When the lake is at a low level, with a small volume of water, it warms up sooner than most lakes in the region, so people can be seen starting to swim the lake in April while bigger lakes even in lower areas of Switzerland remain rather cool.
When the lake is at a low level, with a limited amount of water, it heats up more quickly than most lakes in the area, causing people to begin swimming in April before larger lakes even in lower regions of Switzerland have warmed up.