Baltic eutrophication

Improved knowledge opens for novel solutions

By Anders Stigebrandt, Senior professor in oceanography, Dept. of marine sciences, University of Gothenburg (e-mail:

A correct description of a system is obviously needed to design efficient methods to change the system. The eutrophication of the Baltic Sea has been debated since the 1960s. The suggested solutions to the eutrophication problem have varied in time, mirroring the increasing level and complexity of scientific understanding of the functioning of the Baltic Sea. Interestingly, the proponents of various methods have usually been quite confident that “their method” is the correct one which often has led to harsh debates with scientists with differing opinions. The lesson that can be learned from history is that there may be additional, still unknown, facts about a system that are needed to correctly describe and understand it. The following brief description of the history of methods proposed and applied to cure the eutrophication of the Baltic proper will illustrate this.

In the 1960s and 1970s, the eutrophication of the Baltic proper was thought to be caused by too large supplies of phosphorus (P) from land-based sources. However, Fonselius showed that large internal sources were activated when bottoms became oxygen free. Then followed an extended period when the eutrophication of the Baltic proper by an overwhelming majority of scientists was attributed to a too large supply of nitrogen (N). This conclusion was questioned because extensive blooming of cyanobacteria clearly demonstrated that there was a large P surplus (N deficit) in the surface waters. From about 2010, P has by an increasing number of scientists been considered the main cause of eutrophication although the Baltic Sea Action Plan still includes demands on reductions of N supplies.

The contemporary big discussion concerns the right way to get rid of P in the water column of the Baltic Proper. It is often argued that the only solution to this problem is to decrease the land-based P supply (present day paradigm).  However, the land-based P supply has been halved since the 1980s but the P content in the water mass has increased by 25% during the same time. It has therefore become apparent that a contemporaneous internal P source has more than well compensated for the reduction of the land-based P supply. It has been shown that this source is coupled to oxygen-free bottoms that release P when the “lid” of iron and manganese oxides on top of the sediment disappears when the overlying water becomes oxygen free. Nobody knows how long time it will take before the internal source becomes very small so that the decreased land-based supply will lead to lower concentrations of P in the surface water. If it takes several decades, it will become very hard to enforce additional expensive P reductions as demanded by proposers of the present paradigm.

The new piece of knowledge, that the internal source now is the major P source to the Baltic proper, opens for a new solution of the eutrophication problem. Since the internal source is connected to anoxic bottoms, it may be stopped by keeping bottoms oxygenated. This conclusion is supported by massive empirical evidence. The new method is described in my recent paper “On the response of the Baltic proper to changes of the total phosphorus supply”, published on-line by Ambio, July 2017. The paper can be downloaded here. I recommend that you read the paper if you want to know how the Baltic proper works based on facts I use in the model. As history indicates, harsh debates between proponents of present day paradigm and proponents of the new ideas about the functioning of the Baltic proper, that eventually might become the new paradigm, may be expected. Indeed, they have already started. The good thing with debates of this kind is that they are instrumental for the advancement of science.