funds typically are involved in these projects, and several states with large numbers of lakes have developed their own programs. As problems of lake acidification became more widely recognized during the past decade, restoration of acidified lakes by addition of limestone has become a relatively common practice in some northeastern states, as well as in Scandinavia.
For long-term restoration, it is essential to control the source of the problem. In the case of eutrophication, this means decreasing the loading of nutrients, particularly phosphorus, from various watershed sources. In some cases, this also means that loadings of silt and organic matter must be decreased. Control of external sources is sufficient to return some lakes to their former conditions, but in many cases the changes in the lake have been so dramatic—major shifts in biota, loss of habitat, physical changes in bottom sediments, and lake hydrology—that merely turning off the loadings is not sufficient to improve water quality and ecosystem structure, at least in a reasonable time frame. In-lake restoration techniques must be employed.
Numerous methods have been developed to restore lakes or improve their condition; this chapter describes more than 25 such methods. Available methods range widely in effectiveness, cost, frequency of use, and range of applicability. For example, methods that require addition of chemical agents to lake water are limited to small-and medium-sized lakes for economic reasons. Methods that use biological agents are potentially effective at low cost even in large systems because of low initial costs and the absence of labor and maintenance expenses. Many methods are applicable only to a single type of problem (e.g., liming to mitigate acidification). Others are potentially useful in restoring lakes degraded by a range of stresses; for example, dredging may be used for siltation, nutrient buildup, and toxic contaminant problems. Because eutrophication is the most widespread and longest-studied lake problem, more methods have been developed to restore eutrophic lakes than to address all other problems put together. Aside from removing contaminated sediments by dredging or covering them with uncontaminated sediment, few methods are available to restore lakes degraded by toxic substances.
Our ability to assess the effectiveness of past lake restoration projects and to compare the effectiveness of different restoration methods is severely limited by three factors. First, and perhaps most important, surveillance of lake conditions for an adequate period of time before and after a restoration attempt has been done on relatively few lakes. In some cases, sufficient surveillance probably was done, but rigorous analysis and interpretation of the data were not a part of the surveillance effort. All too often the data are not readily