Pond algae, non-vascular plants, are a major source of oxygen and food for fish and other animals living in a pond. Forms of algae range from the microscopic forms that make water look green to clumps of filaments that float on top of the water. Branched forms of algae sometimes live on the bottom of a pond.
Causes of Algae Growth
Excessive algae in a pond are usually caused by nutrient pollution or excessive plant food. Nitrogen, phosphorus, potassium and carbon dissolved in water are invisible. But combined with water and carbon dioxide plus sunlight on the surface of the water and warm temperatures, they spur the growth of algae.
Types of Algae
Phytoplankto are tiny, free-floating algae that make water look like green soup. During hot summer weather, they can rise to the top of the pond as a yellow-green or reddish scum.
Attached-erect algae, commonly called muskgrass or stonewort, grow on the bottoms of ponds. These look like regular plants because they have what look like leaves arranged on what appear to be long stems. They can annoy swimmers and cause snags for fishermen.
Green filamentous algae grow in clear, shallow water where sunlight can reach the soil. Long strands of these algae grow in fur-like clumps on the bottom and edges of a pond. They can break off and float to the surface, forming dense mats.
Signs of Algae Growth
A rapid growth of algae, called "bloom," will turn the water green. Blobs of slimy green gunk might float on top of the pond. You should be able to see two feet down in healthy water. If you can't see a light-colored object in 10 inches of water, a heavy bloom is taking place.
Methods of Control
Use a garden rake to remove algae growing on the top and sides of a pond.
Deepen shallow areas of a pond to reduce the penetration of the sun to the bottom.
Use less phosphorous-rich fertilizers near the pond.
Keep large groups of ducks, geese and livestock away from the pond.
If the pond is surrounded by a steep bank or drainage area, plant a buffer strip of high grasses or shrubs to reduce the runoff of fertilizers into the pond. Dig diversion trenches to redirect runoff.
Introduce grass carp, also called white amur, which eat filamentous algae.
Barley straw is touted as a solution to algae largely on the basis of a complicated, poorly understood chemical reaction reported by British researchers. The U.S. Environmental Protection Agency has ruled that it may not be sold as an agent to control algae. The effectiveness of barley straw varies, and you should research methods of applying it before you use it.
It is illegal to use a chemical to control pond algae unless it is specifically labeled for that purpose. Those algaecides that have been approved include copper, copper sulfate, diquat dibromide, endothall and simazine. The label should include the word "algae" or the term "algaecide."