High levels of nutrients lead to eutrophication of the lakes. Some of the dead biomass is deposited on the lake bed (sedimented) where it is degraded by bacteria which consume oxygen. An excess of biomass can then cause anoxic conditions in the deep water or on the sediment surface. This can lead to the production of toxic substances such as ammonia and hydrogen sulphide, causing a loss of habitat for lake bed organisms. Fish spawn on the bed (e.g. whitefish) cannot survive. At the end of the summer, the oxygen depletion reaches layers closer to the surface at a time when high temperatures in the surface water threaten fish life (see the indicator "Water temperature of surface waters"). The fish have to move to the cooler deep water, which greatly restricts their habitat.
With the economic boom since 1945, phosphorus pollution of surface waters from housing developments, industry and agriculture increased. The expansion of wastewater treatment from 1980, the ban on phosphates in detergents (1985) and the ecologisation of agriculture from 1990 achieved a reduction in phosphorus pollution. Lakes react individually due to their differing structures (e.g. depth, water exchange or wind exposure), making it impossible to define a general target for phosphorus. At levels below 15 to 20 micrograms of phosphorus per litre, however, the legal limits are generally met. Phosphorus concentrations in lakes in Switzerland have been decreasing since the Eighties. But because the soil is still enriched with phosphorus in catchment areas with high livestock densities, further improvement cannot be guaranteed in all the lakes. If the land is subject to leaching, run-off or soil erosion, high levels of phosphorus continue to be discharged into surface waters. A decrease in soil levels has only been observed in catchment areas where crop requirements are higher than fertilisation application rates. Reducing nutrient levels in soil is a slow process which takes decades.
Switzerland has very clean lakes by international comparison. The European average, for example, is slightly below 0.1 mg/l.
With the water protection measures since 1970, the expansion of wastewater treatment plants, the ban on phosphates in detergents (1986) and the ecologisation of agriculture, Switzerland was an early international pioneer. The Europe-wide ban on phosphates for household detergents will not enter into force until 2013. Discussions are now focussed on extending the P ban in the EU to dishwasher detergents from 2017. Switzerland is expected to follow the EU timetable and procedure in this regard.
Weighted annual average volume of depth profiles or spring circulation levels.