Being a quite narrow and shallow lake, Mälaren has bridge crossings betweenEskilstuna andVästerås with two crossings on the western end atKvicksund and three separate bridges betweenSträngnäs andEnköping in the central part of the lake. On the eastern end, the entirety ofEkerö Municipality is set on islands within Mälaren. That urban area also has a bridge connection to the mainland in Stockholm along with bridges between various islands in the municipality.
The barrow of Björn Ironside (Björn Järnsidas hög) on the island of Munsö, in lake Mälaren,Sweden. The barrow is crowned by a stone containing the fragmentedUppland Runic Inscription 13.
Theetymological origin of the nameMälaren stems from theOld Norse wordmælir appearing in historical records in the 1320s and meaninggravel.[5] The lake was previously known asLǫgrinn, which isOld Norse for 'The Lake'.[6]
By the end of the last ice age about 11,000 years ago, much of northern Europe and North America was covered byice sheets up to 3 km (1.9 mi) thick. At the end of the ice age when the glaciers retreated, the removal of the weight from the depressed land led to apost-glacial rebound. Initially the rebound was rapid, proceeding at about 7.5 cm (2.95 in) per year. This phase lasted for about 2,000 years, and took place as the ice was being unloaded. Once deglaciation was complete, uplift slowed to about 2.5 cm (1.0 in) per year, and decreased exponentially after that. Today, typical uplift rates are of the order of 1 cm (0.39 in) per year or less, and studies suggest that rebound will continue for about another 10,000 years. The total uplift from the end of deglaciation can be up to 400 m (1,312 ft).[citation needed]
In theViking Age, Mälaren was still a bay of theBaltic Sea,[7][8] and seagoing vessels could sail up it far into the interior of Sweden.Birka was conveniently near the trade routes through theSödertälje Canal. Due to the post-glacial rebound, Södertälje canal and the mouth ofRiddarfjärden bay had become so shallow by about the year 1200 that ships had to unload their cargoes near the entrances, and progressively the bay became a lake.[9] The decline of Birka and the subsequent foundation ofStockholm at the choke point of Riddarfjärden were in part due to the post-glacial rebound changing the topography of the Mälaren basin. The lake's surface currently averages .7 m (2.3 ft) abovesea level.
According toNorse mythology as contained in the thirteenth-century Icelandic workProse Edda, the lake was created by the goddessGefjon when she trickedGylfi, the Swedish king ofGylfaginning. Gylfi promised Gefjon as much land as four oxen could plough in a day and a night, but she used oxen from the land of the giants, and moreover uprooted the land and dragged it into the sea, where it became the island ofZealand.Snorra Edda says that "the inlets in the lake correspond to the headlands in Zealand";[10] since modern maps show this to be more true of LakeVänern, the myth has been suggested to have been originally about Vänern, not Mälaren.[11]
One of the characteristic species is theosprey which has one of its strongest presences in Lake Mälaren. The osprey nests in almost all bays of the lake.[12]
Lake Mälaren has 33 species of naturally occurring fish species, which makes it Sweden’s most diverse lake in regard to fish. Other species have been introduced to the environment, including the common carp and the rainbow trout. The rainbow trout in particular is known to compete with indigenous fish for habitat and food, as they have a faster growth rate and predate on local species. However, they are not considered to have made a significant impact.[13]
Crayfish are of large cultural and economic importance in Sweden, withcrayfish parties being a longtime annual tradition for many Swedes.
Lake Mälaren was the first lake in Sweden to be affected by thecrayfish plague (A. astaci Schicora) when infected crayfish imported from Finland were introduced to the lake with the intention of human consumption. The plague spread quickly through the lake, exacerbated by boat traffic, which decimated the indigenousnoble crayfish (A. astacus) population and caused severe economic losses to the local fishing industry. From Mälaren, the plague spread rapidly to all freshwater bodies in Sweden.
After multiple recurrent outbreaks and failed attempts to restore populations of the noble crayfish, Swedish authorities introduced the North Americansignal crayfish (P. leniusculus) to L. Mälaren, a species that is resistant to the plague. Since 1969, Mälaren has been continuously stocked with signal crayfish.[13] However, it was later discovered that signal crayfish were often carriers of crayfish plague. In addition, while populations of noble crayfish and signal crayfish have been known coexist, their larger size, faster growth rate, and aggressiveness often allows them to dominate populations of noble crayfish. This, in addition to habitat degradation,[14] has led to a significant decrease in noble crayfish populations in L. Mälaren.
Signal crayfish have been largely successful in Sweden and rapidly expanded as a replacement population to noble crayfish. Stocking this species is now prohibited within multiple lakes in Sweden in order to protect noble crayfish population. In the Stockholm municipality, which includes L. Mälaren, signal crayfish are now illegal to import, move, or farm.[15] There have been efforts to encourage farming of noble crayfish populations instead.[16] However, illegal farming of signal crayfish continues to be a problem due to greater public demand for the larger signal crayfish.[17]
One of the earliest maps of the lake, made by cartographerAnders Bure in 1614
From the mid 17th century onwards, multiple expeditions to measure and map lake Mälaren were undertaken. The lake was strategically and economically important, and there was a lack of a reliable map to navigate it. However, L. Mälaren’s many islands and bays made it an extensive and laborious task to measure.
In 1687,King Charles XI of Sweden commissioned cartographerCarl Gripenhielm to map Lake Mälaren and its surrounding provinces.[18] From 2 August 1688 to 17 September 1688 Gripenhielm conducted the bulk of his expedition, and he would complete the map in 1689, known in Swedish asGripenhielm'sMälarkarta. The Mälarkarta is unique in that it is surrounded by a frame of 96 small, realistic gouache paintings of cities, castles, mansions, and fairways around Lake Mälaren. The map is hand drawn on regal paper, and measures 3.46 m × 2.14 m (11.4 ft × 7.0 ft). It has been kept in theNational Library of Sweden since 1884. Due to its larger size, it was initially hung in the large viewing room, however in 1931 it was moved to a more secluded wall in a corridor to the map department. Heavy handed preservation techniques have led to some degradation on the illustrations. Since 1961, the map hangs in an area that the public does not have access to.
Gripenheim’s Mälarkarta was valid until 1739 when a new, more accurate map of L. Mälaren was completed by the then surveyor and cartographerJacob Nordencreutz.
^Capinha, César; Larson, Eric R.; Tricarico, Elena; Olden, Julian D.; Gherardi, Francesca (August 2013). "Effects of climate change, invasive species, and disease on the distribution of native European crayfishes".Conservation Biology.27 (4):731–740.doi:10.1111/cobi.12043.ISSN1523-1739.PMID23531056.S2CID25775481.
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