3.2. Possibilities to alleviate harmful effects

3.2.1. Alternatives to restoration

The littoral environment of a regulated lake is under ecological pressure due to erosion caused by ice and waves. Erosion processes are stimulated by the altered water level and enhanced by the water level fluctuation (I, II, III). In general, there are three alternatives to mitigate the negative effects in the littoral zone (Table 1). The first possibility is to trust in natural succession, which ultimately leads to stabilisation through sedimentation and settling down of the eroded shores (Rørslett & Johansen 1996). This alternative includes several benefits: there are no costs and no need for guidance. For example, the follow-up in Lake Kemijärvi, which has been regulated by 7 meters since 1966, shows that the abundance of species was higher in 1998 than in 1983 (Hellsten et al. 1999). Another example of Swedish storage reservoirs showed similar enrichment, although the trend was downward again after 30-40 years (Nilsson et al. 1997). It has been shown in III that succession in the short run does not change the set of species already present in the littoral. The second choice is to apply active restoration methods, including mechanical protection of shores, shoreline modification and planting of new tolerant species (IV). The third route includes changes in the regulation practice, which improve the environment of the littoral subjected to water level fluctuation (V).

All of these methods or strategies of shore management have both positive and negative effects (Table 1). From the lake managers point of view, a combination of different methods is the most suitable way to improve the littoral ecosystem of a regulated lake. It should be noted that, without changes in the regulation practices, the possibility to achieve the desired recovery of littoral is limited (cf. Nilsson 1996).

3.2.2. Revegetation studies

As shown in the previous chapters, the littoral environment of Lake Ontojärvi is bare and vulnerable due to the water level fluctuation. Therefore, the planting experiments carried out on the shore were quite risky since the beginning (IV). After the first summer, the average survival rates were about 45 %, because the seedlings tended to dry up due to the low water level, which was almost 50 cm below the normal. The survival rate was slightly higher in the sheltered area with a lower elevation, where 60 % of the plants were alive. As a result of freezing and ice erosion 10-15 % of the plants died during the first winter. The second year was characterised by extraordinarily high water levels causing a sudden drop in the survival rates. Only in the sheltered area protected by floating timbers did the survival rate stay above 20 %.

In the uppermost part of the littoral, the possibility of drying is also obvious due to regulation (Rørslett 1984, I, II). The effect of extending ice during the wintertime also causes scouring and freezing of the bottom sediment, as has been described for several lakes and rivers (Rørslett 1984, 1987c, Erixon 1981, I). Thus, the rapid decline of planted species was quite expected in the case of Lake Ontojärvi.

The difference between the different soil treatment plots was relatively small; only the erosion carpet provided a better survival rate during the first summer, but the second-year erosion caused a rapid decrease in survival (IV). Oat as a protective plant provided the lowest survival rates for planted helophytes, because wild ducks grazed on the young oat seedlings and concurrently plucked planted individuals (cf. Bache & MacAskill 1984). The bottom substrate is one of the most important factors affecting aquatic macrophytes (e.g. Barko & Smart 1983). Substrate can be improved by erosion barriers (see Allen et al. 1984) or by adding fertilisers or organic matter (e.g. Fowler & Maddox 1974, Broome et al. 1988). In the case of Lake Ontojärvi, both of these methods failed due to the high water level during the second year. This concerns also erosion carpets, which were covered by eroded sand (cf. Allen et al. 1984).

The best results were obtained for the helophyte Carex rostrata, of which 30 % were struggling against erosion (IV). Tall willows (Salix phylicifolia) were also erosion-resistant with a survival rate of 80 %. Phragmites australis and Juncus filiformis disappeared quite soon, even though the latter is quite resistant against erosion (II). The latter helophytes are typical C-strategists, which are good in competition, but slow in extending their living areas (see Grime 1977, Murphy et al. 1990). All the species with good resistance against erosion are small R-strategists (e.g. Ranunculus reptans, Eleocharis acicularis), which are difficult to use as experimental plants and do not provide any natural protection against erosion.

One of the most promising results was achieved with tall willow seedlings (cf. Comes & McCreary 1986). Willows are easily collected and resistant against flooding. On the other hand, the willow carpets dried or eroded during the first year of our study, whereas many other studies have shown it to be a good method (Bache & MacAskill 1984, Bagemann & Schiechtl 1986). Re-design of the shore could also have been a solution against erosion as proposed by Allen & Klimas (1986).

The experimental areas were re-studied again in 1994 and in 1998 (Fig. 4). The observations, those made eight years after the planting, confirmed the results of IV. The tall willows were still alive, but they had not expanded their living area and their survival rate was less than 40 %. On the other hand, the survival of Carex rostrata had increased and was more than 40 %. It seems that wave erosion had transported planted species to the upper part of the eulittoral, where the environment was more stable. It should also be noted that small Phragmites australis individuals were still present (< 5 %) near the shoreline, which proves their tolerance against freezing if the erosion is not too severe.

According to the 1998 results, the average survival rate of the planted species was 9.3%. There were no significant differences between the different treatments, except the slightly higher (17.5 %) survival rate in the peat treatment. The survival rate was also some what higher (13.4 %) on sheltered shores compared to value of 5.3 % on open shores. The follow-up proved the adaptive nature of Carex rostrata, which is also one of the few helophyte species more common in Lake Ontojärvi than in Lake Lentua (II).

3.2.3. Changes in the water level fluctuation regime – ecological regulation practice (ERP)

Changes in water level due to hydropower regulation cause extensive changes in shore processes. The replanting of shore vegetation is not effective without any mechanical wave-protecting barriers, although some species can survive in hard environmental conditions (IV). The effects of different soil improvement methods also seem to be quite unimportant in the area of abundant erosion. Also, the removal of aquatic weeds in a lake with a lowered water level is usually only an emergency measure for the problems caused by altered shore processes. The situation can be changed only by an elevation of the water level or by small-scale dredging operations in the outermost zone of the shore (Keränen 1985, Nykänen 1998).

Ecologically-based regulation (ERP) requires careful investigations and interpretation of the results before any recommendations can be given (e.g. Marttunen & Hellsten 1997). In the case study of Lake Kostonjärvi, the efforts were aimed at a reduction of the area affected by bottom freezing, which covered the whole productive zone in the lake (V). In the ERP, the water was kept at a higher level during the early winter compared to the old regulation practice. Consequently, the water level was lowered during the late winter to minimise the frozen bottom area. Moreover, the minimum water level in the early summer was set to a level, which allowed spring-spawning fishes to reach their spawning grounds. The water level of the open water period was not altered significantly, because the rocky shores were already adapted to the higher water level and the proportion of sandy shores was only 4 % of the total shoreline (V).

The ecologically-based regulation practice should reduce the area of frozen bottom by 24 %, which should increase the growing area of large isoetids with a positive impact on the zoobenthos and the autumn-spawning fishes (V). From the viewpoint of hydropower production, however, the effects were quite significant and caused significant losses during the winter, when discharge was limited. The value of fishery was not calculated in this study, but, apart from its indirect effects, the ecologically-based regulation practice would definitely also increase the area available for winter fishing as well as the fishing of spring-spawning fishes. The value of recreational use was not estimated in this case study.

In spite of the high costs of the ERP, these water level guidelines have been voluntarily followed by the hydropower company (PVO Group Ltd) since 1992 (V). For the following 3 years, the water level successfully remained between the ERP targets with the exception of the year 1994, when the winter water level dropped rapidly due to the dry autumn 1993. The extremely wet summer 1992 caused the water level to be very high in July and August. The short field investigations at Lake Kostonjärvi during the summer 1998 showed that there were some positive changes in the shore biota. Ice-sensitive Isoetes lacustris was found on one sheltered shore, and the presence of erosion- sensitive Equisetum fluviatile indicated an enhanced littoral environment (unpublished data). On the other hand, the exceptional years will probably greatly disturb the succession of isoetids (e.g. Rørslett 1985b).

Another case study at Lake Oulujärvi was designed to decrease the emergent vegetation expanding on the littoral due to the lowered summer time water level (V). In a preceding research the impacts of hydropower production, recreational use, flooding and fishery were estimated (Marttunen & Kaatra 1993). The annual loss caused by the current water level regulation for recreational use was US $ 0.34 million, whereas in the planned ERP it would have been US $ 0.26 million due to the excessively high summertime level (Aittoniemi 1993).

Due to the notable loss in energy production and partly also to the high level of recreational use, the ERP was rejected at Lake Oulujärvi and an intermediate water level regulation plan was developed with a target level of 122.5 m on the 20th of June instead of the beginning of July (V). The target had been reached during 3 of the 4 years, with the exception of the year 1992. During the very dry summer of 1993, the water level dropped below the target level in August. The water level during the summer did not completely prevent the spread of emergent vegetation and more attention should be given to the other management and rehabilitation methods, including the removal of the vegetation from the shores of Oulujärvi under intensive recreational use (Keränen et al. 1992). The small-scale dredging of vegetation was, however, quite useless, because the vegetation returned quite rapidly. The method is economically suitable only to relatively limited shore areas.

Despite the rejected ERP, the tendency towards high summer water levels since the 1980’s has led to improvements in the littoral environment. Nykänen (1998) noted, in a vegetation follow-up, that vegetation has decreased after the raise of the water level. A strategy analysis of aquatic vegetation showed that there was a slight change from competition-oriented to stress-oriented vegetation (Nykänen 1998). The change in succession status showed that the stability of vegetation is slowly increasing.