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Toxic Algae

Toxic algae are naturally occurring. They are particularly prolific in summer when longer days and higher temperatures provide good growing conditions.

In Marlborough the most common toxic algae are Phormidium. They form dark brown mats growing on rocks in the river bed and are mainly found in fast flowing water, such as riffles. When algae mats become very thick they start to detach from the rocks and float in the water, accumulating along the edges of the river. This is when the algae present the greatest risk to recreational users and dogs.

The algae have a musty smell that dogs are attracted to. As little as an area of a 50 cent piece can be fatal to a dog when digested. The neural toxins produced by the algae cause muscular twitching, paralysis and convulsion. If your dog shows any of these symptoms after spending time in a river, take it to the vet immediately. In extreme cases, death can occur within 30 minutes after the first symptoms are noticeable. Small children are also at risk as they might accidentally ingest the algae when playing in the water.

In a number of neighbouring regions toxic algae mats have caused several dog deaths. Although there have been no confirmed cases in Marlborough, we have seen an increase in the occurrence of toxic algae mats in our streams and rivers.

This is a very wide-spread phenomenon and Council is unable to place warning signs at all sites with toxic algae. It is therefore important that you familiarise yourself with what these algae look like and keep children and dogs away from rivers with toxic algae.

There are many different kinds of algae that grow in rivers, but toxic algae have a quite distinct appearance. Below is a link to a video made by Cawthron and pictures of toxic algae from Marlborough rivers to help you with the identification.

​Figure 1: Dark brown toxic algae amongst other algae in the Taylor River (arrows). The green and light brown algae are harmless.​

Figure 1: Dark brown toxic algae amongst other algae in the Taylor River (arrows). The green and light brown algae are harmless.

Figure 2: Toxic algae usually form thicker mats than other algae. The green stringy algae are harmless (Kaituna River).

Figure 2: Toxic algae usually form thicker mats than other algae. The green stringy algae are harmless (Kaituna River).

Figure 3: The toxic algae (arrows) is a lot darker in colour than other brown algae (Omaka River).

Figure 3: The toxic algae (arrows) is a lot darker in colour than other brown algae (Omaka River).

Figure 4: Sometimes the toxic algae can be a slightly lighter colour as in this picture of the Wairau River, but are still significantly darker coloured and form thicker mats than other algae.

Figure 4: Sometimes the toxic algae can be a slightly lighter colour as in this picture of the Wairau River, but are still significantly darker coloured and form thicker mats than other algae.

Figure 5: Toxic algae grow mainly in the faster flowing water such as riffles. These photos of the Omaka River show toxic algae in the faster flow, identifiable by the rippled water surface (left) and no toxic algae in the slower flow with a smooth water surface only meters upstream (right).

Figure 5: Toxic algae grow mainly in the faster flowing water such as riffles. These photos of the Omaka River show toxic algae in the faster flow, identifiable by the rippled water surface (left) and no toxic algae in the slower flow with a smooth water surface only meters upstream (right).

Figure 6: The furry appearance of the smooth algae can be seen in this picture. Note that the algae mat in this picture is starting to detach from the rock (red arrow).

Figure 6: The furry appearance of the smooth algae can be seen in this picture. Note that the algae mat in this picture is starting to detach from the rock (red arrow).

Figure 7: This picture shows toxic algae that have detached from the rocks and accumulated along the stream edge. This is particularly dangerous as dogs will be attracted by the musty smell. Small children might also play with the algae and accidentally ingest some of it.

Figure 7: This picture shows toxic algae that have detached from the rocks and accumulated along the stream edge. This is particularly dangerous as dogs will be attracted by the musty smell. Small children might also play with the algae and accidentally ingest some of it.

If you suspect that you dog or child has ingested toxic algae visit your doctor/vet immediately.

Figure 8: After long dry periods toxic algae can become exposed as a result of dropping water levels. This increases the risk to dogs as they might eat it while in the water.

Figure 8: After long dry periods toxic algae can become exposed as a result of dropping water levels. This increases the risk to dogs as they might eat it while in the water.

Figure 9: Close-up of toxic algae outside of the water. Do not touch the algae without wearing protective gloves.

Figure 9: Close-up of toxic algae outside of the water. Do not touch the algae without wearing protective gloves.

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