Planktonic algae may at first appear as if your pond water is changing
color. As this green or blue-green algae blooms more it can cause your
pond to look like pea soup or paint. It will also cling to things, such as
your shoreline, and leave traces of scum on rocks and other objects in the water.
Some forms of Planktonic algae are toxic and can only be identified with the proper equipment.