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What is internal load?

What is internal load?

There are many questions being asked about internal load and geo-engineering measures. Here, some of them are being answered by Baltic Eye researcher Michelle McCrackin.

hosphorous 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.