Phosphorus in lakes

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It has been recognised since the 1970s that the discharge of anthropogenic nutrients has caused eutrophication in many European lakes. Eutrophic lakes exhibit increased phytoplankton growth (in particular diatoms and blue-green algae) which can make the water turbid and unattractive. Some algal blooms produce toxins and also tastes and odours that make it unsuitable for water supply. Low oxygen concentrations due to degradation of dead algae lead to the exclusion of fish and other animals.

Lakes tend to take longer to recover from eutrophication than rivers since they generally have lower flushing rates and huge reserves of phosphorus that can be released from the sediment. In freshwater bodies, phosphorus is usually the limiting nutrient for algal growth, so its concentration gives an indication of the trophic status of a lake.

The proportion of lakes and reservoirs with low phosphorus concentrations (< 25 µg P/l) has increased in the last 20 years and the proportion with relatively high concentrations (> 50 µg P/l) has decreased. This indicates that eutrophication in European lakes is decreasing. In the past, urban waste water has been a major source of nutrient pollution, but recently treatment has improved and outlets have been diverted away from many lakes. Diffuse pollution, particularly from agriculture, continues to be a problem. Phosphorus enrichment in lakes is a greater problem in accession and western countries than in the northern countries. This is because the northern countries have lower population densities, lower agricultural intensities and a longer tradition of removing phosphorus from waste water.

In two large lakes, which were previously highly polluted by phosphorus, the phosphorus concentrations have steadily decreased over the last decades, particularly in response to control of point sources (such as the Bodensee and Ijsselmeer). In two other examples (Loughs Neagh and Erne) concentrations have steadily increased in spite of reducing point source loads. This is because of a steady build up of a surplus of phosphorus (arising from fertilisers) in the soils in the catchments draining into these two lakes.