The researchers studied how the plants in Lake Constance changed over 50 years. In the past, the lake got dirty from sewage and farm chemicals, but then people cleaned it up. As the water got cleaner, more types of plants grew, especially charophytes. These plants like clean water and they increased a lot in number. The amount of a nutrient called phosphorus in the water was the main reason for these changes. So, in lakes like Lake Constance, the amount of nutrients can tell us a lot about the plants that live there.