Phosphorus concentration in lakes
Phosphorus is a nutrient which is contained mainly in animal feedstuffs and fertilizers in Switzerland. If it reaches surface waters by leaching or soil erosion, it encourages eutrophication of the lakes. This produces more biomass, which consumes more oxygen to degrade. This is turn leads to oxygen depletion and the production of toxic substances, especially in the deep water. This eutrophication then threatens fish life and aquatic biodiversity.
With the economic boom after 1945, phosphorus pollution of surface waters from housing developments, industry and agriculture increased. Since the 1980s it has been declining, thanks largely to the water protection measures since 1970, the expansion of wastewater treatment, the ban on phosphates in laundry detergents (1986) and the ecologisation of agriculture from 1990.
Lakes react individually to changes in phosphorus pollution due to their different characteristics (e.g. depth, water exchange or wind exposure), making it impossible to define a general target for the phosphorus concentration in lakes. At levels below 15 to 20 µg/l of phosphorus, however, the legal limit for maximum average production of biomass is generally met. This is now the case for the majority of lakes, but in many of them the legal requirement that the oxygen concentration must not be less than 4 µg/l at any time and at any depth is not yet achieved, or only achieved with the help of artificial aeration or assisted circulation. For this reason, the state is still not satisfactory.
Further improvement cannot be guaranteed in all the lakes because the soil phosphorus levels continue to be high, particularly in regions with high livestock densities. Even if the use of fertilizers containing phosphorus were reduced in these regions, the phosphorus pollution would only decrease slowly. Reducing the soil reserves is a slow process which takes decades.
In international comparisons, Switzerland has very clean lakes in terms of phosphorus pollution. The European average of phosphorus concentration is slightly below 100 µg/l. With the water protection measures since 1970, the expansion of wastewater treatment, the ban on phosphates in laundry detergents (1986) and the ecologisation of agriculture from 1990, Switzerland was an early international pioneer. In the EU the ban on phosphates for household laundry and dishwasher detergents came into force in 2017. Switzerland will follow the EU timetable and procedure applicable to the ban on phosphates in dishwasher detergents.
The phosphorus concentration is calculated from the volume-weighted annual average of depth profiles or spring circulation levels.