Dense growths of filamentous algae pose serious problems for amenity and recreational exploitation in many Irish watercourses. Excessive algal growth impedes boat traffic, obstructs angling, clogs sluices and lock chambers, in addition to creating unsightly and malodorous masses. The growth of large algal populations can also cause serious diurnal fluctuations in dissolved oxygen levels and result in fish kills. Likewise, the death and decay of a large algal biomass can deoxygenate the water, killing fish and other aquatic fauna. Floating mats of algae can reduce the level of incident light that reaches submerged plants, thereby restricting growth and reducing overall productivity.

Control of filamentous algae in waterways using mechanical methods (cutting, raking, harvesting) has been largely unsuccessful. This is because large numbers of plant fragments remain and vegetation regrowth is rapid. The use of algicides in weed control trials conducted by the Central Fisheries Board on the Royal and Grand Canals provided moderate to good results but, because these herbicides are not selective, they also killed ecologically important submerged plant species.

There is a considerable body of research that demonstrates the antialgal properties of rotted barley straw, properties bestowed without having any discernible adverse impact on higher plants, invertebrate fauna or fish. A primary requirement for the successful use of barley straw is the maintenance of aerobic conditions. Unstable, short-lived algal inhibitors are released during the aerobic decomposition of the straw. These are highly selective against planktonic and filamentous algae and are algistatic rather than algicidal. There is strong evidence that these algal inhibitors are derived from oxidised polyphenolics released from solubilised lignin, although the precise nature or mode of action of the inhibitors remains unknown.

The effects of more than 100 barley straw treatments in the UK and Ireland were assessed, and results reveal that algal control was achieved, to at least some extent, in all types of water body, but was better in smaller watercourses (<5ha).

An algae infested section of the Royal Canal near Abbeyshrule.	Mattress of barley straw being placed in the Royal Canal at Mullingar.

Detailed trials conducted on an algal infested section of the Royal Canal between 1990 and 1993 demonstrated the effectiveness of barley straw in inhibiting nuisance algal growth. Mattresses of loosened straw, retained in garden netting, were anchored along the banks in the trial section. These were spaced at 50m intervals and alternated from bank to bank. A quantity of straw to provide a dose of 10g per cubic metre was applied. The straw was replaced at roughly six month intervals. A contiguous, untreated control section was monitored for comparative purposes.

Algal growth in the section broadly followed a cyclical pattern, with peak biomass between July and September and low production in February and March. In the treated section, however, algal biomass decreased from the time the straw was first introduced. Thereafter, as long as rotted straw was present, no filamentous algae were recorded. The absence of algae in this section between August 1991 and Spring/Summer 1993 permitted the recolonisation of higher plants, which are commonly less troublesome and more ecologically useful in fishery waters than algae. Further trials have produced similar results, and barley straw is now routinely used for algal control in many aquatic situations.

Contact: Dr Joseph Caffrey, Senior Research Officer, 
Central Fisheries Board,
Mobhi Boreen, Glasnevin, Dublin 9; 
Fax: 01-836-0060.