Blue-green algae: look before your dog leaps
CSIRO water scientists have a long-standing active program building up an understanding of the complex chain of events that leads to a blue-green algal bloom, and the aftermath of toxins released into the water.
10 October 2012
Glen Paul: G’day, and welcome to CSIROpod, I’m Glen Paul. Australia experiences algal blooms in all types of water bodies – rivers, estuaries, lakes, billabongs, water storage, and farm dams.
In low numbers algae aren’t generally a problem, and can actually be an essential part of a healthy body of water, providing oxygen, and as a food source for certain aquatic animals. However one type of algae, known as blue-green algae, has the potential to be harmful to humans and animals.
Australia’s inland waterways are vulnerable to outbreaks of toxic blue-green algae, often made worse in times of drought. The coming season is expected to be highly favourable to outbreaks, so joining me on the line to discuss what might lay ahead is CSIRO’s Dr Brad Sherman.
Brad, it’s certainly nasty stuff, but is it actually algae, or is it bacteria?
Dr Sherman: Oh, well strictly speaking it is bacteria, but it’s a photosynthetic bacterium, so they actually do, just like any plant, rely on having light, and nutrients, and water, to provide the energy to sustain growth.
Glen Paul: So what sort of toxins do the algae produce?
Dr Sherman: The algae microcystis produces a toxin microcystin, which has been the subject of a lot of investigation over the last 15 years, and that is hepatotoxin, so it does attack the liver. Now, the other algal species that is very common in south-eastern Australia is anabaena, and this produces a neurotoxin, and the neurotoxins, obviously by their name, attack the nervous system, and they can cause death quite rapidly in organisms if they’ve consumed enough of them.
Now that said, I’m not aware of any human deaths being reported by ingestion of these things. Certainly there haven’t been any in Australia, but there are numerous cases of livestock and animal deaths, typically situations where like a dog might go down and drink water with a scum on the surface which is contaminated and happens to have a high concentration of toxin in it, and in those cases it can kill an animal within half an hour.
Glen Paul: So how can these outbreaks be effectively managed?
Dr Sherman: Well, like all plants, standard bacteria require light and nutrients – they’ve obviously got plenty of water, because you won’t find them otherwise – so management efforts to control algal blooms typically focus on restricting access to either light or nutrients.
Now there’s a third option too, you can poison them, and this has been done for a long time in some water supply reservoirs around the world, and in a few locations in Australia, where you might dose it with copper sulphate, or some other copper compound, and that provides a sort of short term elimination of the aquatic life, but it’s typically not recommended because there can be chronic effects if too much copper accumulates in the environment, and it may impact on target species.
So that leaves pretty much trying to reduce the exposure to light, and to reducing the supply of nutrients, as the most promising strategies for reducing the amount of algae that can grow.
Glen Paul: So how do you then control that light?
Dr Sherman: The light exists up near the top of the water column, so in your lake, or your river weir pool, and typically extends downwards. In a lot of Australian systems you probably would have 99 per cent of the light absorbed in maybe the top six to eight metres, because we have fairly turbid water.
Now in order to grow, that means the algae have to be up in this top five to six metres, and because the blooming algae that cause the problems for us typically are ones that don’t sink, because they have little gas pockets in them, we try to produce deeper surface mixed layers.
Now the surface mixed layers, basically how deep the water is when you first dive in, before you notice the temperature getting colder as you swim down, and there are reasonable rules of thumb that we can apply, such that if you can make that depth of the mixed layer more than three times the depth to which 99 per cent of the light is absorbed you will get something else growing, because the blue-green algae require higher light levels than do a lot of other species that thrive in lower light, when there’s more turbulence in the environment.
So that’s how you’d try to control things with light, and it’s very difficult, and there’s very limited success using that approach. Typically you can make that work for smaller storages, say less than 10 000 megalitres, but even then it can be a little bit challenging, but there are some very dramatic success stories in some locations whereby introducing enhanced mixing, either by injecting bubbles onto the bottom, or using these big fans that pump the surface water down below, the reservoir operators have been able to produce this deeper mixed layer and limit the amount of light that’s available for the algae to grow.
Glen Paul: And what about reducing runoff from farms and other places that could potentially be providing nutrients for the algae?
Dr Sherman: Well, targeting nutrients is the other most commonly applied practice, and there the idea is simply if you reduce the amount of phosphorous in particular, but nitrogen is sometimes used as well, if you reduce the amount of nutrients available in the environment, then the total biomass that can be supported in terms of algal growth is just limited by the amount of nutrient, which makes sense. You know, if you’re growing sugarcane, and you didn’t have any nutrients, you could only grow as much cane as you have nitrogen and phosphorous to support.
Now to reduce the supply of nutrients we rely on a couple of things. Typically what’s been done is you target the obvious sources first, which is improving the level of sewage treatment from sewage treatment plants; when you’ve got large agricultural catchments you encourage best management practice in order to reduce the amount of fertilizer that runs off, and to decrease the amount of erosion that might be carrying nutrients in the sediment that’s washed off.
In most reservoirs, certainly virtually all reservoirs in Australia, they stratify every year – that just means they’re warmer at the top than at the bottom – so if you image the middle of water, the water column is completely mixed, and maybe eight or nine degrees, like it is here in Canberra, and then come late September the days get longer, and the surface of the reservoir starts heating up relative to the bottom, and that’s what produces the surface layer that allows the algae to grow.
And because the bottom stays cold, and you have a temperature change from top to bottom, this actually prevents the oxygen from the atmosphere getting down to the bottom of the reservoir, and the bacteria and the animals soon consume all of the available oxygen, and once there’s less than a very low concentration, effectively say zero oxygen, we start getting nutrients released from the sediments. There are two approaches that are used to reduce the amount of nutrients coming from the sediments, which we call the internal nutrient load.
One is to cap the sediments, you might apply an engineered clay, like Phoslock ®™, which Grant Douglas and CSIRO invented, or you might directly inject oxygen using diffusers, pretty much like these seeper hoses that you use in your garden for drip irrigation, only instead you push pure oxygen through the hoses and create a fine mist of bubbles, and by keeping the oxygen concentration higher in the bottom of the water you prevent the nutrients from coming out of the sediments.
The other way is by mixing the reservoir using big pumps or bubble plumes, and in that case what you’re doing is you’re setting up a very large scale circulation pattern in the reservoir which has the effect of producing a net downwards velocity away from where the bubble plume is, so if you just think of the whole water column just slowly dropping, and that will allow more oxygen from the atmosphere to get down to the bottom. And sometimes that works, sometimes it doesn’t – that’s just an engineering calculation, provided you have enough energy to introduce to the system, you can get the oxygen down into the bottom.
Glen Paul: Well it sounds like there is some hope on the horizon, and we might be able to dip our toe into the water come summer. Thank you very much for explaining it to us today, Brad.
Dr Sherman: You’re very welcome.
Glen Paul: Dr Brad Sherman. And to find out more about the research, or to follow us on other social media, visit www.csiro.au.