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Methods for treatment of lakes in which the water level has been lowered.
In order to obtain access to arable land, the water level of a great number of Swedish lakes has been lowered. However, the aeration of the organic sediment soils made available for agriculture, resulted in rapid subsidence. Therefore, after some decades, this type of water-level lowering mostly created problems and resulted in degraded lakes overgrown by macrophytes, surrounded by areas that were impossible to cultivate because of the ground being too wet. Some of the lowered lakes used to be outstanding waterfowl biotopes and produced both fish and crayfish. Efforts are now being made to restore them to their former value.
The most serious damage following the lowering of water levels in shallow lakes is the development of a root felt in the top sediment layer, and the accumulation of coarse plant material (detritus) produced by the highly productive wetland macrophyte vegetation. (After lowering of a shallow- lake as well as insufficient increase in water levels as a means of "restoration" such areas usually pass through a transient period of fluorishing birdlife). Before the decisive step for restoration of a lake/wetland - raising the water level - is taken, the bottom has to be treated in order to get rid of the coarse plant material and the root felt. If the water level is to be raised over areas overgrown by sedge, reed, bulrush, etc. the gas produced in the bottom would otherwise accumulate in and beneath this root felt. After some time the felt would, therefore, be floating at the water surface and would soon be overgrown by new vegetation (plaur formation).
The goal for most restoration projects in lowered and overgrown lakes is to create an open water area and a mosaic of open water and emergent/ submerged vegetation. The restored lake/ wetland should be brought to a stage of permanence where no future extensive management program is needed. For responsible, long-term planning of wetland restoration, management and protection, this is of the utmost importance.
Fertile wetlands are highly productive and the aging process takes place with high speed in the types referred to here, i.e. shallow lakes in the last phase of their ontogeny, before the transition towards terrestrial conditions. It would be a sisyphean task, demanding continuous funds, to keep pace with and to try to overcome rapid natural processes that work in the opposite direction. It is, therefore, important that the goal for wetland-redevelopment projects is set at restoring the area in such a way that it attains the characteristics of the phase preceding that of the fastest aging.
The Lake Hornborga Case
The Swedish restoration project, which has reached a high degree of notoriety, is the case of the shallow Lake Hornborga (Hornborgasjön, 28 km2). The lake was drained and became completely overgrown by emergent vegetation. According to governmental directives investigations to determine whether or not the lake could still be restored and whether restoration would be sufficiently permanent were to be carried out. The investigations started in 1965 and plans for restoration, based on limnological and ornithological facts, were presented in 1973. The plans were accepted by the Government and the Riksdag (Parliament) and the Water Court granted permission to raise the water by 1.4 meter in two steps. However, since 1973 new planning and replanning have brought the project back to scratch.
The degraded lake was overgrown by large, homogenous stands of the common reed, Phragmites australis, covering 11 km2. Sedges, bushes, etc. covered the remaining area, of which about 16 km2 were predestined for plaur formation following the raising of the water level. With the governmental directives as guidelines, the necessary limnological treatment measures and developmental prognoses were based on the specific conditions in the degraded Lake Hornborga itself. As the conditions in the lake are unique, and not generally comparable to those in any other lakes/ wetlands, planning must be individually designed.
The further the aging processes (filling in with sediments, etc.) have advanced in shallow lake basins, the more difficult it is to restore them by raising the water level. Ultimately, it becomes impossible. It is still possible to restore Lake Hornborga by this process. However, if the lake should be restored by raising the water level, then that part not covered by revegetated plaur and bottom-rooted emergent plants must be large and deep enough to give the area the longevity stressed in governmental directives, and not just temporary duration as wetland. The open water should act as the heart or kidney of the whole area, constituting an integrated part of the complex system, powerful enough to preserve the vitality after restoration. The timescale of the duration of the results would otherwise be a matter of decades, not centuries.
Vague planning concerning the construction, or nonconstruction, of dikes to protect arable land, their location, etc. suddenly (1982) ended in a proposal to raise the water level in the overgrown lake by only about 80 cm. This meant that government directives would not be followed, and that the project would be doomed to failure because of redeposition of huge masses of settled matter and heavy production of detritus-forming vegetation. Such a small rise in the water level is totally inadequate for the severely damaged Lake Hornborga basin. Every rise in the water level would cause a flourishing period for waterfowl, but the lower the rise the shorter the period. Furthermore, after a very limited increase in water level, chain reactions including, e.g. periodic oxygen deficiency and nutrient release will appear and will affect animal survival and fertilization of downstream waters.
In Sweden, there are several examples of lowered, originally shallow, nutrientrich lakes situated in areas rich in limestone. The characteristic sediment deposit in these waters is lake marl, i.e. calcium carbonate precipitated as a result of the photosynthetic activity of a rich submersed vegetation.
If the sediment consists of lake marl (as in some areas of Sweden) there are techniques to lower the bottom in an economically acceptable way. This is possible because lake marl can be used for the treatment of land, brooks, streams, and lakes in adjacent areas that are suffering from severe acidification. In acidified regions the spreading of finely ground limestone/chalk (obtained from quarries and crushed and ground to fine particles) is state-financed. Lake marl is often of high quality and excellent for the liming of acidified areas. Modern automatic extraction methods open up possibilities for efficient sediment-mining aimed at:
For Lake Hornborga, situated on the geological borderline between limestone and gneiss, the characteristic sediment is lake marl, and immediately southwest of the lake lies the area of Sweden where the soils and waters are most severely affected by acidification.
The restoration of lowered lakes implies a general redevelopment of the landscape and of the biological diversity. It is often of great importance for tourism and recreation.
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