Recovery from Eutrophication

Abstract

Many lakes world-wide suffer from eutrophication as a result of high external nutrient inputs from domestic sewage, industry and agricultural activities, Increased demands for water by a growing and developing human population as well as enhanced global warming may escalate the eutrophication on a global scale in the next century. Yet, in some countries large efforts are now being made to combat eutrophication by reducing the phosphorus input. Some lakes respond rapidly to such loading reductions, while others are highly resistant due to high internal phosphorus loading (chemical resistance) or homeostatic effects of the food web altered by eutrophication (biological resistance). Some general models have been developed for the response of lakes to reduced loading, but major advances within this field can be expected in the future when more case studies appear. While these models may be used as a core for evaluating response patterns, local factors should always be considered to avoid wrong and often expensive decisions. To precipitate recovery from chemical and biological resistance, several physico-chemical and biological restoration methods have been developed. The biological methods include removal of planktivorous and benthivorous fish, stocking of piscivorous fish, protection or planting of submerged macrophytes, introduction of artificial structures, or addition of mussels. A 135 M. Kumagai et al. (eds.), Freshwater Management © Springer-Verlag Tokyo 2003 136 E. Jeppesen, M Sondergaard, J. P. Jensen, and T. L. Lauridsen widely applied method is removal of planktivorous and benthivorous fish. In many cases such efforts have yielded major improvements in water quality and the ecological state of the lakes. Yet, the listed restoration methods have mainly been applied to northern temperate lakes and cannot readily be transferred to subtropical and tropical lakes where the eutrophication-related problems are going to be greatest in the future. There is thus a major need for development and adaptation of methods focusing on south temperate, SUbtropical and tropical lakes.