Clever fishing part of the solution

Nutrients are constantly brought from land to lakes and seas. It is a slow and natural process, which has been accelerated by human activities, for example by nutrient leakage from agricultural land. Despite a decreased addition of nutrients in recent years, there are many indications of the worsening environmental state of the Baltic Sea. Researchers at SLU’s Department of Aquatic Resources study ecosystem changes of the Baltic Sea, for example changes in fish communities and how they may be triggered by eutrophication.

Lack of oxygen a vicious circle

As algae and other organic material reach the bottom of the Baltic Sea, oxygen is consumed at its decomposition. When all oxygen is consumed, hydrogen sulphide is formed, and most living organisms are killed. In this oxygen deficient environment, phosphorus and nitrogen are released. This creates a vicious circle since phosphorus and nitrogen are nutrients which algae – the basis of the marine food web – need to thrive. The amount of nutrients which is released this way from the Baltic Sea’s oxygen-free seafloor may be of the same magnitude as the addition of nutrients from land.

The areas of the Baltic seafloor which is oxygen-free are expanding, in summer algal blooms are frequent, and blanketweed (Cladophora glomerata) is outcompeting macro algae, such as bladder wrack (Fucus vesiculosus), along the coasts of Sweden. Many fishermen report of smaller catches of coastal fish and swimmers see the sea turning into a green soup in summer.

Eutrophication benefits carp fish

Are fish communities affected by eutrophication, and if, in what way?

‘Our ongoing research shows that nutrient content and nutrient load can be connected to changes in the composition of coastal fish communities in the Baltic Sea and the Bothnian Sea,’ says Anna Gårdmark, researcher at SLU.

The researchers have noticed an increase of species which are favoured by a high nutrient content, for example carp fish. They have also observed a lower presence of perch (Perca fluviatilis) and pike (Esox lucius) fry in turbid waters with a low visibility depth, that is, in eutrophic bays.

‘It’s a matter of clarifying what depends on what. The Baltic Sea is not only becoming increasingly nutrient-rich, but also warmer. Climate change, which generates warmer seas with a lower salt content, can also be connected to the observed changes in fish communities. Several species which are favoured by an elevated water temperature are then also favoured by eutrophication. And farther off the coast, fish is also affected by oxygen depletion and overfishing,’ says Anna Gårdmark.

Increased eutrophication in the absence of fish of prey

The symptoms of eutrophication seen in the Baltic Sea are not only a result of a high nutrient load. The absence of fish of prey may also lead to increased amounts of phytoplankton and growth of filamentous algae, as shown by research on the role and development of predatory fish carried out at SLU’s Department of Aquatic Resources.

´A chain of effects starts when predators, such as cod (Gadus morhua), disappear. Then, their prey such as sprat (Sprattus sprattus) is favoured, which in turn results in lower amounts of zooplankton, which in turn decreases the amounts of phytoplankton, and so on along the food chain. The result is eutrophication symptoms and we have observed such chain reactions both along the Baltic coast and farther off the coast,’ says Anna Gårdmark.

Therefore, eutrophication mitigation measures may include both a Baltic Sea management plan which lowers the nutrient load, and fishery management which makes sure that populations of predatory fish are restored.