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Research offers clues about causes of toxic algae in region’s rivers

Press Release – Greater Wellington Regional Council
New research into toxic algal (Phormidium) blooms in the Wellington region’s rivers gives our river users more information on what causes it.

GWRC have completed a report summarising toxic algae monitoring and research over the last decade and the results are interesting says Environmental Scientist Mark Heath.

Nutrients such as nitrogen and phosphorus running off the land have been identified as important drivers of toxic algal blooms but not in the way you might expect.

The data shows toxic algal blooms generally occur in rivers with low phosphorus levels.

Hutt, Waipoua and Waikanae rivers all have had several serious blooms in recent years. What they all have in common are they’re cobbled bedded rivers and have low phosphorus levels. In these low phosphorus rivers, the more nitrogen inputs there are the greater likelihood of toxic algal blooms.

In the Hutt River, blooms occur more frequently in the middle reaches around Silverstream which has moderate levels of nitrogen compared to the head waters where nitrogen levels are low. Nitrogen concentrations in the Hutt River are however still below the norm.

“Limiting the amount of nitrogen getting into our rivers may be one way to help reduce the occurrence of toxic algal blooms. Investigations of where nitrogen in the Hutt River comes from shows groundwater from under Upper Hutt City, as well from as rural areas of Mangaroa and Pakuratahi river tributaries are all key sources of nitrogen. “Combine this with conditions like less rain fall and warm weather and all are likely to produce perfect environments for toxic algal growth,” says Mark

“Unfortunately, there is no quick easy fix to the toxic algae problem. The best thing you can do is to know what toxic algae looks like so that you can keep your family and your pets safe and away from areas where it is,” says Mark.

Toxic algae is easy to identify by the dark brown/black mats it forms on rocks and its distinctive musty odour.

The full report and summary are on our website.

Find out more about toxic algae.

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