The conditions of Järlasjön

April 6, 2018

Outlets for surface water.jpg

Järlasjön is the largest lake in the municipality of Nacka. It is located in a large catchment area with five other lakes, where Järlasjön is located at the bottom of the river. The lake consists of four pools separated by narrow outlets. The lake is naturally nutritional, but today it is nutritious and there is a need to reduce the supply of nutrients and other pollutants. Measurements of oxygen levels also show that there is oxygen deficiency in the bottom water.

The lake is situated in between areas with high exploitation rates, with dwellings, roads and industries. The lake is nutritious since it has received pollution from surrounding households and industries for a long time. Both nutrition and high levels of heavy metals have therefore accumulated in the sediment at the bottom. This is especially true outside the Järla industrial area. Today, Järlasjön still receive large amounts of contaminated water. Several pump stations have disaster emissions to the lake.

The lake is heavily layered in summer, which means that the surface water rarely comes into contact with the nutritional and contaminated bottom water. This reduces the risk of algal blooms but still creates problems for underwater life. The visibility depth is small (1.0-2.5 meters). Oxygen deficiency often occurs in the bottom water around 10 meters deep, and is often higher towards the end of the growing season and lower in the wintertime.

Over-fertilization has negative effects on the biodiversity of the lake. The relationship between organisms is disturbed in the water, vegetation that provides protection for birthplaces for parr and other animals is affected and large amounts of plant material can fall to the bottom, creating oxygen defiency, which in turn strikes out aquatic animals and fish. The shortage of aquatic animals and fish leads to less food for other animals in the lakes ecosystem, for example for the mallards that inhabit the lake.

As part of the work on achieving good status under the EU Water Directive, 2015 was a description of the proposed plan for the Järlasjön. The report was later supplemented with a study of water interchange in the lake.

To change the conditions of the lake the proposal was for example to limit pollutants in the feeds from surrounding areas and to use aeration, pumping or aluminium deposition. In the study of water interchange it was proposed that the negative trend of water transparency diminishing was probably caused by plankton conditions and may possibly be solved by biological methods (biomanipulation), such as implantation of fish. To ensure it would work, the recommendation was to take samples of environmental pollutants and other harmful substances that may affect the conditions for the fish. The water exchange between sub-basins was also recommended to be investigated to know how the sub-basins affect each other and whether there is enough directed effort in one part of the lake or not. Possibly, an improved water exchange between the pools could be a measure to improve oxygen conditions.

Felicia Svensson




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