| Environmental factors and aquatic macrophytes in the littoral zone of regulated lakes: Causes, consequences and possibilities to alleviate harmful effects | ||
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The methods of ecological regulation practices (ERP) were mostly based on the results of the studies I and II, originally presented in Hellsten et al. (1989) and in Alasaarela et al. (1989a, c). These practices were based on voluntary work by the permission owners and the water authorities. Regulation was developed within the allowed water level fluctuation regime without changing the legal permissions. In this study, ERP methods were applied to Lake Kostonjärvi and Lake Oulujärvi (for details, see paper V). In the ERP procedure, the responses of the littoral to the water level fluctuation were analysed carefully, and the following measurements were then made.
During the open water period there was a tendency towards a declining water level in Lake Kostonjärvi, where, due to the raised water level, the littoral was bare and unstable. In addition to the mean water level, special attention was paid to the dynamics of the water level, which should follow the natural lowering trend during the summer, offering optimal possibilities for proper development of the emergent vegetation. On the other hand, the beginning of regulation in Lake Oulujärvi resulted in a lowering of the water level, which led to a rapid invasion of shore vegetation on the gently sloping sandy or silty shores (Keränen et al. 1992). Therefore, the main aim of ERP after the lowest water level was to lift the water as early as possible and to keep it at a sufficient level to reduce the plant invasion.
Another aim was to minimise the effects of the lowering of water level during the winter by shifting the lowest level towards the spring. The final goal of the procedure was to keep the productive littoral partly non-frozen (I, II). This was considered especially important for Lake Kostonjärvi, where the productive part of littoral was completely frozen. This aspect was also taken into account in Lake Oulujärvi.
The annual range of water level fluctuation was also used together with the Secchi disk transparency values to estimate the production of the macrozoobenthos biomass (Bm, mg m-2) in the 0–3 m depth zone according to Palomäki (1994). This calculation method was used to compare the differences between the different regulation practices.