The development of the lake from its natural state till today
- The entire catchment area affects the lake’s condition

Jaakko Syrjämäki

Lake Tuusula has hardly ever been clear watered. The catchment area of the lake is 92 square kilometres, of which quite large area of 64% is made up of loams. About ten streams and drains run into the lake carrying fine light-weighted clay soil which does not stratify on the bottom immediately but floats in the water making it murky. Clay soil leaching gradually increased as agriculture, that has continued till today, started on the fields around the lake. Some humus water also drained into the lake from the two bogs on the east and west sides of the lake, Tuomala bog in the east and Ruskela bog in the west. Humus waters gave the lake a slightly brownish colour that started to disappear as the bogs were drained and was later covered under the colour of the algae.

As the population and use of fertilizers around the lake increased (first animal manure, then artificial fertilizers) also the fertilizers in the water increased and this slowly started to have an effect on the micro-organisms of the lake, mainly on phytoplankton and through that also zooplankton and other organisms saw an increase. Järnefelt (1937) notes that this phase was evident already in the early 1930s when from time to time algae gave the water a greenish colour. Also the warm summers in the 1930s increased the environment’s loading on the lake. As the population grew after the wars also the wastewaters increased and as they were drained into the lake or into the streams leading to the lake untreated or insufficiently treated, like the town of Järvenpää did, it accelerated eutrophication of the lake, which was especially fast in the 1950s and throughout 1960s.

In addition, in 1959 a dam was built to regulate the water level e.g. during spring floods and as a result the submerged lakeside vegetation further increased the nutrient loading of the lake.

The lake was contaminated and spoiled by the end of 1960s. The water was deep green in colour, mainly due to green algae and blue-green algae and at worst it lacked oxygen already at two meter’s depth. Even the surface water had very little oxygen in it. The condition was at its worst at late winter when organisms and dead organic matter had used up the oxygen of the water and the ice prevented oxygen intake from the air.

The lake was dying slowly. The heavy loading of the lake also led to a different kind of threat. The plankton algae production presupposes that the water is rich in certain nutrients, mostly phosphorus and nitrogen.

The excess produce is then deposited at the bottom of the lake where the microbe activity uses up the oxygen and releases nutrients. The lake thus fertilizes itself and in addition to the nutrients carried in by the streams and drains also the lake’s bottom silt reserve produces and releases nutrients into the water.

At the moment, the bottom of Lake Tuusula is covered in a thick layer of putrid mud. The start of the restoration has slowed down the deposit rate, but it used to be as high as several centimetres per year and it was estimated that the lake would have silted up in a couple of hundred years.

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