All choked up

Barley straw boom across a river

Barley straw boom

11 January 2013 by Jonathan Newman

Algal blooms mean trouble for the water industry and pond-owners alike - and they're on the increase as more pollution reaches our fresh water. Jonathan Newman explains two techniques to keep them under control.

It may be toxic blue-green cyanobacteria, or masses of floating filamentous algae, but too much of it can block sluices, pumps and weirs and deprive pond-life of oxygen. Algae bloom, which means they grow rapidly, when there are lots of nutrients in their environment, usually phosphates. Blooms are getting bigger and more frequent in the UK as more and more nutrients reach our watercourses, principally from fertilisers washing out of agricultural soils and into our rivers and lakes. This has led to algae becoming the dominant group of aquatic plants almost everywhere in the UK, and especially in static water like reservoirs, gravel pits, lakes and ponds. They might be easy enough to remove from a garden pond, but to UK water companies they represent a major and increasing cost.

It's not a new problem - in fact research into controlling algae has been underway since the 1950s. The first government guidance on managing aquatic vegetation, published in 1958, lists chemical treatments using copper sulphate, chlorination, sodium arsenate and 2,3-Dichloronapthoquinone. Common sense and the European Plant Protection Products Directive of 1991 have since combined to stop more chemicals being thrown at the problem. It's a positive move, but the problem of algae growth in water-industry sites, recreational lakes and our own garden ponds remains. So can we find a way to tackle algal blooms that doesn't add to the chemical burden in our waters?

Barley straw has been known as an effective and sustainable treatment for years, but the reasons why it works remained something of a mystery. I have researched the anti-algal properties of all types of straw - lavender straw, organic and non-organic straw, brown and white rotted wood and rice straw - with experiments in canals, lakes, reservoirs and laboratories. All of them proved effective against algae to some degree - but why?

As the straw decomposes, microbes set to work on lignin, a chemical found in the cell walls of the straw. Analysis at the Centre for Ecology & Hydrology (CEH) found that this microbial action released more than 800 compounds from the straw, most of which had some anti-algal effect. We needed to find out which compounds were most important, and exactly how they affected the algae.

Research in South American rivers had already established a link between organic compounds called polyphenols and the amount of hydrogen peroxide in the water. Absorbing light causes these compounds to decompose, and as they do so they produce hydrogen peroxide. We knew hydrogen peroxide is toxic to algae at very low levels, and when we took a closer look at our barley straw we found that background hydrogen peroxide levels in the water increased by 30 per cent as the straw decomposed. So it looks like the magic ingredients in the decomposing straw are the polyphenolic compounds.

Another proven way of controlling algae is with low-frequency, low-power ultrasound. It's not exactly a new technique - it was developed by the Marconi company in about 1912, to protect the hulls of submarines from fouling by marine algae. Ultrasound is still a very new technique in the commercial world and is being tested by water companies, with other industries starting to look at it too. But we still don't understand the science behind the process, and CEH is running a small research project to try to find out.

We know that ultrasound affects biological systems in various ways. For example, resonating sound waves disrupt gas pockets, or vesicles, in blue-green algae - but this does not explain the effects on other algal groups. Other theories relate to a process called ultrasonically induced cavitation, which often causes pitting on brass propellers.

The damage is caused by the collapse of small gas bubbles, which produces very high pressures and temperatures and releases light - a process called sonoluminescence - as well as nitrate and hydroxyl radicals. But generating these radicals takes more power than the equipment currently used for algae control can probably generate, so the answer must lie elsewhere. In fact, we have again found increased levels of hydrogen peroxide in algae exposed to ultrasound, so there appears to be a common factor involved in these two very different methods. It seems that if you need to control algae, hydrogen peroxide is the key.

But chemistry aside, the important point is that all this research has led to a practical solution becoming available for gardeners and water companies alike, and you will find barley straw - if not yet a handy ultrasound device - in most aquatic garden centres today.

Jonathan Newman is head of the Aquatic Plant Management Group at CEH Wallingford