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Newsletter and Technical Publications

Lakes and Reservoirs vol. 3

Water Quality: The Impact of Eutrophication

Water Quality management and Eutrophication in Some Lakes Around The World

Lake Biwa

Photo 23: Lake Biwa, the largest lake in Japan during summer time showing overgrowth of aquatic plants.

The eutrophication of Lake Biwa (Photo 23) began in the 1960s when the post-war economic growth of Japan began. The plant biomass concentration in the 1980s was about 10 times than that in the 1950s. 13 million people depend on the water supply from the Lake Biwa/Yodo River system. Since 1969, unpleasant odors in tap water from Lake Biwa has annoyed users every summer.

Plankton biomass peaked in the late 1970s, when blooms of red algae also appeared as a “freshwater red tide”. This has occurred almost annually since then. Blooms of blue-green algae have appeared since 1983, a sign of more advanced eutrophication.

Trends in the degradation of water quality in Lake Biwa have now more or less leveled off, following cooperative efforts of the residents and local government of Shiga Prefecture. Advanced wastewater treatment was introduced. The use of detergents based on polyphosphates was banned. And wetlands were constructed to cope with drainage water from agriculture. As a result of these measures, the degradation of water quality stopped, but no signs of further improvements have yet appeared. A more extensive abatement of diffuse pollution is probably needed before water quality improves significantly.

Lake Fure, Denmark, a typical example of water management in Northern Europe

As in many other European lakes, the eutrophication of Lake Fure began in the 1960s. Lake Fure (Photo 24) is situated only 15-20km from Copenhagen in an attractive area with several lakes and forests. The population close to Lake Fure was therefore growing in the decades after the Second World War, with increased impact on the nature in the area, including lakes. At the start of the century, the transparency of the lake water was several meters, while in the late 1960s it was only 1.2m during the spring and summer blooms.

Photo 24: General view of Lake Fure, Denmark

In the early 1970s it was decided to expand treatment of the wastewater from about 30,000 inhabitants to include nutrient removal (98% removal of phosphorus). Wastewater from another 100,000 inhabitants was diverted to the sea. By these measures the phosphorus loading was reduced from 33 to 2.5 tonnes per year. The remaining phosphorus loading comes from stormwater overflow, from the treated wastewater and from diffuse sources.

As a result of these efforts, the transparency has almost doubled since the late 1960s. However, the lake has a water retention time of 20 years, which explains why even larger improvements have not yet been observed. Only a little more than 20 years have passed since measures were commenced, and two to four retention times are usually needed to see the full effect of the measures taken. While, the external phosphorous loading (mainly wastewater) has been reduced to 2.5 t/y, the internal loading, i.e. the loading from the sediment is still about 12 t/y. Consequently, other methods have been considered to restore the lake (see the list of methods in Table 5). However, on a long term basis, it is still beneficial to reduce external loads of phosphorus to less than 1 t/y. This can done by proper treatment of stormwater overflow, and by increasing phosphorus removal efficiency by the wastewater treatment to 99% or more. In Lake Fure, diffuse pollution is less important, as the lake is more or less surrounded by wetlands and forest.

Extensive wastewater treatment involving nutrient removal has been introduced for many lakes in northern Europe, but, as Lake Fure shows, a long time will elapse before the full effect of this treatment can be observed. In addition, further reduction of nutrients is needed before an adequate reduction in eutrophication can be expected. In most cases, non-point, diffuse pollution will be needed to be reduced considerably -clearly a much more difficult task than reducing point source pollution.

North American Great Lakes

Approximately 30% of Canada’s population and 20% of the population of the U. S. live in the Great Lakes drainage basin, some 520,000km2. The Great Lakes comprise: Lake Superior, Lake Michigan, Lake Ontario, Lake Erie and Lake Huron. 24 million people depend on these lakes for drinking water. Industrial growth in the 1940s and 1950s resulted in oil pollution and eutrophication accelerated in the 1960s. By the late 1960s, water quality had deteriorated to a critical level. Massive algal blooms were frequent, and severe oxygen depletion occurred, even in the central bottom water of Lake Erie (Photo 25). Massive fish die-offs took place in Lake Michigan and Lake Ontario.

Photo 25: General view of Lake Erie, Canada and U.S.A.

Photo 26: General view of Lake Ontario, Canada and U.S.A.

In response to this situation, specific effluent standards were established in the early 1970s. Phosphorus removal was introduced in wastewater treatment plants, and phosphorus content in laundry detergent was reduced from 30- 40% to 5%.

By the early 1980s, the phosphorus loading approached the target established 10 years earlier. In Lake Erie and Lake Ontario (Photo 26) it was reduced by one-fifth, but overall the reductions in the Upper Great Lakes were only about 50%. These reductions were also reflected in the total phosphorus concentrations and phytoplankton concentrations in the open water of Lakes Erie and Ontario. These reductions were only to about one-third of the 1970 peak values. Again, reductions of non-point sources of phosphorus and nitrogen to the same extent as for municipal wastewater were not possible. However, measures to abate eutrophication of the Great Lakes are among the most successful lake management case studies, because point source pollution was the major source of nutrients discharged to the lakes.

Table of Contents

• Brochure
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• International Year of Forests
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• World Environment Day
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• UNEP Campaign
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