The Baltic Sea has been an important shipping route for thousands of years. Therefore it has been also of great strategic importance for the navies of many countries. No wonder that on its bottom there may be more than 100 thousand wrecks, both ancient and modern ones.
The Baltic is an enclosed shelf sea, thus it is relatively shallow - its average depth is just 52 msw. None of its parts has the average depth greater than 70 msw, and about 80% of its bottom lies at a depth of 50 - 100 msw. This creates favourable conditions for exploration.
The Danish straits (the Great Belt, the Little Belt, the Sound), through which the Baltic connects with the ocean, are also very shallow. Their maximum depth is only 38 msw. This causes significant difficulties in the exchange of water masses and, apart from a huge fresh water supply from rivers and little evaporation, is one of the main features that decide about the oddity of this sea. The Baltic is in fact the largest stable brackish water habitat of the world. Its waters are of much lower salinity (7 - southern Baltic) than typical ocean waters (35), which has a substantial impact on the conservation status of objects lying on its bottom. In the Baltic Sea, in comparison to other saline seas, metal objects remain in a much better condition for longer periods of time.
Another important characteristic related to hindered water exchange through the Danish straits, is oxygen content. Infusions of oxygen-rich ocean water are rare (tend to happen every few months or even every several years) and of short duration (from a few to several days). Over time, oxygen dissolved in the water overlying bottom gets completely utilized and in many deeper located places so called “dead areas” or “biological deserts” with limited biological life are formed. From the point of view of archaeology however, this has a beneficial effect on the state of preservation of materials of organic origin (e.g. wood, leather, textiles, basketry) since decomposition of organic matter effectively isolated from the oxygen proceeds very slowly. Other abiotic factors which should be also mentioned is the cold and lack of light, but the most important is the fact that the low temperature fluctuations at the bottom are minimal which positively affects the artefacts. Higher temperatures favour growth and development of microorganisms that can have a devastating effect on most of the archaeological material.
Perhaps the most important example of biological life that threatens shipwrecks is a naval shipworm Teredo navalis – a mussel feeding on wood. Its negative impact on submerged wood elements was recorded since the eighteenth century in many places of the world as well as in the North and Baltic sea. The mass appearance of this species, lasting several years, took place in the Baltic Sea near Germany, Denmark and southern Sweden in the 30-ies and 50-ies of XX century. Since 1993 shipworms were observed along the west coast of Mecklenburg-Vorpommern. Some sources have reported it even in several places along the Polish coast, but most likely it was dragged there with wood by the currents. Currently, the occurrence of T. navalis is limited to Germany, Denmark and southern Sweden. Further propagation of the species is impossible due to the low salinity prevailing in most parts of the Baltic Sea. Its reproduction is possible only in waters with salinity higher than 8. That is more or less up to the island of Rügen. Thanks to that wooden wrecks of sunken ships, such as the Swedish galleon Vasa, were not completely destroyed and survived under water for several centuries.
Piotr Bałazy - marine biologist & scientific diving officer at the Institute of Oceanology Polish Academy of Sciences in Sopot. Moved to the seaside from south part of Poland nearly 15 years ago and still believe its one of the best places to live.. Why? See above. Passionate about diving (wrecks, polar ecosystems) and uw photography...
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