INTERNAL LOADING

Aeration

As a result of oxygen deficiency, large amounts of phosphorus can be released from the sediment into the water. In order to prevent the oxygen depletion, the lakei has been aerated ever since 1972, initially only during the winter, but nowadays also through the summer. From 1998, several Mixox aerators have been run simultaneously, with the aim of maintaining a continuous water flow. Since then, the concentrations of oxygen have remained higher and those of phosphorus lower in the summer. In the winter months, only one aerator in needed.

Fish removal and management of fish stocks

The goal of the fish removal is both to restrict the growth of cyanobacteria and to make the lake more attractive to recreational fishing.

The fish removal affects the internal loading of the lake by reducing the cyprinid fish stock especially bream and roach, that stir up nutrients from the bottom and thus increase the growth of the cyanobacteria. Another advantage of the fish removal is that by a lower density of planktivorous fish the zooplankton is able to graze the algae more efficiently and the turbidity of the water caused by the algae is reduced.

The fish removal in the lake was started in 1997 in co-operation with municipalities, the local angling club Tuusulanjärven Urheilukalastajat and the local fishing associations. The total catch (1997 – 2021) is over 1000 tons consisting mainly of roach and bream. Nevertheless, the cyprinid stock still remains far too high. Fishing has to be continued until the sustainable situation, where the naturally reproducing predatory fish stock will be able to control the cyprinids, is achieved. This may not be possible until the external nutrient load is clearly decreased.

The improvements in the fish stocks will be maintained by stocking of predatory fish as eel and pike. The population of pikeperch, the most favoured species by fishermen, has been sufficient in most years thanks to a mesh limit of 55 mm decided by the local fishing associations.