Algae in Pennsylvania Ponds

Algae is an essential part of the pond ecosystem. Found in many forms in a typical Pennsylvania pond, it functions as a base of the food chain, provides habitat for fish and benthic organisms, and supplies needed oxygen to the water environment. The requirements for algae to thrive in a pond are similar to any other aquatic plant – nutrients, sunlight, and the ability to grow without being inhibited. It is these characteristics and its ability to grow profusely at times that often cause algae to become a nuisance plant in ponds.

Types of Algae

Planktonic algae are microscopic single-celled organisms living throughout the water column. It can color the water green, reddish, or brown and is sometimes mistaken for silty or muddy water. Planktonic algae provide a food source for tiny aquatic organisms called zooplankton, which in turn become food for larger macroinvertebrates and fish. In warm summer weather, this algae can multiply rapidly in blooms that seem to appear overnight to change the appearance of the water quickly.

Planktonic algae can make pond water look murky or brown. Photo: Bryan Swistock

A subset of this microscopic algae that can be of special concern for ponds is cyanobacteria, also given the misnomer of blue-green algae and often referred to as a HAB or harmful algal bloom. These cyanobacteria have the ability to produce toxins that can be potentially dangerous to humans or animals utilizing the pond water. HABs can make water appear as if green paint or pea soup has been spilled on the surface, or they can also look like teal or green-colored puff balls or grass clippings in the water. If you suspect a HAB in a pond, avoid contact with the water until a few weeks after the bloom subsides. You can learn more about HABs in this short video.

Harmful algae blooms or cyanobacteria can resemble spilled green paint on a pond. Photo: Tom Davis

Branched algae or macroalgae are another form of algae that looks more like a rooted aquatic plant. Two of the most common in Pennsylvania ponds are Chara and Nitella. Both types of algae are important habitat plants that support the life cycles of small pond fish, salamanders, newts, and other macroinvertebrates. They are similar in their pattern of growth, featuring branched leaves growing in whorls around a central stem. Chara has a gritty feel due to encrusted calcium carbonate on its leaves, a musty, skunk-like odor, and a preference for hard water ponds.  Nitella has a smooth feel to its stems and leaves, does not have any odor, and is found in soft water ponds.

Nitella spp., a common branched algae. Photo: Bryan Swistock

Filamentous algae species are the ones most considered to be a nuisance in Pennsylvania ponds. They begin as growth on the pond bottom in shallow water or attached to rocks, branches, or other aquatic plants. As the algae grows, it can rise to the water’s surface and form dense mats resembling thick, stringy hair or cotton-candy-like masses. In ideal growing conditions, these mats can completely take over a pond, inhibiting recreational uses and making the pond water look unsightly. These algae are an important habitat for small fish and protozoans in the pond. More information on filamentous algae can be found on the Penn State Extension website.

Filamentous algae forming mats on a pond. Photo: Bryan Swistock

Algae Control Methods

The overabundance of nutrients entering a pond is the main contributor to algae growth. Nutrients from runoff of surrounding land introduce phosphorus and nitrogen to the aquatic ecosystem, providing ideal growing conditions along with sunlight. The first step to algae control should be reducing the amount of nutrient transport by making changes to surrounding land uses or by the use of riparian buffer plantings to help slow and filter the influx of nutrients. More information on reducing nutrients in waterways can be found on the Penn State Extension website.

Aquatic herbicides can also be effective in algae control, but steps should be taken to ensure they are being used properly.

• Positively identify the algae and decide which herbicide will be most effective for control.
• Take pond measurements of area and volume.
• Obtain the state permit to apply the herbicide.
• Purchase the herbicide and apply it according to label directions.

Natural algae control methods, such as barley straw, can also be used. While barley straw will not kill existing algae, applying it to ponds early in the growing season can prevent new algae growth. Pond fish used for aquatic plant control, such as triploid grass carp, do not prefer algae as a food source and will only eat it if nothing else is available, so these fish are not a recommended natural control method.

More detailed information on aquatic plant control can be found on the Penn State Extension website.

Many resources exist for pond owners on aquatic plant identification as well as help with pond water quality issues, wildlife, and fisheries. For resources and access to Extension webinars, publications, and workshops, please visit the Extension Pond Management website.