Chapter 1. Introduction

Finland is a land of thousands of lakes; there are more than 187, 000 lakes with an area of more than 0.05 ha (Raatikainen & Kuusisto 1988). This amounts to a total area of 32,600 km2, which is approximately 10% of the total area of Finland. In over one-third of this lake area (11,900 km2, including nearly 220 lakes of > 1 km2), water levels are regulated (Alasaarela et al. 1989a). In northern Finland, a larger proportion of lakes are regulated than in the other parts of the country, and the main objective of lake regulation in this region is to generate hydroelectric power, with an option for flood protection. In a typical northern regulation scheme, the water level is raised by 0.5 - 3.5 m in the summer and lowered by 2 - 7 m in the winter, whereas in southern Finland the water level in the open water period remains unchanged or is lowered (Alasaarela et al. 1989b, Marttunen & Hellsten 1997).

The littoral undergoes considerable geomorphologic changes during the initial stages of lake level regulation, especially if the water level is raised to increase the storage capacity of the lake (Sundborg 1977, Nilsson 1981a, Newbury & McCullough 1984, Mark 1987, Mark & Kirk 1987, Rørslett 1988a, Hellsten & Alasaarela 1984, Alasaarela et al. 1989b, Wilcox & Meeker 1991). This leads to major geomorphologic changes, including breakdown of the organic surface layer and erosion of minerogenic matter. The water level is usually lowered during the winter, when the need for electricity is greatest and the enhanced storage capacity for the spring flood is important. As a consequence, the ice layer may extend down to the bottom, causing the sediment to freeze and to be partly eroded by scouring (Nilsson 1981a, Erixon 1981, Rørslett 1985a, Palomäki & Koskenniemi 1993, I). As the water level is kept relatively low during the early spring, the spring flood is much lower than before and tends to shift towards the midsummer (Alasaarela et al. 1989a, Wilcox & Meeker 1992).

Aquatic macrophytes are an essential part of the productive zone in northern lakes (Pearsall 1920, Hutchinson 1975, Spence 1982). The littoral vegetation provides a living habitat for benthic fauna and zooplankton and a feeding area for fish (e.g. Tikkanen et al. 1988, 1989, Wilcox & Meeker 1992). Moreover, it serves a spawning ground for several fish species and is also a favourable habitat for fish fry (e.g. Huusko et al. 1988, 1989, Wilcox & Meeker 1992). Alterations in the structure of the vegetation tend to bring about dramatic changes in their living conditions, leading to a decrease in the production of zooplankton (e.g. Lindström 1973, Selin & Hakkari 1982, Huusko et al. 1988), benthic fauna (e.g. Grimås 1961, 1962, Palomäki 1994) and spring spawning fishes (e.g. Gaboury & Patalas 1984, Huusko et al. 1989, Wilcox & Meeker 1992). The littoral vegetation, especially emergent plants, is also a main component of the shore landscape. If it disappears due to water level regulation, the lake becomes less attractive for recreation. On the other hand, the excessively dense vegetation often stimulated by the lowered summertime water level hinders the use of the shoreline, which can be considered a negative impact (e.g. Pieterse & Murphy 1990). In this context, it should also be taken into consideration that the littoral vegetation protects the shoreline against erosion by waves and currents (Foote & Kadlec 1988, Raspopov et al. 1988, Coops et al. 1991, 1994, Keränen et al. 1992, Juntura et al. 1999).

A majority of the changes in the littoral zone of regulated lakes result from changes in water level fluctuation. These erosional processes affect directly the littoral zone, causing destruction of vegetation and affecting the successional status of vegetation, as reported in several Scandinavian lakes (Nilsson 1981a, Rørslett, 1985a). The new vegetation on eroded shores consists of disturbance-tolerant species (e.g. Ranunculus reptans, Eleocharis acicularis) adapted to the altered ecological environment (e.g. Murphy et al., 1990), which is under succession for several decades (Koskenniemi 1987, Nilsson & Keddy 1988). On the other hand, the disappearance of large isoetids due to the increased effect of ice has been reported in several cases (Quennerstedt 1958, Rørslett 1984, 1985a,b, 1987b,c, 1988b, 1989, Renman 1989, Rintanen 1996). The effects of water level regulation are also related to water quality. When the water is transparent, the negative effects of a fluctuating water level are less severe, due to the wider productive zone, than in the case of turbid water (Rørslett 1988a, Alasaarela et al. 1989a, Palomäki 1994). The effects of water level regulation on lakes have been under intensive research in Finland since the early 1980s (Alasaarela 1988, Alasaarela et al.1989a). However, reports dealing with littoral vegetation are rare, and most of them have been published in Finnish (Granberg & Hakkari 1980, Granberg & Ruohonen 1985, Hellsten & Joronen 1986, Hellsten et al. 1989).

The main objectives of the present study were:

1. to identify and describe the relationships between aquatic macrophytes and the environmental factors affecting the littoral zone of a lake following regulation (I, II),

2. to develop a preliminary model in order to estimate the effects of water level regulation practices on aquatic macrophytes (II, V),

3. to compare the stability of aquatic vegetation by means of permanent plots (III),

4. to estimate the possibilities to regenerate the shore vegetation by means of planting (IV) and

5. to develop and apply an ecological regulation practice (ERP) to northern regulated lakes (V).