Keeping tabs on the blue-green menace

This is unlikely to occur in the UK, where medical water supplies are more rigorously checked. However, poisoning can occur during recreation, or where contaminated drinking water supplies are inadequately treated.

The World Health Organisation’s (WHO’s) working party on risk assessment for blue-green algal toxins will publish a book on the subject later this year. The group has already set a guideline level of 1µg/l in drinking water for the hepatotoxic microcystin LR (MC LR). Many blue-greens, such as Microcystis and Anabaena, are capable of producing dangerous combinations of hepatotoxic microcystins and neurotoxic anatoxins.

New water treatment methods could be in the pipeline, said Professor Geoff Codd of Dundee University. “Methods are still being developed, and will be effective, given adequate tuning and monitoring of the procedures. At the moment, we are investigating the breakdown of blue-green algal toxins in water by sunlight and bacteria.”

He added: “We carry out regular toxicological analyses for the Environment Agency (EA), Scottish Water Authorities and the private sector. We have developed immunoassays sensitive to most of the toxins, which can be used to complement existing chemical methods of analysis.”

Commenting on control measures Jan Krokowski, algal scientist with the EA said: “Once a water body has got blue-green algae in significant numbers, they are generally there to stay. However, there are a number of popular options for control. Some are only effective in small water bodies, and in many cases a combined approach is the most suitable.”

Options include artificial mixing, biomanipulation and nutrient input control. The first two options are considered here in more detail.

Artificial mixing is one of the most effective methods for limiting growth of blue-green algae in large reservoirs. Dr Petra Visser from the University of Amsterdam reported success from Lake Nieuwe Meer in Holland in 1996, where Microcystis aeruginosa abundance was reduced by artificial mixing and no blooms were observed, while blooms continued to form in adjacent lakes.

“During two years of artificial mixing, the phytoplankton changed in summer from a blue-green algae dominated community to a mixed community of flagellates, green algae and diatoms.”

Factors implicated were reduced sedimentation rates of non-buoyant species, the entrainment of buoyant blue-green algae in the turbulent flow, and decreased pH, following enrichment by carbon dioxide raised from the hypolimn-ion.

Biomanipulation is a popular option for small reservoirs, where populations are simpler to control. Zooplankton graze algal populations, so in theory, increasing the number of zooplankton should help to control algal growth. Limited success has been reported in studies where planktivorous fish such as roach (Rutilus rutilus) have been removed, and where predators of small fish, such as pike (Esox lucius), have been introduced.

Barley straw (Hordeum vulgare) is another option, which, according to Dr Jonathan Newman of Bristol University, can inhibit the growth of Microcystis by up to 94 per cent in the laboratory. Antibiotics released from the cell walls of the straw, or from fungi during decomposition were suggested as the cause, but the identity of the active compounds remains unknown.

Unfortunately, barley straw cannot be relied upon for a rapid effect, and in a study of Loch Airthrey, near Stirling, Dr Liam Kelly of Heriot-Watt University, Edinburgh found barley straw had no effect at all.

To try and restrict algal growth, bales were placed in the 6.9ha loch on two occasions, in April and September. However, severe blooms continued to occur throughout August and November.

EFFECTS ON SUPPLY

Terry Bridgman, senior scientist with ThaMES WATER IN WEST LONDON SAID: “Our dissolved air flotation filters and dual-media filter beds cope easily with the algal loadings we get IN WATER FROM OUR RESERVOIRS.”

However, the picture is not so clear-cut in remote areas, where there are fewer opportunities for treatment.

For example, Loch nam Brac, near Scourie in north west Scotland, has been experiencing a severe bloom of Oscillatoria agardhii for the last 18 months.

During this time, extraction for drinking water has been banned by the local environmental health office following identification of the bloom by the Scottish Environmental Protection Agency (SEPA).

Dr Shona Marshall of the West Sutherland Fisheries Trust said: “Blue-green algae do not always produce toxins, so the only safe approach is a complete ban. Even fishing is banned here at the moment, in case of accidental human contact. Some villagers were without a water supply for some time.”

Attempts to curb the bloom using barley straw bales have repeatedly failed. Fortunately for the villagers, north west Scotland is a water-rich region with many alternative sources.

To understand why blooms occur and how they can be controlled, the biology of blue-green algae must be considered. According to Professor Codd: “Blue-green algae are really bacteria, called cyanobacteria.”

Many blue-green algae have a gas vacuole, which expands or contracts in relation to photosynthetic activity. In over-intense light, the vacuole contracts and the cell sinks rapidly. This protective swim bladder mechanism gives blue-greens an advantage over less buoyant species, particularly in seasonally stratified water bodies.

Many species release dangerous levels of toxin when present in large numbers, which can occur naturally, given sufficient nutrients.

However, blooms are much more likely in water-bodies where phosphorus levels have been boosted by human activity, particularly where the total phosphorus level is greater than a fifth of total nitrogen.