A cyanobacterial bloom in Lake Taihu, China, in October 2010.
Credit: Science/AAAS
A cyanobacterial bloom in Lake Erken, Sweden, in June 2011.
Credit: Science/AAAS
BRISBANE: Controlling nutrient run off in our waterways might help decrease the incidence of toxic blue-green algae blooms, researchers have said.
Rising global temperatures and increased nutrient loads can drive toxic blue-green algae, or cyanobacterial, blooms which, when they enter waterways, can be very costly to control. These blooms also pose significant health risks to humans and livestock that drink contaminated water.
Now researchers from Cornell University in New York and the University of Adelaide in South Australia have suggested that the toxic blooms can be managed more effectively if nitrogen and phosphorus pollution is limited.
"We found that increasing nutrients and temperatures have a synergistic effect on cyanobacterial blooms, but only when nutrient levels are high. Ultimately nutrients are the primary factor influencing bloom formation," said co-author Cayelan Carey from Cornell of the paper published in Science today.
Competitive phytoplankton
Abundant wherever there is water, cyanobacteria are the only prokaryotes (organisms that lack a cell nucleus) that use plant-like, oxygen-generating photosynthesis. Many cyanobacteria have gas-filled vesicles that enable them to rise to the water surface, causing massive blooms.
"The cyanobacteria are able to out-compete other phytoplankton when the nutrient level in the water is high," said Carey.
An increase in cyanobacterial and algal biomass because of high nutrient levels leads to a phenomenon known as thermal stratification, by which water separates into layers as it is heated and the less dense warm water sits at the surface. This provides a favourable environment for the buoyant cyanobacteria, who reduce the available light for other phytoplankton competitors by dominating the water surface.
Booming blooms
These toxic blooms can be toxic to both humans and livestock. In some cases, livestock have died from cyanotoxins traced back to the water source.
They also represent substantial economic costs in water treatment, loss of tourism and recreational revenue and in the decline in value of bloom-infested waterfront properties.
The main factors that may contribute to an increase in blooms are climate change and changing land use. Nitrogen- and phosphorus-based fertilisers being run off into waterways increases the nutrient content available for cyanobacterial growth. Previous research had suggested that as global warming is inevitable, so too must be the increase in cyanobacterial blooms, but Carey disagrees.
"Some studies have shown that a decrease in nutrients can hinder the occurrence of blooms despite an increase in water temperature," she said.
Controlling cyanobacteria
This is not news. Cyanobacteria (blue-green algae) have been present for millions of years, in fact are largely responsible for the oxygen in Earth's atmosphere. It has been known for a long time that nutrients, (particularly phosphorus), light, warmth and quiescent waters are the prerequisists for the formation of blooms. Prevention of nutrients entering receiving waters that tend to be stagnant - especially in warmer months when there is more daylight - has long been advised as a means of preventing blooms. How best to achieve this aim is still being argued. Meanwhile researchers still churn out papers that present old evidence in new ways.
In the short term there are few practical methods of preventing blooms. Once blooms occur, algicides can be used to kill the bacteria - which then release their toxins. The most promising control measures appear to be mechanical; products such as AquaArmour that aim to prevent light penetration of the epilimnion, or SolarBee reservoir circulators that disturb the preferred habitat of the bacteria by continuous circulation above the thermocline. Both methods can be deployed right now while we figure out how to reduce nutrient loads in the future.
Knowledge of cyanobacteria and their effect on the aquatic environment has been around for decades; practical means of preventing blooms still appear to be overlooked or ignored. Why?
Algal Blooms- Control
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