As the Bureau of Meteorology has already warned us, Australia is in for a hot, dry summer as the current El Niño takes hold. Those conditions are ideal for blue-green algae to bloom in lakes, ponds and reservoirs.
Photosynthesising bacteria, also known as cyanobacteria, are found in all aquatic environments from the tropics to the poles. Most species have no adverse impact on the environment, but a few have nastier effects, and some are toxic to humans and animals.
Blue-green algae can form vast “blooms”, some large enough to be seen from space. In the Australian drought summers of 2009 and 2010, for example, hundreds of kilometres of the Murray River suffered major cyanobacterial blooms, which hampered the use of water for drinking, agriculture and recreation.
These blooms occur mostly in still water bodies and can be found throughout Australia. Some blue-green algae form visible surface scums, while others remain hidden in the water column. Some live in freshwater; others float in the open ocean or even live on the sea bed.
Tiny and toxic
The toxins produced by some blue-green algae can affect the nervous system, the liver and kidneys, or be toxic to cells more generally. Humans can be affected by drinking contaminated water or eating affected shellfish. Direct contact with water can also cause itching and rashes. Worse still, the toxins can remain in the water even after the blue-green algae themselves have vanished - in some cases for weeks, depending on the conditions.
Larger blooms tend to occur where there is an excess of nutrients, often the result of fertiliser runoff from intensive agriculture or other degradation of the catchment system. This means that, throughout Australia, the potential for blooms is increasing.
Water temperature also influences algal blooms, for two reasons. First, blue-green algae grow faster in warmer water and, second, warmer temperatures increase “thermal stratification”, in which a warmer surface layer overlies deeper, cooler water. Stratification allows cyanobacteria to flourish in the warmer sunlit surface waters because of their unique ability to make themselves float.
So how do you steer clear of blue-green algae? The obvious tips are to avoid drinking untreated water from still, calm water bodies, and to be mindful of children or dogs playing by the water.
Green surface scum is the most obvious tell-tale sign of an algal bloom, but not all species of cyanobacteria form scums. Discolouration of the water, particularly a green colour, can also indicate the presence of blue-green algae. Some species, such as Microcystis, give off a distinctive odour, although some other blue-green algae also create musty-smelling chemicals that are non-toxic.
It is comforting to know that if water quality is at risk, your local water authority is probably on top of it already, and will typically erect warning signs each summer. Many lakes and reservoirs are routinely closed for recreational use to protect the public from toxic blooms during the hotter months.
The forecast hot, dry summer is likely to be a boon for blooms, given that blue-green algae prefer warm, still water. This means that areas that typically get algal blooms might find they are bigger and longer-lasting this summer. In Australia’s southern states, the blooms might also start earlier in the summer and last longer into autumn.
But the scale of blooms also depends on nutrients, so reducing the amount of nutrients that are washed off the land during rainfall events can provide a way of controlling them. This can be done by reducing land degradation, for example, reducing erosion, creating vegetation buffer zones along river banks, and avoiding excessive fertiliser use.
Some of these processes will take time to implement and therefore won’t help us this summer. But combating cyanobacteria in the longer term will help to protect the environment, allow continued recreational use of water and, most importantly, protect our precious drinking water.