Large freshwater lakes: present state, trends, and future

The large freshwater lakes of the world are an extremely valuable resource, not only because 68% of the global liquid surface fresh water is contained in them, but because of their importance to the economies, social structure, and viability of the riparian countries. This review provides decision makers with the knowledge of large lakes (≥ 500 km 2 ) essential to establishing policies and implementing strategies compatible with sustainable development. This is achieved by considering the present state of the lakes, the extent of changes and factors causing them, long-term consequences of these changes, major threats and possible states of the lakes into the year 2025. Case studies of lakes are presented, namely the St Lawrence Great Lakes of North America as representatives of glacial scour lakes of North America, northern Europe and Asia, and the African Great Lakes as representatives of tropical tectonic lakes. Lake Baikal is also included because it is unique for its species, great age, and largest single volume of liquid surface fresh water. The Aral Sea is further included because of the ecological disaster following diversion of water away from its basin. The major impacts on large lakes are diversions, eutrophication, invasive species, land-use change, overexploitation of resources, and pollution. These impacts can or do affect all the representative lakes, but to varying degree. The St Lawrence Great Lakes have been severely impacted by eutrophication, land-use change, overfishing, invasive species and pollution. Eutrophication has been reversed for these lakes and constraints are now in place on land use change, such as shoreline alteration and destruction of wetlands. With the demise of most commercial fishing, overfishing is no longer as important. Invasive species have become a major problem as increasingly non-indigenous species gain access to the lakes. Pollution continues as a major impact. These problems are likely to continue and seriously impact use of the resources as well as bring about changes in the biota. Among the African Great Lakes, invasive species are a major problem in Lake Victoria, and eutrophication associated with land-use change and overexploitation of resources is a growing problem. Many endemic species have been lost and many are threatened, so that species associations will have changed by 2025. The Aral Sea continues to disappear and in the future, the remaining largest part of it will continue to become increasingly saline and eventually disappear. A small body of water will remain as a freshwater lake with a productive, although small, fishery. Lake Baikal shows evidence of pollution in the southern basin and is likely to be impacted by land-use changes, primarily logging. Some non-indigenous species are present, but so far, they are not a major problem. Overexploitation of resources in the watershed could lead to adverse impacts on inshore waters. Overfishing has been recognized and appears under control. The major threat to Baikal is continued and growing pollution. Climate change and pollution are global problems that will affect all lakes, large and small. At present, while some warming has occurred, climate change appears not to have impacted large lakes. Present studies on the Laurentian Great Lakes predict possible major impacts. Pollution, especially from persistent toxic substances such as PCBs, is a global problem. Diversion of water out or away from large lakes will become more of a threat as global human population growth continues and water supplies from rivers and ground water become depleted.