The Baltic Sea’s bottom sediments have stored large quantities of nutrients, especially phosphorus, most of which has come as external loading from e.g. farming and communal waste waters. In poor oxygen conditions sediment can no longer retain phosphorus, and it leaks back to the water and causes eutrophication. Decomposing dead algae consumes oxygen in the sea bottom, which in turn makes the oxygen conditions even worse. This creates a vicious cycle, where oxygen is consumed, phosphorus leaks, the amount of algae increases and the condition of the sea worsens.

To ensure the future of the Baltic Sea, combating external loading is important. Today’s internal loads are a consequence from past external loads. External loads have already been cut somewhat successfully, for example by developing more efficient waste water treatmet techniques and by reducing the use of fertilizers in farming. However, the state of the Baltic Sea hasn’t improved much in spite of these actions, due to nutrients leaking from the sea bottom. In the future climate change will increase precipitation, which will lead to more runoff from land. Rising temperatures might also accelerate eutrophication. This is why current methods for mitigating external loading are not by themselves sufficient for improving the condition of the Baltic Sea.

Finland has committed to the objective of the EU Marine Strategy Framework Directive to achieve good environmental status in the Baltic Sea by the year 2020. This goal cannot be reached with current measures, which focus solely on mitigating external loads. BSAG sees combating nutrient leakage from internal storages as a crucial step towards a healthier Baltic Sea.

So called sea-based measures offer possibilities for decreasing internal nutrient loads. Phosphorus leakage from sediments could be prevented by treating the sea bottom chemically (e.g. with aluminum), or by pumping oxygen to the anoxic bottoms. Another possible measure is dredging, where phosphorus-infused sediments are removed from the sea. Biological measures, such as mussel farming, might also have a beneficial effect on the sea’s nutrient dynamics.

However, these measures may still contain many risks and uncertainties. Bottom areas suffering from poor oxygen conditions are large, and should not be tampered with in an unpremeditated fashion. Due to the Baltic Sea’s fragile ecosystem and simple foodwebs, interfering with its ecological processes should be done extremely carefully. BSAG wants to be involved in advancing research on both internal nutrient storages and sea-based measures. Different measures could be tested in a smaller scale with pilot projects done in Baltic Sea’s archipelago and small bays. This might also have local benefits for these areas, if the tested measures prove to be successful.

Internal nutrient storages and possible sea-based measures were examined further in a report commissioned by Finland’s Ministry of the Environment. BSAG participated in the report as part of an international consortium of experts. The full report can be found here.