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Green Algae Blooms in Ponds

Algae are a group of microorganisms that make their way into water bodies of all sizes. Algae cells can be carried to water by bird, land animal, boat, fishing gear, and even the air! The proliferation of algae depends on nutrients and sunlight to grow.

Types of Pond Algae

  • Filamentous Algae
  • Planktonic Algae
  • Macroalgae

All three types of algae impact ponds in a variety of both positive and negative ways. Moderate algae blooms can have benefits to the pond ecosystem while excess algae can be a threat to other aquatic life.

Filamentous Algae

Filamentous algae is commonly called string algae, hair algae, or pond scum. String algae begins growth at the bottom of the pond. There are many species of filamentous algae so not all algae blooms will look the same.

String algae can look like green mossy mats, slime or long hair-like strands. The color of filamentous algae is normally bright green to dark green.

Pithophora is a type of filamentous algae that has a cotton-like consistency. Spirogyra feels like wet silky hair.

As string algae matures it eventually releases to the surface of the pond. Why does algae float at the surface? Filamentous algae becomes buoyant from the oxygen it is producing.

Algae bloom of green string algae

Filamentous algae forms clumps on the pond surface with the consistency of wet hair

Is string algae bad for a pond?

While filamentous algae may not be visibly appealing, it can provide some benefit to the pond. Algae is an essential part of a pond's ecosystem. It is a food source for invertebrates, protozoans, and fish.

If algae is covering the majority of the surface, this is nature's indicator that excess nutrients are present. Steps should be taken to reduce nutrients levels within the pond or lake. Find Out How

Exessive aglae blooms also pose real threats to the oxygen concentration in the water and can lead to fish kills if not addressed.

String Algae bloom treatment

Planktonic Algae

Planktonic algae is commonly referred to as green water algae or pea soup algae. These single-celled algae float freely throughout the pond. Coloration of planktonic algae forms can be green, yellow, blue-green or brown.

Planktonic algae blooms can be a sign of a serious imbalance in water. If visibility of pond measures less than 18 inches from the surface, steps should be taken to improve water conditions.

Algae blooms can lead to fish kills

Like all forms of algae, planktonic blooms produce oxygen during the day. Algae consumes oxygen at night. A heavy planktonic bloom can result in a fish kill from oxygen uptake at night. These kills can also occur when there are consecutive overcast days. When sunlight is blocked, the planktonic algae consumes oxygen during the day instead of producing it.

Green pond water algae bloom plankton

Planktonic, single-cell algae is found throughout the water column. This type of algae bloom can boost the productivity of a pond for food chain improvement for fish. However, it is a delicate balance with substantial risks due to low dissolved oxygen in the pond.

Chara and Nitella


Chara does not have roots and floats freely in ponds.

Chara and nitella are types of macro-algae. They are commonly misclassified as submerged weeds but are forms of algae. Chara is also known as muskgrass.

Chara algae produces stems with leaves growing in whorls around the stems. There is a gritty calcified outer coating. Chara has a skunk-like odor. It is typically found in hard water.

Nitella has similar looking growth to chara but lacks the gritty calcified coating and has no odor. Nitella algae is more delicate than chara. Nitella is typically found in acidic or soft water.

There is no root system with macroalgae forms. Just like string algae, growth begins at the bottom of the pond and then releases to the surface in mats.