Internal loading returns nutrients that have already ended up in the sea and are bound to bottom sediments back into the water column. The internal load cannot, therefore, be equated with the load flowing in from the catchment area and deposited from the air. Only external anthropogenic loading and natural leaching introduce new nutrients into the sea.

The oxygen situation in the seafloor affects the sediments’ ability to bind phosphorus

On average, more phosphorus is bound to the seabed of the Baltic Sea than is released from it. However, in years where the oxygen levels are low, more phosphorus can be released from the bottom than that deposited on the seabed and bound to the bottom sediment.

Thus, the ability of bottom sediment to bind phosphorus varies. This ability depends on the oxygen situation of the seabed. The oxygen conditions on the bottom, in turn, are affected by the level of marine eutrophication and the internal properties of the sea, such as the tendency of water masses to form layers, i.e. stratification.

A marine area that is enriched by nutrients produces a lot of organic matter that ends up on the seabed. In turn, the decomposition of this organic material consumes the oxygen resources in deep off bottom waters. At the same time, strong water stratification limits the mixing of water caused by weather conditions, such as storms. In such cases, oxygen cannot be transported from the surface layers to the bottom where it is most needed.

The depletion of oxygen in off bottom waters impairs the ability of the sediments to retain phosphorus. This results in phosphorus being released from the bottom deposits and accumulating in large concentrations in the deep water layers close to the seabed.

The sea eutrophicates itself

In the Baltic Sea, the release of phosphorus from the bottom usually takes place in late summer. However, the water stratification prevents the transfer of phosphorus to the surface layers.

The full circulation of water which occurs in autumn and winter eventually mixes the phosphorus released from the bottom throughout the entire water mass. In this way, it affects the amount of phytoplankton produced during the next spring and summer. Thus, the sea area itself is the cause of its own eutrophication.

The release of phosphorus from the seabed into the water column makes the ecosystem nitrogen-limited. This condition favours the occurrence of cyanobacteria (blue-green algae) and their blooms.

Eutrophication caused by internal loading cannot be controlled by man

If there is abundant internal loading, the eutrophication status of a marine area is no longer dependent on the external nutrient load. For example, the total amount of phosphorus in the Gulf of Finland can vary annually considerably more than is possible due solely to the external variations in phosphorus loading.

In the Gulf of Finland, the variation in the amount of phosphorus is largely explained by climatic conditions, such as winds and differences in barometric pressure, which regulate water flow and oxygen situation. As a result, the amount of phosphorus in the Gulf is clearly more affected by the internal processes of the Baltic Sea Proper and the Gulf of Finland than the external phosphorus load.

In the Gulf of Bothnia, internal loading is not a problem due to the good oxygen situation of its seafloor areas.