Tree cover cuts algal growth

6 September 2010 by Tamera Jones

Some of Britain's cleanest rivers could suffer larger and longer-lasting algal blooms by 2080 due to climate change, researchers have discovered. But they say growing trees on riverbanks will probably be more effective at limiting this algal growth than cutting the amount of fertilisers and sewage going into them, and could save millions.

Algal scum

Algal scum in river.

The research focuses on the relatively clean rivers of the River Ouse network in northeast England. It reveals that reducing how much phosphorus (from sewage) gets into the Swale tributary in Yorkshire is only likely to limit algal growth by 11 per cent. But planting trees on the Swale's riverbanks as well quadruples this figure to 44 per cent.

Similarly, limiting the amount of nitrate from agriculture reaching Yorkshire's River Ure would probably cut algae by just 10 per cent. Instead growing trees on the riverbanks could cut it by 47 per cent.

Authorities currently spend millions of pounds on cutting the amounts of both phosphorus and nitrates getting into our rivers. But the researchers estimate that over a 20-year period, while cutting phosphorus from sewage would cost £28·1m and limiting agriculture would cost £20m, planting trees would cost a mere £270,000 in the River Ouse network.


When rivers contain too many nutrients like phosphorus and nitrogen they're more prone to algal growth and become what scientists call eutrophic. While this isn't necessarily a bad thing, eutrophication can lead to much lower oxygen levels, which can be problematic for fish.

Dr Mike Hutchins from the Centre for Ecology & Hydrology wanted to explore how important light and temperature, and river nutrients levels are in controlling how much phytoplankton (or algae) grows.

Hutchins' question was part of a bigger project, which seeks to understand the links between agriculture and river pollution both today and under a number of 'what if' scenarios.

Scientists had always assumed that higher nutrient levels were strongly linked with more algal growth. "We knew that nitrate, phosphorus, light and temperature all play a part in phytoplankton growth," says Hutchins.

But before this study, which is published in Science of the Total Environment, no one had figured out how important each of these was.

To find out, Hutchins, colleagues from CEH, researchers from engineering and design consultancy Atkins Ltd, and the University of East Anglia set about testing a well-established river model against river flow and quality data from the River Ouse under a number of scenarios. These included controlling sewage, controlling agriculture and planting trees on riverbanks.

They found that under climate change, the model predicted rising temperatures and more sunlight would encourage more algal blooms to grow.

But they also found that light levels encourage algal growth much more than nutrients, "which was a surprise," Hutchins says. "It's exciting because how much light gets to rivers is easier to control than nutrients. But of course this is addressing a symptom of eutrophication rather than its cause."

"Since 2000, under an EU directive, there's been a big drive to reduce phosphorus going into rivers from sewage treatment works, but we found that cutting phosphorus isn't as effective as planting trees," he explains.

"Tree planting is a radical alternative that appears to be both cheap and effective at reducing eutrophication, although putting this kind of scheme into a practise throughout a river catchment would need the support of lots of stakeholders and landowners," adds Hutchins.

He's keen to point out that the River Ouse may not be as prone to climate change as rivers in the south of Britain, and his next task is to run a similar study on the River Thamas where he says, "data to test the model against are more comprehensive."

The project was funded under a programme called Rural Economy & Land Use.

'Which offers more scope to suppress river phytoplankton blooms: Reducing nutrient pollution or riparian shading?' - MG Hutchins, AC Johnson, A Deflandre-Vlandas, S Comber, P Posen, D Boorman, Science of the Total Environment vol 408, p5065-5077, published online 9 August 2010, doi:10.1016/j.scitotenv.2010.07.033