What Causes Cyanobacteria? 7 Factors That Cause Pond Algae to Grow

What Causes Cyanobacteria? 7 Factors That Cause Pond Algae to Grow

If you have a pond in or near your home or business complex, you are probably well-versed in the struggles that come with having a standing water feature, along with the aesthetic benefits. If you are worried about cyanobacteria—commonly known as blue-green algae—this post will give you explain the seven reasons it grows. Plus, it will offer some advice that can help avoid it.

Excessive Soil Nutrients in the Water

In any ecosystem, living organisms need nutrients to survive. When a pond is in close contact with agricultural soil or ordinary lawns that are given large amounts of phosphorus or nitrogen as fertilizer, this can cause an overgrowth of algae in the pond. Combined with erosion and land clearing, this brings a great deal more soil nutrients into the waterways.

This abundance of extra nutrients in water is called eutrophication, although it makes for healthier crops and more beautiful grass, it can cause big problems as far as pond algae go.

Climate Change

Blue-green algae grow best when water temperatures are above 25°C. This means that blooms typically appear in the spring and summer months. However, with global temperatures rising each year, blooms can sustain into the fall or even winter months.

Climate change also affects rain patterns, which contribute to higher amounts of cyanobacterial growth. The more rain there is the more erosion and runoff, which make more nutrients from the soil present in water systems, thus creating more algae blooms.

Carbon Dioxide from Algae Growth

Many of these causes are cyclical in nature, and CO2 emissions are one of those. Cyanobacteria bring down carbon dioxide from the atmosphere, creating literal sinks of the chemical in the freshwater where they bloom. These blooms produce large amounts of greenhouse gas during photosynthesis, and large amounts of methane gas have been observed during varying conditions of light and oxygen availability.

The cycle happens when higher temperatures (as in the climate change noted above) create more blue-green algae, which off-gas methane, which then, in turn, exacerbates global warming.

Light Conditions

When ponds with large algae populations are exposed to intermittent amounts of high-intensity light, blue-green algae flourishes. Underneath the surface of pond water, light fluctuates, which makes optimal conditions for the growth of cyanobacteria. Cyanobacteria can also adapt, though, and is known to grow in conditions with low light or turbid water movement.


There are many types of blue-green algae, and most thrive when pond conditions have low flow, long retention, and little to no wind and turbulence. However, other forms are quite adaptable and can survive well when conditions are mixed.

Environmental and man-made conditions often contribute to algae growth in ways that are higher than normal. Droughts, irrigation extraction, excessive human and animal consumption, and weir and dam construction all combine to decrease water flow.

Extreme water stability can also be caused by thermal stratification. This happens when the top layer of water stays at a warm temperature and the lower layers stay cool. This prevents the layers from mixing and encourages algae growth. Thermal stratification is also often associated with oxygen deprivation of the water, which is a catalyst for the soil to release extra nutrients, causing eutrophication (see above).


When floating particles and organic matter are suspended in water, it causes turbidity. High turbidity can be caused by large amounts of rain and runoff, and low turbidity can be caused by slow or stagnant water allowing particles to settle outside of the water column. Low turbidity is a condition that allows light to penetrate, which, as noted above, is an optimal condition for algae growth. Then, a high presence of algae creates slow-moving water which is then a condition for turbidity, and the cycle continues.

Failing Septic Systems

Every home or building must have a septic system installed or be connected to a communal system. When they are properly maintained, they should not at all contribute to algae growth. However, if a system is too close to a pond or it begins to fail, the septic system can seep into the pond water and cause harmful amounts of cyanobacteria to form.

If you notice unusually lush grass growing on or around your septic tank, foul-smelling odors coming from your grass, or drains that are backing up, you should have your septic system inspected immediately.

What You Can Do to Prevent Algae Growth

All of this might seem overwhelming, but there are many simple steps you can take to prevent excessive cyanobacteria from a pond on or near your property.

Limit Fertilizers 

This is a great place to “go green.” Although fertilizers might make for more luscious plants and larger crops, strong fertilizers run off into ponds and encourage algae growth. There are other ways to help plants grow well without using fertilizer! You can set your grass mower to 3” or higher so that the root structures have the chance to develop a complex, healthy structure. Long grass is preventative against weeds, as well, so leaving it long reduces the need for extra chemicals to remove weeds. Lastly, don’t bag your grass clippings. Leave them on the lawn so that the natural nutrients can return to the soil, and the soil does not have to overproduce.

Proactively Plant a Buffer

If you have the right to do so, plant some vegetation around the pond to create a natural buffer that will prevent erosion. Doing so will also keep excessive soil nutrients and pollution from entering your pond while making it look beautiful at the same time.

If you have a pond in your living or working area that you don’t own but in which you notice excessive algae growth, find out who owns the rights to that portion of the land. Contact them, and suggest a buffer of around 15 feet of natural vegetation. This can block up to half of unwanted phosphorus and nitrogen from creating the overgrowth of blue-green algae.

Use a Fountain or Aerator

Adding a fountain or aerator on the edge of your pond keeps the water from remaining so stagnant, thus inhibiting algae growth. They also bring in extra oxygen to the water, creating less inhabitable conditions for cyanobacteria. It is also an extra point for aesthetic beauty!

Final Thoughts

There are many reasons why a pond would grow excessive amounts of algae, but there are also plenty of things you can do to prevent it! Take it one step at a time to ensure healthy air and water around and from your pond.

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