Why Do I Have a Film on Top of My Pond?

As summer starts to wind down and we prepare for the inevitable cold season ahead, our mind inevitably turns towards our backyard ponds. It is a perfect time of year to make sure everything is in order before winter sets in.

One question that often comes up this time of year is why do I have a film on top of my pond? This blog post will address this question and provide you with some tips on how to prevent it from happening next year, and what exactly is this biofilm.

A lot of times, people are surprised when they see a white or milky film or scum at the top of their pond. Pond keepers know that algae can grow really quickly if there are too many nutrients in the water so sometimes an algae bloom happens naturally without any human intervention.

Other times, a buildup may occur because you over-stock your pond and didn’t maintain it properly, or perhaps the weather was hot for an extended period of time, which stunted the turnover that usually occurs in the fall when cool nights and rain return. All of this can cause a biofilm to develop on top of your pond.

What is that film?

Before we jump into prevention, let’s talk about what the film is exactly. It’s a natural occurrence that occurs all over the world every year, but at different times. It typically forms in late summer and into early fall, although it’s not uncommon to see it later into the season or even year-round if conditions are ideal.

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The cause of this “film” on the top of your pond is actually a type of algae, but it’s different from the types that usually grow in your pond. It’s not like a blue-green algae bloom where you see large patches of green floating on the surface or clinging to plant leaves and stems below the surface. This film is more akin to a scum that forms at the top of your pond.

The film you’re seeing is actually a combination of algae and bacteria. The algae provide the structure for this community, but it’s the bacteria that gets all the credit.

Without bacteria to break down organic matter, our ponds would become stagnant places that wouldn’t support aquatic life or be as aesthetically pleasing as they are today. It is beneficial to have a little bit of this bacteria on top of the pond. The presence of bacteria can indicate that your pond is healthy and balanced, but sometimes there are too many nutrients in the water for it to be beneficial.

What does it mean when you have a lot on top?

If your pond has lots of algae film at the surface, it’s usually a sign that there are elevated nutrients in the water column. Oftentimes, this occurs when pond keepers overstock their ponds or don’t manage the pond fertilizers and food properly during the summer.

If you have lots of algae film on your pond it could also be a sign of how much-dissolved oxygen is in the water. To put it simply, algae needs more dissolved oxygen than fish. If your pond doesn’t have enough dissolved oxygen because there are too many fish in the water, then the film will continue to build and become thick.

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Algae won’t die off if your pond is low on oxygen but rather start to group together in this film on top of the water. The film serves as a protective barrier to help the algae grow faster and reproduce. If dissolved oxygen levels are low, then adding aeration or other treatment options may be necessary to bring those levels up so the pond can start to break down this algae film.

Treatment Options – how to remove biofilm from a pond

  • Remove it manually – Scraping the top of the pond when it is still thin and easy to remove will provide for an instant cosmetic benefit. But, if conditions are right and you keep adding fertilizers or feeding your fish then the film will return quickly.
  • Remove as much as possible – If you have a skimmer, consider using it on a daily basis to pull the film up and out of the water column. It will decompose in your skimmer and then you can dump it into your yard waste bin.
  • Add plants/cover – If you have a large pond this may be an option for you, especially if the water is clear already. Adding floating plants like hydrilla or ludwigia will help shield and shade the surface of the water, preventing sunlight from penetrating.
  • Aeration/Turnover – If you have algae in your pond due to low dissolved oxygen levels, then adding some aeration or a fountain will help with turnover and increase dissolved oxygen in the water column.
  • Water changes – Although this is not recommended, a water change may help reduce the number of nutrients in your pond if you have a heavy fish load or high levels of fertilizers in the pond. Be prepared to repeat this process several times over the course of a few weeks for the best results.
  • Reduce feeding/lower fish load – If you simply stop feeding your fish and don’t add any new fish, then the pond should stabilize in a couple of weeks. Once you start adding food or more fish back into your pond, the algae will grow right back.
  • Add beneficial bacteria – Adding beneficial bacteria to your pond can help with breaking down all those nutrients in the water. There are many types and brands available at most pond supply stores.
  • Adjust fertilizer dosing/timing – Take a look at your dosing and make adjustments based on pond size, fish load, and water conditions. Avoid using too much in the spring or fall since algae are more likely to form when temperatures are warmest. A good rule of thumb is not to exceed 4 pounds of actual nitrogen per 1000 gallons of water. Flourish Excel, Milorganite, and Humate are examples of good organic sources of nutrients for your pond.
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Summary

Biofilm is a thin layer of organic matter that accumulates on top of the water. It can be found in ponds, rivers, and streams all over the world.

The white film on top of your pond or lake may be caused by algae blooms, which are actually quite common during summers with higher temperatures.

The result is often an unsightly appearance. Thankfully biofilm removal methods are available to help you remove this buildup from your pond or other body of water so it won’t affect how people view your property any longer.

References:

Biofilm https://en.wikipedia.org/wiki/Biofilm