Algae levels down but still dangerous

Weather conditions have caused toxic algae in Nelson’s Maitai River to detach from rocks, creating a danger to dogs.

The naturally-occurring organism forms on rocks in the river bed, creating thick brown or black mats with a musky smell.

Nelson City Council team leader science and environment Jo Martin said the algae growth peaked in spring and had dropped off, but there was still a risk for dogs.

‘‘It’s not that there is more of it, in fact there is less of it, it’s just that it’s more accessible for dogs,’’ Martin said.

‘‘Normally it would have rained and it all would have washed away.’’

Martin said they were ‘‘pretty pleased’’ with the lower levels which she attributed to to efforts to lower pollutants, increase plantings and remove sediment as part of Project Maitai. The main drivers of toxic algae are warm weather, elevated nitrogen levels and fine sediment.

But the river needed ‘‘one decent rainfall’’ to move the algae on and away from the banks, she said.

In the meantime, the council is warning dog owners to stop their pets from eating any algae floating on the surface or at the edge of the river.

Martin said dogs were attracted to toxic algae because of its smell, but it was very dangerous if ingested.

Tasman District Council spokesman Chris Choat said staff had found toxic algae levels very low.

‘‘However, in the lower Waimea and Wai-iti and Motupiko we still expect people to be careful, but toxic algae gets going a lot later in Tasman.’’

New Zealand Association branch Veterinary president Todd Field said the region hadn’t seen any cases of dogs ingesting toxic algae in the last couple of years.

‘‘I think that’s come down to people actually understanding the risks,’’ he said.

When dogs did eat toxic algae, or drink water close it, there would be acute neurological signs. ‘‘Tremoring, shaking, vomiting, the first four or five cases we saw in the region resulted in death, they just died,’’ he said.

Advice for dog owners was to get good at recognising the algae and check out areas of the river where the dog might swim first.