It is not possible to give general recommendations as to which restoration to apply in a specific case. Most restoration problems are associated with eutrophication, and it is necessary in each individual case to use a eutro phication model to assess the effect of the restoration method and compare the effects and the costs to decide which method gives most 'pollution abatement' for the money. It is, in other words, necessary to set up a cost/ benefit analysis.
The following modifications in the eutrophication model must be carried out to account for the effect result ing from the application of the restoration method:
1. Diversion implies that the forcing functions (a) input of nutrients and also (b) the hydraulic retention time will be changed.
2. Removal of superficial sediment implies that the sediment contains less phosphorus and nitrogen, which will of course change the release rate of these nutrients from the sediment to the water phase.
3. Removal of macrophytes corresponds to a removal of the amount of phosphorus and nitrogen in the harvested plants.
4. Coverage of sediment by inert material will have the same effect as (2) but will in many cases be more cost moderate, particularly for deeper lakes.
5. Siphoning of hypolimnic water corresponds in the model to removal of more nutrient (the concentra tion in hypolimnion to replace the concentration in epilimnion) with the outflowing water. It is of course necessary to examine what the effect of the higher nutrient concentration will be downstream. If there are other lakes downstream, it is inevitabe that the nutrient must be removed, which is possible by a number of methods. For instance, for phosphorus, adsorption on activated aluminum oxide and/or precipitation of phosphate with aluminum sulfate, iron(III) chloride, or polyaluminates can be used; see Figure 1. This method is obviously only applic able to lakes with a thermocline at least for a part of the year.
6. Flocculation of the phosphorus in the water phase implies that the phosphorus is once removed from the water phase to the sediment. Usually, it is neces sary to apply this method several times.
7. and 8. Circulation and aeration of hypolimnion implies that the release rate of phosphorus and nitro gen from the sediment to the water phase is changed. Aerobic conditions usually imply that the release rate is lower particularly for phosphorus than under anearobic conditions.
9. Changes of the hydrology means that the forcing function hydraulic retention time in the model becomes shorter.
10. Construction of a wetland to cope with the non point pollution of nutrients implies that the input of nutrients is reduced. The forcing functions in the model expres sing the input of nutrients are changed correspondingly.
11. Shading by the use of trees changes the photosyn thetic activity in the lake. The forcing function, solar radiation, in the model is reduced corresponding to the shading effect.
12. Biomanipulation is often a cost moderate method with a good effect, provided (as mentioned above) that the phosphorus concentration is in the range of c. 50-130 mgPl 1 when removal of planktivorous fish is actual and c. 100-250 mgPl 1 when plantation of submerged vegetation is actual.
Numerous eutrophication models have been published in the literature. It is of course necessary to apply a model with a complexity that is in balance with the problem, the ecosystem, and the data.
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