The Fishing Lake

The upper lake at Weiswampach has been closed to swimmers due to water quality concerns, while the lower lake remains open. The closure applies to the so-called "fishing lake", the upper one, where swimming has only been permitted in recent years and only when conditions allow. The lower lake has maintained good water quality over the years and remains available for swimming and pedal boating.[1]

The cause is blue-green algae. Lakes are especially prone to heating up during warm weather because the water is still, according to the deputy head of the Surface Water Department at the Water Management Agency. Lakes also accumulate nutrients, which can be washed in from surrounding farmland or enter through partially treated wastewater. Warm temperatures and sunlight then encourage the growth of blue-green algae.[2]

The problem is not easily solved. Agitating the water may disperse the algae temporarily, but it does not address the underlying issue. During their growth, blue-green algae release cytotoxins. Even when the algae are no longer visible, these toxins may remain in the water, posing health risks to humans and animals. "Just because the algae are no longer visible does not mean the water is of good quality," the deputy head warned.[3]

The two lakes differ in their aquatic plant life. The lower lake is naturally better at cleaning itself, while the upper lake requires more attention. The local council plans to improve the upper lake's condition using natural methods, planting reeds at stream inlets to act as a filtration system. Reeds help remove nutrients, particularly phosphates, from the water. The hope is that as water plants recover, overall water quality will improve.[4]

This is the same pattern as the Upper Sûre Lake fire ban: warm weather, still conditions, and a lag between the problem arriving and the response being coordinated. Weiswampach is in the north, near the Belgian border, and its lakes are popular summer swimming spots. The closure of the upper lake during peak swimming season is not a minor inconvenience. It is a signal that the combination of agricultural runoff, warm temperatures, and still water is producing conditions that natural systems can no longer absorb on their own. Reeds at stream inlets are a good solution. They are also a slow one. The lake will not fix itself by next weekend.[5]