Birds are a major contributor to many water quality imbalances in pond, lakes and dams.
The faeces they produce adds large amounts of nutrients and they can also bring in weeds and pests by flying in from other water bodies.
Like scarecrows, there are aquatic equivalents that perform a similar function. Floating alligator heads look very realistic and will scare many types of birds away.
Water sprays with motion activated sensors are another useful tool, as is the humble bird net.
Regardless of the methods or tools you implement it’s a good idea to keep birds out of the water if you want to minimise water quality problems.