Eutrophication: Fixing of phosphorus for a better archipelago environment – a mecocosm experiment

Fixing of phosphorus for a better archipelago environment – a mecocosm experiment

Eutrophication is a major problem in the Baltic Sea, where it is regarded as one of the greatest threats to the marine environment. The present-day nutrient levels result from a high external load of nitrogen and phosphorus that took place throughout the 20th century and that in many areas is still in progress today. We have now become better at reducing our emissions, but the historic load remains stored in the bottoms and phosphorus leaches back up into the water mass. This phosphorus leaching increases when the oxygen levels become low, which makes the situation worse as oxygen-free bottoms are becoming increasingly common in the Baltic Sea. This means that it will take a very long time to improve the condition of the Baltic Sea solely by reducing the external load. New methods must instead be found to also reduce nutrient leaching from the bottoms.

There are a number of proposed methods to find phosphorus in the bottom sediment, one of the most strongly advocated of which is precipitation by the addition of aluminium salt. This method has previously been used to restore eutrophicated lakes, where it has proved effective and is considered to entail little risk to the ecosystem. However, the method has not yet been tested more widely in a brackish water environment. For this reason, BalticSea2020 conducted a major mesocosm study in the summer of 2011 aimed at evaluating on a small scale the effectiveness of the method and its impact on the ecosystem in brackish water. The study is a first step in a more long-term project to develop a powerful measure against eutrophication on a larger scale.

The study shows that phosphorus precipitation with aluminium salt has had the intended effect on phosphorus levels in the treatments. The reduced supply of nutrients was found through reduced phosphorus levels in the water mass, limited leaching of phosphorus from the sediments and consequently a generally reduced internal load. In addition, precipitation resulted in reduced deposition of phosphorus in bladderwrack and algae growing on it (epiphytes). The reduced supply of nutrients has also resulted in significantly less growth of these filamentous epiphytes, as well as reduced growth on the experimental facility.

The quantity of dissolved aluminium in the water increased as a consequence of the treatment, but was not stored in the organisms, and does not appear to consist of bioavailable forms. In addition, the treatment does not appear to lead to any increase in fish or mussel mortality, which together with other results from biological measurements does not suggest any measurable toxic effects of the addition of aluminium. Nor was primary production and respiration in organisms affected by the method, which otherwise might suggest impaired health. However, these results must be treated with great caution as the experiment is not designed as a pure toxicity study.

The results are relevant from a system perspective as phosphorus appears to be the limiting nutrient in the experimental system. The experiment shows that the aluminium treatment has direct effects on the eutrophication systems that are to be counteracted in the Baltic Sea. The method generally seems also to work in brackish water, and not to lead to any unforeseen disturbances to the system. Based on these results, the next step would be to test the system in an open system with a greater depth of water and a stratified water mass.

To read the whole report (in Swedish), click here. (New version: 2011/2012).