Publications: Research reports and publications

Phormidium blooms - relationships with flow, nutrients and fine sediment in the Maitai River

15 June, 2015
Cawthron Report 2723. Prepared for Nelson City Council.

EXECUTIVE SUMMARY

Blooms of the mat-forming benthic cyanobacterium Phormidium have become an annual problem in the lower Maitai River. A range of environmental parameters have been linked to Phormidium blooms in other regions of New Zealand, but there was no data on what variables promote blooms in the Maitai River.

In this study we evaluated summers (2013 to 2014, and 2014 to 2015) of monitoring data from seven sites on the Maitai River. Weekly monitoring of Phormidium and other benthic algae, and a range of physico-chemical parameters was undertaken at three sites (Avon Terrace, Downstream (D/S) of Dennes Hole and Campground [near the Maitai Valley Motor Camp]) that experience Phormidium blooms in the lower Maitai River. Monthly monitoring of the same variables was undertaken at four sites (Smiths Ford, Downstream (D/S) of Backfeed [Maitai Reservoir], South Branch [control site] and North Branch) in the upper river that either do not, or only very occasionally, experience Phormidium blooms. Sediment traps were used at selected sites to investigate sedimentation rates. Sequential extraction was used to quantify biologically available phosphorus within this sediment. River gauging and nutrient analysis data was collected to enable a preliminary (summer low flow) estimate of nutrient loads from key tributaries.

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