Swiss lakes are in climatic stress 2

Swiss lakes are in climatic stress

Warming up threats to Swiss lakes before overgrazing, warn researchers. Now the government suits.

Sven Altermatt

It cools down in the hillsides of the brewery in Lake Geneva: During the heat of many lakes in Switzerland, bloom comes: Laurent Darbellay / Keystone (Geneva, June 25, 2019)

Cool in Bains des Pâkis in Geneva Lake: During the heat, many lakes in Switzerland are in a great mood.
Image: Laurent Darbellay / Keystone (Geneva, June 25, 2019)

Pesticides. Fertilizers. Micropollutants. These keywords fall when it comes to the impact of Swiss water on the environment. The lakes are particularly affected. They provide habitat for numerous plants and fish and serve as a source of drinking water. It is true that water quality has improved considerably over the last few decades, for example thanks to the ban on phosphates on detergents.

But there's still a lot of work to do. According to the federal government, the lakes are still overburdened, particularly in areas with numerous cattle breeding and intensive farming. Some should be artificially ventilated. One of the main reasons for this is phosphorus, which enters the lake through the fertilizer. Phosphorus promotes algal growth. If they die, they sink to the bottom and bacteria that consume oxygen break down. That then lacks the fish.

While the federal government is still discussing whether farmers can use less fertilizer in their fields or how pesticide use can be reduced, experts focus their attention on another problem: rising temperatures cause loss of balance in the lakes. They suddenly warm up during climate change, some even faster than ocean and atmospheres. The confederation now assumes that heating can endanger the lake more than overfishing.

Although it is difficult to balance two factors, the Federal Office for the Environment (BAFA) emphasizes on demand. However, the trend is clear: "The impact of climate change created by man is growing, while the impact of over-fertilization has been reduced thanks to the great efforts of the last decades to protect water." "According to forecasts, it is expected that the effects of climate change will increase significantly in the coming decades."

Temperature must not be collected throughout the country

In the same direction comes the Eawag Research Institute for Water, which the federal government has commissioned to investigate which changes in lakes are subject to and what is expected there. Climate change can become "the most important cause of changes in marine ecosystems," warned scientists around ecologist Martin Schmid.

What happens when the lakes are under the climatic stress? It is known, for example, that the summer temperature of Lake Zurich since 1980 has increased by 0.4 degrees Celsius. According to researchers, warming leads, inter alia, to "reduced winter mixing". Lakes consist of two layers of water. Water on the surface is full of oxygen but with little nutrients. The reason is just the opposite.

Only when the lakes can cool enough, the two layers are mixed so that the oxygen from the surface reaches the depth and the surface comes from the nutrients from the depths. If the lake can no longer turn, in the end it will literally miss the breathing air.

In Switzerland, the temperature of the sea is not collected systematically throughout the country today. In order to understand exactly how warming affects the lake, it is necessary to observe temperatures at all depths. The Confederation therefore examines the establishment of a national sea-monitoring network. Baf confirms the relevant information on these newspapers. "The climate affects all lakes in Switzerland, is not regionally or cantonal," explains authority. At present, the lake temperature data are only by-products of chemical research for which the cantons are responsible. "Only some nationally important issues can answer that information," says Bafu. His own measurement network today is limited to the river.

Little oxygen, little food

There are about 1,500 lakes in Switzerland, the top ten being 30 to 345 square kilometers. Lake heating is not necessarily bad. Dangerous indirect effects whose consequences stretch over the edge of the water. On the one hand there is a problem when the lake is not cool in the winter; when the upper and lower layers are difficult to mix. Oils and nutrients no longer reach where they are needed. Because of warming and poisonous blue-green algae for longer bloom. Also, invasive shells can be spread. And finally, too hot water disrupts annual cycles of many organisms; For example, fish can no longer migrate when foods are available for their offspring.

Even small changes in water temperatures can negatively affect the ecosystem. Depending on the type and area, the lake reacts differently to heating. However, not all are equally sensitive to temperature changes, Baf knows. For some lakes, over-fertilization is still a bigger problem, while for other climate change, "today is the main driver".

The federal government and Eawag are currently assessing how the new metering network will work and which lakes will be included. Synergies with the cantonal authorities are also considered. According to Eawag researchers, today's measurement programs, which are typically carried out on a monthly basis, are generally insufficient for accurate and early detection of changes in the thermal structure. And it is not enough just to put a thermometer in the lake to determine the surface temperature. Continuous surveys at selected depths require anchoring devices directly in the lake.

How much water can sometimes heat up showing the view below the water surface of Zurich. Last August, at a depth of ten meters, the temperature was still at 25 degrees Celsius. New record. After all, in the winter before Zurich, the lake could be mixed again for the first time in many years.

When it gets worse

Climate changes sometimes jeopardize Swiss Lakes more than over fertilization. But these two factors can be intensified, as the Federal Office for the Environment (BAFU) knows. On the one hand, productivity increases – especially algal growth – in the lake if the intakes of nutrients are too high. Result: At the bottom of the lake, more oxygen is consumed. On the other hand, climate changes in deep lakes exacerbate mixing or replacement of oxygen-rich oxygen in the upper layers with deeper layers of poor oxygen. Depth of oxygen is reduced, living beings are difficult. "Climate change therefore has the potential to exacerbate the undesirable effects of over-nutrients on the lake," explains Bafu. In this context, it would be even more important to strive for natural concentrations of nutrients in the lakes. (SVA)