biodiversity.
Eutrophication causes changes in fish stock. Large predators aren’t able to see their prey in the murky water, and the number of benthivorous and planktivorous fish grows. As zooplankton gets eaten by fish, it no longer controls the amount of phytoplankton and cyanobacteria. This increases algal and cyanobacterial blooms further. These fish also stir bottom sediments in their search for food, which can further release harmful substances from the bottom.