The species of filamentous algae that perhaps causes the most concern in fresh water systems around the world is Cladophora glomerata. This is called blanketweed. However, there are over 150 different species of cladophora. Blanketweed can grow at tremendous rates when the conditions in the pond are suitable. In very high light levels with high levels of nutrients, blanket weed can grow at over 2 metres per day. Other filamentous algae, also referred to as blanketweed, that cause problems in ponds are species of Spirogyra, Rhizoclonium and Enteromorpha. Worldwide, there are over 400 different species of the genus Spirogyra. Spirogyra tends to show in ponds as a tangled pond scum. It is also called "water silk", "silk weed" . On sunny days, the mats of spirogyra filaments usually float on the surface of the water. They are kept afloat by tiny bubbles of oxygen arising from photosynthesis. These algal mats then sink when the sun goes down and the process reverses as photosynthesis is reduced.
As a result, the strands of Spirogyra consume oxygen for cellular respiration. Carbon dioxide is then produced as a waste product. Where there are thick algal mats present, large fluctuations in the dissolved carbon dioxide and oxygen levels in the water can occur. This can lead to rapid changes in the pH of the water that in turn can cause stress and even death to other organisms, eg fish, living in the water.
Enteromorpha intestinalis is another form of blanket weed that occurs in ponds. It is tubular in form and rarely grows longer than about 40cm. There are over 50 different species of the genus Enteromorpha. There are over 20 species of the filamentous genus Rhizoclonium.
Control of Pond Algae
The presence of algae in ponds need not necessarily cause a problem. However, in many cases it does and the pondkeeper is advised to remove it.
Planktonic algae can be controlled in garden ponds by installing an ultra violet filter. The UV light is very effective in agglomerating these single celled algae into clumps that then can be "caught" in the physical filter attached to the pond.
However, UV filter lights have no effect on filamentous or macrophytic algae and other control methods have to be used. These methods include the use of chemicals and microbial products. Many pondkeepers prefer not to use chemical methods.