So, you have a lot of algae on your dam, and there has been a lot of media recently about toxic blooms, so you are worried about the consequences, and want to know how to get rid of the algae.
Algal blooms can be unsightly, smelly, and frankly, a little scary. But did you know that they are simply a product of natural processes and occur when these get out of balance.
Photo by Bernard DuPont, Flickr.
To begin with, it’s not the end of the world. So let’s think about why the algae has appeared, because without knowing this, you are very likely going to throw money and energy away and not fix the ‘problem’. I say ‘problem’ because algae is actually not the problem, but a symptom of the problem.
It is important to realise that in a healthy dam, you will always have algae. There are two main types: string algae and planktonic algae and they are both essential to the natural processes in any waterway or waterbody. Unfortunately, sometimes the system gets out of balance, and this can result in algal blooms, bad smells and in extreme cases, the death of aquatic life.
Caution: I would like to add a note about blue-green algae here. These are misnamed as algae and are in fact cyano-bacteria. They can be very toxic to animals and humans. Thankfully, the best treatment is the same as for algae.
Algae typically require three essential conditions for growth, so check your dam for these.
- Warm water temperatures.
- High levels of nutrients (eutrophication) – both in the water column and benthic layers (the sludge on the bottom). Phosphorous and nitrogen are common contributors.
- Direct sunlight in large amounts.
Other influencing factors include high levels of carbon dioxide (anaerobic conditions) and still water, which often creates temperature stratification with a warm upper layer, and it also lowers oxygen levels as the water is not being mixed.
The key to managing algal blooms lies in preventing the combination of these three main conditions for growth. In most cases, you need to:
- Reduce the water temperature (not that easy to control).
- Reduce nutrient inflows before they reach the dam (much easier to control).
- Promote the growth of natural competition for the nutrients in the dam such as plants and good nitrifying bacteria (fairly easy to manage but can be expensive).
- Increase water circulation and mixing or aeration (fairly easy to control).
- Reduce the amount of direct sunlight on the water surface (not that easy to control).
It’s actually very similar to managing a domestic sewerage system where the focus is on promoting aerobic bacteria that breaks down nutrients (organic matter) without creating nasty smells.
So here are your options:
- Install a dam aerator. This will increase the oxygen levels and promote the growth of good nitrifying bacteria. It will also help to mix the water and stop temperature stratification by reducing the temperature of the surface water.
- Plant around your dam. Plants like sedges, bullrushes and lomandras in the inflow areas will filter out nutrients and also reduce sediment entering your dam. You can use hay bales here too, because hay contains nitrifying bacteria. Tall trees will help shade the water, reducing the amount of direct sunlight received and lowering the water temperature. It’s best not to plant large trees on the dam wall as they could impact the structure of the wall. Your Land for Wildlife Officer can assist with selecting plants suitable for your area. You can also have a look at the iNaturalist Guide: Logan Native Plants for Dams. https://www.inaturalist.org/guides/15391
- Introduce and foster competitive bacteria such as Nitrosomonas and Nitrobacter to actively compete with the algae for available nutrition. They use up excess nutrients and break down organic matter without stripping oxygen from the water. There are a number of products available online such as EcoClear, Biostim, Diatomix and The Water Cleanser.
- Restrict or remove livestock access to the dam. Nutrients and sediment from soil trampling and manure will be reduced when off-stream water is provided.
- Create floating islands. These are simple floating vegetation beds that hold nutrient-hungry plants. The beds allow for many more plants than simple waterside plantings, giving significantly more root mass for taking up excess nutrients. They also have the added advantage of providing brilliant dog, fox and cat-free habitat for nesting waterbirds! Instructions can be easily found on the web.
- Poisons (algaecides) are not recommended (by us) for treating algal blooms. This method only treats the symptoms of the problem. Mass killing of algae quite often results in increased dead matter which robs the water of oxygen as it breaks down, kills aquatic life and starts the algal process off again.
Several Local Governments in SEQ run workshops whereby Land for Wildlife members make floating wetlands for dams on their properties. Floating wetlands are a relatively quick and easy way to get more plant root mass working to take up nutrients out of a dam, thereby reducing the risk of algal blooms. Here are photos from floating wetland workshops in Brisbane and Moreton Bay regions, and an installed floating wetland.
To be clear, most cases will require more than one of these management approaches in combination to ensure long term success. Which ones will depend on your situation. If the above methods do not work for you, we recommend that you seek professional advice and begin by identifying which type of algae you have.
For a much more detailed understanding of the above, we encourage you to have a look at the following excellent websites.
- Waterqualitysolutions.com.au
- Sustainablefarms.org.au
- Fytogreen.com.au
Plus the Land for Wildlife Technical Note W2 Healthy Dams available at lfwseq.org.au.
Article by Peter Copping (Logan City Council)
with assistance from Michael Goode (Redland City Council), Stefan Hattingh (City of Moreton Bay) and Andrew Wills (Brisbane City Council).