<p><p>Eutrophication has been a problem in aquatic systems for a long time. It is well known that the key nutrient to control the eutrophication process is phosphorus. Lately, it has been possible to reverse the process and recover lakes from eutrophication.</p><p>During a long time Lake Näsnaren in Katrineholms kommun, Södermanland, has been exposed of a large amount of phosphorus from the city's wastewater. The lake has a great value as a habitat of rare species but also a high recreation value for people living in Katrineholm and Södermanland. Lake Näsnaren is situated in a Natura 2000 area. This work aims to investigate Lake Näsnarens condition today, reasonable measures to reverse the eutrophication process and Lake Näsnaren's situation in the future.</p><p>Lake Näsnaren has today reached a very high level of phosphorus. Reducing the external load of phosphorus from fertilizers can reverse the eutrophication process. Increasing the depth also has a reversing effect.</p><p>If no measures are taken, it is likely that the lake overgrows. To reduce the lake's phosphorus values, more extreme measures have to be taken, like dredging or decreasing the load of phosphorus. However, Lake Näsnaren is shallow and reacts quickly to changes in the external load.</p></p>

Eutrophication has been a problem in aquatic systems for a long time. It is well known that the key nutrient to control the eutrophication process is phosphorus. Lately, it has been possible to reverse the process and recover lakes from eutrophication. During a long time Lake Näsnaren in Katrineholms kommun, Södermanland, has been exposed of a large amount of phosphorus from the city's wastewater. The lake has a great value as a habitat of rare species but also a high recreation value for people living in Katrineholm and Södermanland. Lake Näsnaren is situated in a Natura 2000 area. This work aims to investigate Lake Näsnarens condition today, reasonable measures to reverse the eutrophication process and Lake Näsnaren's situation in the future. Lake Näsnaren has today reached a very high level of phosphorus. Reducing the external load of phosphorus from fertilizers can reverse the eutrophication process. Increasing the depth also has a reversing effect. If no measures are taken, it is likely that the lake overgrows. To reduce the lake's phosphorus values, more extreme measures have to be taken, like dredging or decreasing the load of phosphorus. However, Lake Näsnaren is shallow and reacts quickly to changes in the external load.