Information about the Baltic Sea

Eutrophication is the greatest and most visible environmental problem faced by the Baltic Sea. Even though we have managed to halve the nutrient discharges that eutrophicate the sea from the levels of the 1980s, the visible signs of eutrophication, such as the mass appearance of cyanobacteria (or blue-green algae, as it is more commonly known), cloudy waters, slimy shores, and an oxygen-depleted seabed still pester the Baltic Sea.

Eutrophication is the greatest problem of the Baltic Sea

Eutrophication is the greatest and most visible environmental problem faced by the Baltic Sea. Even though we have managed to halve the nutrient discharges that eutrophicate the sea from the levels of the 1980s, the visible signs of eutrophication, such as the mass appearance of cyanobacteria (or blue-green algae, as it is more commonly known), cloudy waters, slimy shores, and an oxygen-depleted seabed still pester the Baltic Sea.

Eutrophication is caused by nitrogen and phosphorus discharges, which feed algae and water plant growth in the sea.  Phosphorus in particular plays a key role in the growth of blue-green algae blooms: in contrast to other algae, they can take the nitrogen they need directly from the atmosphere.

 Nitrogen and phosphorus end up in the sea via e.g. urban wastewaters and rainwater from forests and fields. Some of the nitrogen discharges from traffic also end up in the Baltic Sea as atmospheric depositions. Agriculture is accountable for a large share of Finland’s nutrient discharges: its share of nitrogen and phosphorus loads is more than half of Finland’s entire load (source: Finnish Environment Institute SYKE).

The effects of climate change further accelerate the eutrophication of the Baltic Sea. According to forecasts, increasing rainfall and winters with only light snow will increase nutrient runoff from land to the Baltic Sea. As the climate heats up, seawater, too, will become warmer, boosting algae growth in the sea.

Warmer water may also accelerate the introduction of non-indigenous species to the Baltic Sea. Some of the Nordic species that are well-adjusted to the conditions in the Baltic Sea cannot, on the other hand, adapt to the speedily advancing changes. This will lead to changes in the biome of the Baltic Sea.

The abundance of nutrients in the water is favourable to the growth of both microalgae and filamentous algae on coastal rocks. Increasing growths of filamentous algae suffocate the perennial algae of the Baltic Sea, such as bladderwracks, which are very important to the ecosystem of the Baltic Sea. Bladderwrack grows in abundant belts in the coastal waters, providing spawning areas for many species of fish, and a safe haven for juvenile fish.

The impacts of eutrophication and climate change also reach the fish stocks that are financially important, not to mention the damage done to e.g. the tourist industry. To curb discharges, we need concrete action, and the immediate cooperation of all countries in the Baltic Sea area.

Oxygen depleted seabeds

The rich growth of microalgae, i.e. phytoplankton, in the surface waters leads to increasing oxygen depletion in the waters near the seabed.  Dead algae fall to the seabed, and their decomposition uses up oxygen from the seabed.  In oxygen-depleted conditions, the decomposition process starts to generate poisonous hydrogen sulphide, which kills the benthos in the area.  Phosphorus is also released from the seabed’s reserves. This phenomenon is referred to as the sea’s internal load. If the water is mixed strongly, phosphorus from the seabed will enter the water’s surface layers, and boost the creation of voluminous blue-green algal blooms.

 A vulnerable inland sea

Water in the Baltic Sea is low-saline brackish water – a unique mix of salty and fresh waters. The saline content of the water in the Baltic Sea is only about a fifth compared to the salinity of oceans (35 per mille). Moreover, the saline content of the surface waters of the Baltic Sea diminishes towards the north, and in the Bay of Bothnia and the eastern end of the Gulf of Finland the water of the Baltic Sea is actually almost fresh.

Compared to oceans, the Baltic Sea is small and shallow. The average depth of the Baltic Sea is only 54 m, whereas the average depth of the Atlantic, for example, is approximately 4 km, and even the Mediterranean is 1.5 km deep.  At its deepest, the Baltic Sea reaches 459 metres. The Baltic Sea is connected to the North Sea via the narrow Danish straits, and its water turnover time is extremely slow. It has been calculated that it takes roughly 30 years for the entire water mass of the Baltic Sea to change. Because of the slow turnover of the water, environmental toxins and nutrients that cause eutrophication in the sea will stay in the Baltic Sea for a long period of time.

The water of the Baltic Sea is permanently stratified, according to salinity.  The salty seawater, entering from the North Sea, is heavier, and sinks down to the seabed and deeper areas of the Baltic Sea basin. The top layer of water is low in saline content, diluted by rain and the rivers that run to the Baltic Sea. Stratification hinders water turnover between the bottom and the top layers. The surface waters, which have high oxygen content, cannot mix with the deeper layers of water, and, consequently, the abysses of the Baltic Sea often experience periods of oxygen depletion.

In terms of oxygen, the situation does improve from time to time with saline pulses, which occur approximately once in a decade: a pulse introduces saline water with high levels of oxygen from the North Sea to the Baltic Sea abysses, mixing with the waters near the seabed of the Baltic Sea. If the oxygen-depleted seabed is large, the oxygen-rich water flowing to the abysses is, however, not necessarily able to sufficiently improve the oxygen status of the Baltic Sea abysses.

The biome of the Baltic Sea is a peculiar mix of ocean and freshwater flora and fauna. The number of species of flora and fauna that have adapted to life in brackish water is small, but individual species can appear in abundance. Compared to oceans, the food chains of the Baltic Sea are simple. Moreover, the number of species diminishes as we move from southern Baltic Sea towards the north. The low salinity of the northern Baltic Sea, cold winters, and the sea freezing over all set challenges for species adaptation. Many species that live in the Baltic Sea push their capability to adapt to the extreme. The flora and fauna of the Baltic Sea are also very sensitive to changes in the environment.