Algae is a large and highly diverse group of organisms that ranges from unicellular to complex multicellular forms. The ocean dwelling organisms we commonly refer to as seaweed are for instance algae, just like the green slime that can turn a clear garden pond into “pea-soup”.

Some algae grow attached to a surface such as a rock or plant leaf, some drift around freely in the water and some float on the water's surface. Algae also come in many different colours, including red, brown, green, green-blue and black. 

Many different types of algae can colonize a pond and trying to completely eradicate all forms of algae from the pond is rarely a good idea. Algae are a natural part of the mini-ecosystem that is your pond and they can be beneficial for inhabitants such as fish and frogs. There are however situations where algae starts growing exponentially and threatens the well-being of the other pond creatures. Such situations should be dealt with as soon as you notice the first signs of trouble and preventive care is even better.

Pond algae are commonly divided into three main groups: microscopic algae, filamentous algae and attached-erect algae.

Microscopic algae float around freely in the pond and gives the water is characteristic green colour. Having a healthy amount of microscopic algae in the pond is beneficial because it gives of oxygen as a by product of photosynthesis and will use the waste products produced by fish and other animals as nutrients. Microscopic algae can however turn into a nuisance if it starts to bloom excessively. Algae blooms will typically occur during the middle of the summer when the water temperature is high and there is plenty of light. The bloom can be

Algae

visible in the form of yellow-green or reddish scum that floats on the surface of the pond. The most dangerous thing about this type of algae bloom is that if the algae suddenly die, the dead matter will cause the oxygen levels of the pond to sink dramatically as it decomposes. Algae mass-death can be easily set off by natural factors, such as a lowered water temperature or a few days without much sun light.  

The filamentous algae that can occur in garden ponds is typically green and known to be amazingly cold tolerant. Unlike the type of pond algae described above, filamentous green pond algae can bloom during early spring while the water is still fairly cold. Filamentous algae will form long hair-like strings which can develop into fuzzy clusters. Sometimes this type of pond algae will only line the bottom and edges of the pond, but if it starts to break loose the clusters will float to the surface and create an unsightly mess. Just like microscopic algae, filamentous algae are especially dangerous when they suddenly die since their decomposition consumes oxygen.  

Attached-erect algae will rarely become a nuisance in garden ponds, but this type of algae can start to bloom just like the other types and cause the same type of problems.