Cyanobacteria in Maitai River

Nelson City Council and Cawthron scientists are working with students this summer to better understand what local conditions encourage cyanobacteria blooms (otherwise known as blue/green algae).

They are particularly focussed on monitoring the cyanobacteria growth in the lower Maitai, where it is mostly found during periods of low rainfall.

Earlier this month, Council warned dog owners against allowing their dogs to drink from the river or swim in any areas of the Maitai where cyanobacteria is present.

Generally, research shows that cyanobacteria is more likely to spread when river flow and water levels are low, during warm and stable weather, and if there is fine sediment in the river.

Monitoring by Cawthron scientists last summer showed the area of the river by Avon Tce and the Hardy St bridge had the highest coverage of cyanobacteria and also the greatest level of sedimentation.

Council’s Fresh Water scientist Dr Paul Fishers says unfortunately there is no quick fix for removing cyanobacteria. “We hope our continued research will help us further pin point the factors so we can work out how best to look after our river.”

Nelson City Council’s is focussed on four key initiatives to reduce cyanobacteria blooms as part of Project Maitai/Mahitahi. These are: changing dam operations to increase river flows helping to wash the algae off the rocks, improvements to gravel management in the river, riverside planting to help stop erosion and reduce sediment from getting into the river, and a forestry review to encourage best practice forestry operations to minimise nutrients and sediment getting into the water.

Cyanobacteria for beginners

  • In the last ten years, problems associated with cyanobacteria blooms have increased in rivers and waterways throughout New Zealand.
  • This naturally occurring algae likes clean, healthy water - its presence does not mean a river is unhealthy.
  • There is little that can be done to remove this algae apart from waiting for the river to clear it naturally in heavy rainfall.
  • Cyanobacteria is present in all of Nelson’s waterways but has particularly high coverage in the lower Maitai.
  • Cyanobacteria produces anatoxin, which is potentially lethal to dogs, stock and humans when ingested. Dogs tend to eat the algae, therefore they are most at risk.
  • More than 100 dog deaths across New Zealand were attributed to cyanobacteria in the last five years.

Find out more from our factsheet on cyanobacteria