The Baltic Sea - unique and sensitive sea

27 February, 2024

The Baltic Sea is a unique and sensitive sea where organisms live on the edge of what they can tolerate due to the low salinity. It has also been called the world’s most polluted inland sea due to eutrophication and the chemical cocktail caused by human activities.

The Baltic Sea consists of the Gulf of Bothnia, Gulf of Finland, Gulf of Riga, and the Baltic Proper. The sea has a fascinating history of being both a sea and a lake, and today it is a bit of both with its brackish water. Before the last ice age, the Baltic Sea was much saltier than it is today, explaining the coexistence of both marine and freshwater species. The high sea level at that time connected the Baltic Sea with the salty waters of the North Sea and the White Sea. During the ice age, the entire Baltic Sea was covered by several kilometers of thick inland ice. It was only around 10,000 years ago that the ice retreated, making the Baltic Sea a relatively young sea.

The transition to the current brackish water is due to significant changes the Baltic Sea has undergone since the last ice age, such as land uplift, a sinking water surface, and reduced connection to the North Sea. The Baltic Sea is also unusually shallow, with an average depth of 54 meters compared to the Mediterranean’s 1500 meters. Like the Mediterranean, the Baltic Sea is an inland sea because it is only connected to larger seas through the narrow Danish straits. This means that it takes about 30 years for the water in the Baltic Sea to be completely exchanged, allowing old pollutants to persist in the area.

Today, the Baltic Sea is characterized by significant differences in salinity, ranging from around 25 parts per thousand in the Kattegat to two parts per thousand in the northern Gulf of Bothnia. The only influx of saline water occurs through the narrow straits of the Øresund and the Belts. The low salinity in the north is due to the numerous rivers and streams that flow into that area. This influences the types of species that can thrive in different areas. The brackish water is too low in salinity for marine species while it is too saline for the freshwater-origin species in the Baltic Sea. This leaves only a small number of species that are able to handle these challenges. Some species, like blue mussels, have adapted to the low salinity, resulting in them staying small, around three centimeters, compared to up to ten centimeters on the west coast where salinity is higher.

Because there are relatively few species in the Baltic Sea, the food chain is simpler. It has taken thousands of years for the few species living in the Baltic Sea to adapt to the low salinity. This makes the Baltic Sea a particularly sensitive environment because if one species disappears, there may not be another species to take over its role, which can have significant consequences for the entire ecosystem, as different levels in the food chain influence each other. The loss of key species, such as bladderwrack and top predators like seals and pike, can have particularly large consequences.

In addition to the natural challenges faced by the Baltic Sea, humans have long polluted and exploited the sea beyond its capacity. Over 90 percent of the Baltic Sea’s marine area is in poor condition, according to the latest assessment by the Baltic Sea collaboration HELCOM. A significant portion of human impact originates from land.

At the beginning of the 20th century, the Baltic Sea was a nutrient-poor marine area. However, as the population around the Baltic Sea grew, the pressure from intensive forestry in the north and agriculture in the south increased. Today, the sea is affected by an area (catchment area) about four times the size of the Baltic Sea itself. Around 90 million people from nine countries in this area impact the sea with nutrients, pollutants, and other land-based contaminants flowing into the sea. Sweden has the longest coastline among the countries. Excess nutrients, such as nitrogen and phosphorus, lead to eutrophication, radically reshaping the ecosystem.

Eutrophication primarily leads to proliferation of phytoplankton. Phytoplankton, often referred to as blue-green algae, are actually cyanobacteria that bloom in the summer due to favorable warm and calm conditions. They are also known for producing toxins that can be harmful to animals and children. Algal blooms are a natural process, but when intensified by eutrophication, the algae increase, leading to more oxygen consumption by the bacteria breaking them down when they reach the bottom. This can cause oxygen deficiency and dead zones.

Today, 97 percent of the Baltic Sea is affected by eutrophication, and 21 percent of the seabed is oxygen-free, leaving only bacteria and fungi able to survive in these areas. This corresponds to an area more than twice the size of Småland. The oxygen deficiency that largely characterizes the Baltic Sea is partly due to it being a brackish sea where oxygen-rich and fresher surface water does not mix well with the saltier and heavier bottom water. Therefore, the Baltic Sea is already sensitive to oxygen deficiency, and in combination with external stressors like eutrophication, it can have devastating consequences.

The lack of oxygen is a problem for many organisms, such as cod, which needs oxygen-rich water to reproduce. Oxygen deficiency also releases phosphorus from sediments, worsening eutrophication, known as internal loading, and creating a vicious circle of nutrient leakage.

To counteract the negative consequences in the Baltic Sea, biodiversity is crucial. It acts as an insurance, making the sea more resistant to external influences. The Baltic Sea is estimated to have around 100 fish species, 450 species of macroalgae, 1000 benthic animals, 3000 plankton species, and thousands of unknown bacteria and viruses. This unique biodiversity is threatened by both eutrophication and climate change, as it alters the marine environment. This deteriorates the living conditions for some species, such as bladderwrack, while other species, such as fast-growing filamentous algae, take over their habitats.

Harbor porpoises are one of the species currently classified as critically endangered in the Baltic Sea. Like cod and herring, harbor porpoises are threatened by fishing, as they are caught as bycatch. The porpoise, which uses sound for communication, is also disturbed by the intense traffic generating underwater noise. This has led to the protection of some of the porpoise’s important reproductive areas.

Currently, around 16.5 percent of the Baltic Sea is protected, but to fulfill Sweden’s commitments under the UN Convention on the Law of the Sea, the protected areas should almost double by 2030. By reducing the burden on the sea, scaling down large-scale fishing for species like herring and cod, and establishing new marine protected areas, one can support both individual species and the overall biodiversity in the Baltic Sea.

Despite facing significant challenges, the efforts made in the past have yielded results. For instance, pollutants like PCB and lead have decreased after being banned. This has allowed animals like sea eagles and seals to recover from the effects of these toxins. In recent decades, nutrient input into the Baltic Sea has also significantly decreased. However, due to the large amounts of nutrients stored in the water, it takes a long time for the effects to become visible.

Text: Lina Mattsson
Foto: Johan Candert, Kimmo Hagman, Simon Stanford

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