often reduce the catchment areas of smaller lakes by redirecting surface water to Lake Mälaren or Lake Saltsjön. While nutritious substances such as phosphorus and nitrogen are mostly derived from agriculture, urban areas produce high amounts of metals and organic compounds. In Stockholm, this mostly applies to central bays – such as Klara sjö, Årstaviken, Ulvsundasjön, Riddarfjärden, and Hammarby Sjö - but also waters surrounded by bungalows and villas – like Långsjön in Älvsjö. Bridges and viaducts The historical name for Stockholm Old Town was "The city between the bridges" (Staden mellan broarna), a name which is still used for the entire city which spans over many islands, islets, and hills. During the course of centuries, the city has seen many bridges relieve each other. In an urban code dated 1350, King Magnus IV (1316-1377) prescribed the bridges leading over Norrström and Söderström to be built and maintained by the city of Stockholm together with six other cities surrounding Lake Mälaren, as they were the only land passage between the provinces Uppland and Södermanland, north and south of the city respectively. Apparently, in the view of the king, the city, a hundred years after its foundation, was still unable to afford maintenance of its own bridges. These first bridges were in no sense technically complicated or physically impressive, but rather simple wooden bridges, either floating bridges or beam bridges resting on poles or stone caissons, in either case with spans of no more than a few metres. The width probably corresponded to the directions for public roads, eight ell or 4,8 metres, which was probably more than enough for many centuries. The long and narrow bridges were easily demolished in case of siege, which besides the drawbridges, also necessary for the passing of ships, was an important defensive strategy. As the accounts of the city tells, spring floods and ice break-ups resulted in the frequent destruction of the bridges. By the mid 17th century, the population of the city had resulted in settlements north and south of Gamla stan, on Norrmalm and Södermalm, and the number of bridges had grown considerably, if not their dimensions or quality. In a map dated 1640, three bridges connects Stadsholmen to Norrmalm passing over Helgeandsholmen, at the time still a group of islets; while two bridges close to each other lead over to Riddarholmen. Several new bridges of considerable length connected Norrmalm to the