How do lakes suffer from climate change?

Despite the spring rains, lakes in Germany have been suffering from heat and drought in recent years. Algae and invasive species bother them. What does that mean for next summer?

At six in the morning, Catherine Kerner prepares her boat to go out on the lake. On the Obersee – as the inhabitants call the southern part of Lake Starnberg in Upper Bavaria – she was alone on the water on a cold morning. She wants the fisherman to haul her net.

But it’s had a problem for some time: While it’s been too warm on average in recent years, it’s too cold this spring. The result: fish remain in the deeper layers of the water and are difficult to catch.

The example shows: Climate change, rising temperatures and increasing extreme weather conditions are affecting the situation in the lakes. True, more water flows to them through the rivers due to heavy rains in the spring. However, they suffer – in other ways.

Toxic algal blooms

Scientists from the Technical University of Munich want to get to the bottom of the problem. Jürgen Geist conducts research in the field of aquatic systems biology, trying to understand how nature is changing underwater – the consequences of climate change and human intervention on our lakes and rivers.

For example, blue-green algae multiply over and over again. “This is because lake mixing often no longer works,” Geist explains. “Only when sea water has the same temperature at all depths can it mix.”

This occurs when the lake is allowed to cool during the winter and the water is about four degrees all around. Then strong winds are enough to mix the oxygen-poor water from the bottom of the lake and the oxygen-rich surface water.

If oxygen is not transported to the depths in this way, blue-green algae can develop aggressively there. They like low-oxygen water and secrete toxins that can be toxic in higher doses. “There are reports of dogs that are said to have drunk seawater and died,” says Geist.

Groundwater remains a problem

But what about lakes that are not fed by rivers but mainly by groundwater, such as Lake Starnberg or Lake Steinhude in Lower Saxony? The rains have improved the water levels in the lakes, and they have returned to normal levels after the dips. But groundwater levels are still low – and this could become a problem again in the future.

Until a few years ago, Marcus Diess of the Division Chair of Hydrology and River Basin Management focused on flood protection as a research focus. Today, however, he is working more on anti-drought measures. “We have to make sure that the soil can absorb the rain and more groundwater forms, which also benefits these lakes.” Des explains.

For example, drainage channels next to fields and roads are a problem. “Ditches ensure that water drains away, collects it in rivers and drains it away.” Eventually it will flow into the sea and will no longer be available as groundwater. That’s why farmers and water management agencies recommend filling in trenches. The soil then stores the water and releases it only slowly, even during periods of drought.

Trout needs ideal conditions

the Global Warming But they also cause problems for animals in lakes and rivers. In order to investigate this, the Weihenstephan researchers are raising hucho in large tanks. This is a huge type of trout that is found only in the Danube and its tributaries. The length of the fish reaches 1.5 meters. But they are threatened with extinction.

“Our research shows that even a slight 1.5-degree increase in temperature causes fish eggs to die,” explains Dees. The result is a significant decline in fish populations.

Brown trout in Bavarian rivers have a similar situation. She retreats further and further into the headwaters, where the water is still at her usual temperature. In the lower reaches they are then replaced by rainbow trout, which are better able to handle the conditions. This type of trout originally comes from America.

Nothing is normal again

Hunter Kerner also noticed such displacements in everyday life. In recent years I have discovered more and more lumps stuck together, little brown mussels in their nets – zebra mussels: these sharp-edged clams really shouldn’t be here. They were brought in years ago. “There are more and more of them. One of the few predators is the Gura. Mussels have also destroyed the mussels in our ponds. They sit on them and then the pond mussels no longer breathe. Zebra mussels stick to everything. They are on every boat chain, everywhere says Kerner.

“The same applies to all species that we call generalists,” explains biologist Geist. “They are better adapted to rough weather and as a result spread widely in the area. It’s a worldwide phenomenon. It’s also known as McDonaldization: the fact that you always find the same species in the most diverse bodies of water, all over the world.”

Hunters want to persevere

This summer, the trend could intensify if it gets too hot again. However, the weather may be wetter than last year, say some weather models, which are also used by the German Weather Service. This means that heavy rain can avoid lows like last summer. However, high temperatures can heat the water again and harm animals and plants. However: Such long-term projections are not yet ripe, according to a warning from the Department of Human Development (DWD).

Catherine Kerner catches the last net of the day, which is also nearly empty. But whatever the situation, she will continue to live, just like her ancestors have for generations. “Fishing at Lake Starnberg will remain,” Kerner is sure of, no matter how the climate changes.