Introduction

Freshwater lakes and reservoirs (lakes) constitute a vitally important resource for humans. They supply water for consumption and irrigation, are used for harvesting fish and other food resources, and for recreational activities such as angling, boating, and swimming. Moreover, lakes and to some extent also reservoirs add significantly to biodiversity on Earth and act as important foraging areas for many terrestrial animals and waterfowl.

For fifty years or longer eutrophication has represented the most serious environmental threat to lakes worldwide. High loading of nutrients of lakes has resulted in turbid water, excessive blooms of often toxic cyanobacteria, and loss of biodiversity. The shift towards blooming reflects in part the increasing nutrient loading to and ensuing enhanced nutrient concentration in lakes (reduced resource control also called 'bottom-up control' of phytoplankton). However, eutrophication is also a result of higher predator control (also called 'top-down' control) of invertebrates by fish. Increasing nutrient concentrations leads to a major increase in fish density, a shift in dominance from piscivorous fish to plankti-benthivorous fish and, consequently, a higher predation on the vital large-bodied zooplankton, such as Daphnia, and with it less grazing by zooplankton on phytoplankton. Moreover, benthivorous fish stir up sediment, which results in enhanced turbidity in the water column above and less light for growth of submerged plants. An increase in fish predation also reduces snail abundance and thereby also the snail grazing of epiphytes on plants, which further impoverishes the growth conditions for the submerged vegetation. The plants disappear and the food source and feeding habitats of aquatic birds become reduced.

Although many countries in the developing world at present face an alarming increase in lake eutrophi-cation as a result of the fast economic development, major efforts have been made in the Europe and North America to combat eutrophication. Substantial investments have been made to improve wastewater treatment and other pollution-combating measures. However, despite reductions in nutrient loading, eutrophication remains a major problem. Today, the highest pollution input to lakes in the developed world is often derived from diffuse sources in the lakes' catchment areas, mainly from cultivated land, while sewage water and pollution from in-lake fish or crab/shrimp farming often play a more central role in the developing countries.

Restoration of eutrophicated waterbodies will always require reductions of external nutrient loading and often supplementary in-lake measures to speed up the recovery process.

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