Shipwrecks as artificial reefs

The first images that come to mind when thinking about reefs are the multicoloured coral reefs of the tropics. Are there any reefs in the Baltic Sea? What is an artificial reef?

Sanna Kuningas

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Sanna works as a research scientist at the Natural Resources Institute Finland

Actual coral reefs are tropical oases of high biodiversity and are found on both sides of the equator. Although reefs can also be found in the Baltic Sea, they are not formed from corals. In the coastal areas of Finland, reefs are often found in the outer archipelago, where the water is clearest and hard rocky surfaces remain clean thanks to the scouring action of the waves.

Composed of rocky or stony ridges and covered by various macroalgae, such as bladder wrack and red algae, the reefs of the Baltic Sea create a bustling life all of their own. In the Bay of Bothnia, where the water salinity is very low, reefs are instead covered with water mosses and various green algae. Among them thrive many of the benthic animals which are also known from saltwater areas. 

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Hard seafloors

Human activity creates artificial reefs

Artificial reefs are man-made structures that provide attachment surfaces, as well as protection for underwater organisms. Artificial reefs can include, for example, different concrete structures or even sunken ships, i.e. shipwrecks.

Many organisms spread to new habitats almost randomly by drifting passively in water currents. Any sufficiently hard and coarse surface can be used as a growing surface. It is most important to get a firm grip on the substrate. After that, the development into either an alga or a mussel can continue.

Many artificial reefs have been built around the world. They are needed because many species have lost their natural habitats due to the destruction of coral reefs.

There are still hundreds of wrecks in the Finnish marine area because they are well preserved here

Throughout time, the Finnish sea area has always been an important maritime area. Shipping traffic in the area is busy and navigation has always been challenging. As a result, there are hundreds of shipwrecks at the bottom of the Baltic Sea.

Compared to other sea areas, old wooden wrecks have remained in remarkably good condition in the Finnish marine area. This is because the shipworms that use the carbohydrates in wood for food do not thrive in such low salinity seawater. As a result, many wooden shipwrecks have been able to rest undisturbed in the protection of the sea, some for more than several hundred years.

In Finnish sea areas, these wrecks areas also act as artificial reefs. They provide an attachment surface for macroalgae, mussels, barnacles, and other organisms. Wrecks, especially those located on soft seafloors, can provide organisms with a hard bottom that is otherwise lacking in that habitat. The composition of the biota that attaches to a wreck’s surface is significantly affected by the depth at the wreck site, as well as the surrounding habitat.

Shipwrecks mainly provide shelter for fish, and no breeding grounds are found in the Finnish marine area.

Did you know?

Shipworms which burrow into and destroy the wood in shipwrecks are not actually worms! In fact, they are bivalve molluscs, whose external appearance makes them look like worms.