Castle Semple Loch has high levels of blue-green algae although the type of algae present has changed to a species called Gloeotrichia. This species forms a colonies of around one thousand cells and is visible in the water as small greenish spheres. Swallowing the water or algal scum can cause stomach upsets or more serious health effects. Contact with the water or with algal scum can cause skin problems. Immersion watersports such as swimming are not advised.