As the use of fertilizers around the lake increased (first animal manure, then artificial fertilizers) also the nutrient concentrations in the water increased and eutrophication of the lake got started. The first signs were noted already in 1930s when phytoplankton occasionally coloured the water greenish. As the population grew after the wars also the nutrient loading caused by untreated or insufficiently treated sewage from urban areas increased. This accelerated eutrophication of the lake since the 1950s and especially fast in 1960s and 1970s. The lake was spoiled by the end of 1960s. The water was deep green in colour, mainly due to green algae and cyanobacteria.
The microbial decomposition of organic matter consumes oxygen. At the worst in the 1970s the water lacked oxygen already at two meter’s depth. Even the surface water was poor in oxygen. The condition was at its worst at late winter when decomposition of organic matter had used up the oxygen of the water and the ice cover prevented supplements from the air. As a result of oxygen depletion phosphorus was released from the sediment and the lake thus fertilized itself.
At the moment, the bottom of Lake Tuusula is covered in a thick layer of putrid mud. The catchment management has begun to slow down the deposit rate. 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.
Regulation of the water level of lake was started in 1959. This prevented the spring floods and consequently the reed beds were expanded. In the beginning of the 1990s the regulation was adjusted to follow better the natural range of the water level.