Why are some algal blooms more toxic to our drinking water than others?

Our tap water is safe to drink and that we can swim in our freshwater reservoirs. We take this for granted. But our water is treated before it arrives in our homes and every summer our reservoirs are closed because blooms of toxic blue-green algae poison the water (watch this 60 second video to learn more about blue-green algae).

water closure
Warning: there are blue-green algae present! But, how toxic is the water?

However, not all blooms are equally toxic and we would like to know why some blooms are worse than others. Dr Willis from the Australian Rivers Institute has been working on Brisbane’s water supplies to figure out why (get a pdf version here).

Brisbane’s drinking water is stored in large reservoirs that collect the rain water that falls in the surrounding catchment; it is then treated before being piped to our homes. The Brisbane catchment includes the hills, forests and farmland that surround the Brisbane River and this landscape has changed drastically in the last 150 years of European settlement. The forests have been cut down to make way for towns, cities and farmland. These changes mean extra nutrients are traveling across the land and entering the creeks, rivers and ending up in the water-storage reservoirs.

Warning: there are blue-green algae present! But, how toxic is the water?

The nutrients are food for the microscopic plants (phytoplankton) but too much and the natural balance is disturbed. This can lead to blooms – when a single species takes over the reservoir. The species that bloom are known as ‘nuisance species’ and in freshwater are usually blue-green algae. The most common nuisance species of blue-green algae around Brisbane is called Cylindrospermopsis raciborskii, and it can be toxic. It blooms in summer and is responsible for the closure of the reservoirs to recreational use. When the species is present in large numbers our water also needs more treatment to make it safe to drink.

The reservoirs are closely monitored for the presence of Cylindrospermopsis raciborskii so the public can be warned as soon as they are appear. Currently water agencies monitor the water by counting the number of blue-green algae individuals: if there are too many then the reservoir is closed. However, although there is only one nuisance species, there are different strains that despite all looking the same can be either toxic or non-toxic. This means that a bloom may not necessarily be toxic and dangerous but actually harmless. Without testing for the toxin itself we have no way of knowing this from the current monitoring of individual counts. This is where the research of Dr. Willis is important.

When Dr. Willis first started investigating the question of how to improve monitoring to predict the amount of toxin, it was thought that changing nutrient conditions would change the amount of toxin produced by the toxic strains. Dr. Willis took different strains and grew them under different amounts of nutrients and measured how much toxin was produced. What she found surprised her and the scientific community.

Different to expectations there was no change in toxin production with changing nutrients: the amount of toxin remained constant for each cell. But the amount of toxin in each cell of the different strains can be different. When blooms consist of mixes of different strains, each producing different amounts of toxin, the amount of toxin produced has to be measured directly.

Willis in lab
Dr Willis inspects an experimental bloom in the lab

Dr Willis’ research has shown that blooms of toxic algae are more complicated than previously thought. There is no easy way to monitor blooms and determine their toxin yield. Dr Willis’ and Professor Burford’s group at the Australian Rivers Institute are working hard to work this out. Ultimately this will help us better manage our water ways so that clean water is something we can continue to take for granted.

Read more about algal blooms on The Conversation 

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