When a body of water becomes discoloured with a super-abundance of free-floating, microscopic plant or, in rare cases, animal life, it is said to develop a ‘water bloom’ or algal bloom.
Algae are primitive plants and include the seaweeds, fine hair-like green forms and microscopic single cells or colonies.
Some of the microscopic species are the most dangerous and can multiply rapidly to produce prominent green, red, yellow and other discolouration of water.
The most spectacular blooms in fresh water dams are produced by a group of algae known as blue-green algae and several of these can produce toxins when conditions are favourable.
Development of algal blooms
Blue-green algae blooms thrive in warm, calm, shallow bodies of water where the water is hard, alkaline and rich in nitrogen, phosphates, carbonates and organic matter.
Water inflow from fertile agricultural land and from sewage or certain industrial wastes also encourages algal growth.
Livestock poisoning attributed to blue-green algae usually, but not always, occurs during summer. The ponds or lakes involved have been found to be enriched in some way by the inflow of water from arable land or by animal excreta.
Blooms of the blue-green alga Nodularia spumigena form a scum on sheltered shorelines when concentrated by winds or currents and otherwise can form a suspension in the water.
These blooms have occurred in most spring-summer periods since the 1960s in the brackish to saline waters of the Peel-Harvey Inlet.
One of the most spectacular blooms of Nodularia occurred in November 1990 when a 1000 kilometre stretch of the Barwon-Darling river system in NSW was affected. These blooms can be toxic but fish, crabs and birds seem able to avoid the floating algal mats.
Nodularia blooms have been blamed for stock losses on farms in Western Australia, but the species more commonly implicated in livestock, waterfowl and fish deaths are Microcystis (formerly known as Anacystis) aeruginosa and Anabaena circinalis.
Two other species known to be toxic, Aphanizomenan ovalisporum and Cylindrospermopsis raciborskii, are less common.
Livestock are poisoned during drinking when the algae are swallowed. Fish are often safe until a pond or dam dries or is drained down, bringing them into contact with floating algae.
Anabaena and Nodularia have been implicated in skin and eye irritations in people and dogs, while Microcystis, Anacystis, and another less commonly encountered alga, Lyngbya, have been reported to cause hay fever symptoms.
There is also a suggestion that toxic products released from blue-green algae may be the cause of unexplained forms of human gastroenteritis.
Fish deaths during algal blooms may be caused by the toxin in the algae, by the depletion of oxygen in the water, by the liberation of hydrogen sulphide and ammonia caused by cell decomposition or by clogging of the gills.
Intoxicated fish are usually found dead or gasping at the water surface.
Microcystis aeruginosa forms an emerald green scum in the water bloom stage and when dried looks like pale blue paint. Anabaena circinalis spreads to a greater depth (to 70cm) but forms scum when winds and currents concentrate the water bloom in bays and backwaters.
Microcystis aeruginosa, Anabaena circinalis and Nodularia spumigena blooms produce a characteristic pungent, musky or earthy smell not unlike the insecticide Gammexane.
Signs of poisoning vary among livestock. Sheep poisoned with Nodularia spumigena suffer difficulty in breathing, muscular weakness and may show paralysis or nervous twitching. They may lapse into a coma before dying quietly.
Most commonly, they are simply found dead near affected water. Sometimes the algal scum can be found on the forelimbs, lips and muzzle.
Post mortem examination will often reveal a pronounced haemorrhage from the minute vessels under the skin and between the muscles.
Both Nodularia spumigena and Microcystis aeruginosa produce hepatotoxins, which cause severe liver damage and often may only be detected by examination by microscope.
Anaebena ciricinalis, on the other hand, produces neurotoxins and death results from paralysis of skeletal and respiratory muscles.
It is sometimes difficult to make a confident diagnosis of algal poisoning, since the clinical signs and post-mortem findings resemble a variety of diseases.
Any change to the colour of the water should be investigated and because the algae are mobile all points of the dam should be inspected for evidence of scum. If algal poisoning is suspected, prevent access by stock and arrange access to a different water supply.
If stock must be moved to another paddock, do it at a very easy pace.
Once the stock have been moved, seek veterinary advice to confirm the diagnosis and send a sample of the bloom for identification to the veterinary practitioner investigating the problem or the Department of Primary Industries and Regional Development (DPIRD).