Efforts to Save the Lake
There is no easy way to restore the lake and different methods have to be used to achieve the better state of the lake. Restoration measures of L. Tuusulanjärvi aim to decrease the current level of diffuse nutrient loading, correct the disturbances in the food-chain structure and provide better access to the lake for recreation.
The following is a brief introduction of the planned and launched actions to be taken for the improvement of the state of the lake. Most of them are commonly used around the world in lake restoration.
Reducing the non-point loading
The most significant factor behind the eutrophication of L. Tuusulanjärvi is the external phosphorus loading. It is on average four tonnes per year. This is too much: about twice of the critical load. The basis of permanent improvement of water quality of lake is thus a sufficient reduction in the external loading from diffuse sources. It consists of leaching from agricultural areas, untreated sewage of scattered settlements, stormwater from densely populated areas, in addition to the natural run-off and precipitation.
In recent years, there have been large efforts to reduce the nutrient loading from the fields of the catchment area. Most of the farmers are entitled to environmental compensatory aid of EU directives and protective zones bordering drainage channels have been established. The project has offered financial support to farmers for planning and establishing more efficient water protection measures, as protective belts wider than the protective zones awarded under agricultural environment protection aid.
One way to reduce the non-point loading is by building wetlands and settling ponds. Ten wetlands have been built at the brooks in order to withhold the stray solids and nutrients. Wetland construction has been funded by municipalities, government, EU special funding and also local private funds. The wetland of Rantamo-Seitteli (24 ha), finished in 2009, is one of the largest water protection wetlands constructed in Finland.
Decrease of direct inflow of storm waters from urban areas has been supported and municipalities of Järvenpää and Tuusula have built sedimentation pools in new residential areas. Järvenpää has directed the main inflow of stormwaters through the shoreline vegetation.
There are still several hundred households in the catchment area the lake which are not connected to the municipal sewery. This does pose a threat to the lake’s condition since the phosphorus contained in their wastewaters is more available for the algae than the one contained in agricultural run-offs. Therefore, it has relatively a larger effect on the eutrophication. In little townships household sewages could be connected to the municipal sewage disposal system by transfer sewage or to a joint local cleaning system along a joint sewage. In scarcely populated areas individual solutions have to be found for the treatment of wastewaters.
Reducing the internal loading
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 – 2012) is over 100 tons, i.e. 1200 kg/ha 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.
Low-nutrient water is pumped in summer at a rate of 200 l/s to Lake Rusutjärvi from the tunnel that supplies the capital urban district with raw water from Lake Päijänne. This supplementary water makes up about 10% of the total inflow of L. Tuusulanjärvi. At present, the direct dilution effect of this extra water on the quality of the lake is small, but is does prevent the water levels from falling too low during dry spells both in L. Tuusulanjärvi and in the Tuusulanjoki river downstream.
Restoration of the shores
Water-flow in the reed belts has been improved by dredging the shoreline and mowing of the dense reed vegetation. This helps to slow down the rate at which shallow water edges turn to solid shoreline and it was also found beneficial for the reproduction of the pike. Some of these efforts were part of EU Life-project “Management of wetlands along the Gulf of Finland migratory flyway”. Also hornworth have locally been removed in order to ensure recreational use of lake.