Michelle McCrackin reading a policy brief.

Phosphorous being released to the water from the sediment is called internal load and it contributes to the eutrophication of the sea. Many innovative geo-engineering measures have been proposed to enhance the recovery of the sea through decreasing the amount of nutrients already there. But there is no evidence any of them can work on a large scale, why it's more important to support work that aims to decrease the loads of nutrients still flowing to the sea from land.

This of course raises a lot of questions. Some of them will be answered below. Enjoy!

What is the internal load?

The term “internal load” refers to phosphorus that is released from oxygen-free sediments on the sea floor (also called “dead zones”) to the water column. In the Baltic Sea this occurs mainly in the deep areas of the sea, such as in the Gotland basin, between the east coast of Gotland and Latvia.

What causes the internal load?

It is a consequence of eutrophication, not a cause of it. The cause of the internal load is phosphorus from land sources, such as sewage effluent and agriculture, that has accumulated in the sea for many decades.

If nutrients in the Baltic Sea were in balance, the input of phosphorus would equal the amount exported to the North Sea and the increase in long term phosphorus storage in sediments. Currently, however, this balance has been severely disturbed, building up the pools of phosphorus in the sediments and water column.

How big is the internal load?

First, the internal load recycles of a portion of old “sins”, which are the inputs of phosphorus over the past century. The magnitude of the internal load is difficult to measure because it varies in a complex way, depending on, for example, the amounts of organic matter, biological activity, minerals, metals, and oxygen.

If there is oxygen at the sediment surface, a significant amount of phosphorus is stored in the top sediments bound with iron oxides. When there is little or no oxygen, the bonds break and phosphorus is released back into the water column.

In the Baltic Sea today, up to 0.1 million tons of phosphorus can move back and forth like this, numerous times.

Is this the largest source of phosphorus to the sea?

No. The internal load should not be confused with the external load, which refers to phosphorus from land sources that is delivered by rivers and by the discharge of treated sewage effluent into the sea. The internal load is not a source of new phosphorus, it is the phosphorus that has accumulated over the past century and that moves between the sediments and water column.

The external load from land is the input of “new” phosphorus to the sea. The internal load recycles “old” phosphorus that has accumulated.

How do we stop the internal load?

If the external inputs of phosphorus to the sea continue to decrease, the sea should return a situation where the external load equals the sum of long-term storage in sediments and export to the North Sea. Recent modelling results indicate that after decades of increases, the amount of phosphorus in the Baltic Sea has started to stabilize. This means that external load reductions are starting to have a positive effect, although it could take decades before we see recovery from eutrophication in the sea.

How does the internal load relate to the dead zone?

Studies of deep sediments indicate that dead zones are naturally occurring in the Baltic Sea. In the past century however, the size of the dead zone has increased 10-fold. This increase is attributed to the external load of nitrogen and phosphorus from land as well as increased water temperatures due to climate change.

The accumulation of phosphorus in deep areas of the sea is closely linked to the accumulation of organic matter (living or dead plant and animal material) that contains nitrogen and phosphorus. After algae, fish, and other organisms die, they sink and are decomposed. While microbes decompose this organic matter, they consume oxygen. In some areas of the sea bottom, there is so much organic matter to decompose that all oxygen is consumed. This results in a “dead zone”.

What is oxygen pumping?

There is a proposal to speed up the recovery of the sea from eutrophication by using many large wind turbines to pump oxygen-rich water to deep, oxygen poor areas. It is thought that the addition of oxygen will bind phosphorus to sediments through chemical reactions with iron that will “shut off” the internal load after 10 years.

Could oxygen pumping stop the internal load?

We don’t know. There is not consensus in the scientific community that oxygen controls the internal load. The processes that store phosphorus in sediments and release it into the water are complex and depend on the interaction of organic matter, minerals, metals, oxygen, and the movement of water around the sea. For example, some research argues that artificial oxygenation could increase the short-term storage of phosphorus in sediments and result in a massive release of phosphorus during future low- oxygen events.

But isn’t oxygen pumping used in lakes?

Oxygen pumping has been used extensively as a restoration technique in lakes. Evidence on the effectiveness of oxygen pumping is mixed. Some research has found that artificial oxygenation can increase the area of fish habitats and reduce the amount of phosphorus in the water. But in other cases, 10-20 years of artificial oxygenation did not reduce phosphorus levels or maintain sufficient oxygen. In a number of lakes, when the pumps are turned off, the internal load returns.

Are there other proposals to stop the internal load besides oxygen pumping?

Yes. For instance removing phosphorus-rich sediments and adding substances to bind phosphorus in the sediments. Both of these techniques have been used in lakes, but not at the scale of the Baltic Proper, the location of the largest dead zone.

Removing sediments seems creative. Could it work?

The challenge with removing sediments is that phosphorus is distributed over very large areas of the sea, so it is relatively diluted. It is not known if there are relatively small areas where large amounts of phosphorus have accumulated and this needs further study. In addition, removing sediments would not have a direct effect on the amount of phosphorus in the water (because of the amount of phosphorus already in the water). The phosphorus in surface waters, along with temperatures and wind, contributes directly to the size of algal blooms.

What about adding chemicals?

It is not known if the chemicals would be effective in deep areas of the open Baltic Sea, because of interactions with the salty water. In addition, there are legal issues associated with adding chemicals to the sea that need further investigation.

Adding chemicals to bind phosphorus has been used in enclosed bays of the Baltic Sea, such as Björnöfjärden; close to Stockholm, Sweden. Here, water clarity has improved and the amount of phosphorus in the water column has decreased, but normal levels of oxygen have not yet returned.

How does the internal load affect local coastal areas?

The internal load is a symptom of eutrophication, as are the high concentrations of phosphorus that are seen in most areas of the sea. In the past century, the amount of phosphorus in the water of the sea has increased about 2.5 times. This increase results from discharge of untreated or poorly treated sewage and fertilizer management practices in agriculture.

The internal load has shifted the amount of phosphorus that is stored in sediments of the Baltic Proper into the water, primarily in deep areas. This does not directly affect coastal areas. The amount of phosphorus in the water along coastal areas is most strongly affected by nearby open sea. For example, if the nearby open sea has a relatively high concentration of phosphorus, the coastal area will as well.

Internal loading can also occur in certain coastal areas, such as enclosed bays, as a result of historical inputs of phosphorus from land. When this happens, the coastal areas lose their ability to “trap” land-based nutrients from entering the sea. In these cases, there could be the possibility of using restoration measures.

There are opportunities to pilot-test restoration measures such as oxygen pumping, sediment removal, and the addition of chemicals to bind phosphorus in enclosed bays and in lakes to increase our understanding of the effectiveness of these techniques.

What are effective ways to address eutrophication in the sea?

The most effective way to mitigate eutrophication is to reduce external nutrient loads to the sea.

According to HELCOM, nitrogen inputs have decreased 19 % and phosphorus inputs have decreased 24% since the mid-1990s. Although the eutrophication status of most parts of the Baltic Sea is still poor, improvement is nowadays seen in some large areas, such as the Eastern Gulf of Finland, Kattegat, and Danish Straits.

This shows that reducing the nutrient input from land works in the long run. And in the Baltic Sea region, there are still great opportunities to achieve major reductions – by aiming for the main input sources, agriculture and sewage.