Aasee (Münster) – Wikipedia - "Swimming in Aasee is currently prohibited and, due to poor water quality, especially during hot summer months, not recommended. However, there are efforts to improve the water quality. The main problems are blue-green algae, which multiply massively due to over-fertilization, as well as coliform bacteria. Contact with Aasee water in the summer can cause skin irritation, and ingestion can lead to life-threatening illnesses. Additionally, safety is compromised by the shallow water depth, which is further limited in some areas by bicycles lying on the bottom." - some things just have to be left uncommented ...