Periphyton and macrophytes in seven Hauraki-Coromandel rivers

Report: TR 2017/28

Author: Fleur Matheson and Rohan Wells (National Institute of Water & Atmospheric Research Ltd (NIWA))

About this report

Periphyton are plants that grow in by attaching themselves to the gravel or rocks on the bottom of rivers and streams, or to other plants living there. The government’s National Policy Statement for Freshwater Management 2014 lists periphyton as a key attribute for assessing the ecological health of rivers, and identifies a limit to the amount of these plants that can be present in a river.
Previous work by WRC has shown that periphyton are present in some rivers in the Hauraki and Coromandel areas. This study was commissioned to determine whether the amount of periphyton is likely to exceed the government’s limit in the different river types found in this part of the region.
One site was surveyed on each of five rivers, while two sites were surveyed on a sixth river (i.e. a total of seven sites). The sites were all visited monthly for a year. Stones or rocks were present on the bottom in three of the rivers, and limited amounts of periphyton grew on these. At the other sites the bottom material was sand or mud; here, the periphyton grew on other plants called “macrophytes” (often called “oxygen weeds”). At two of these “soft-bottomed” sites, the government’s limit to the permitted amount of periphyton was exceeded.
The report concluded that it would be difficult to control the amount of periphyton present at these sites by managing inputs of the plant nutrients nitrogen and phosphorus to the rivers, both of which are highly nutrient-enriched. Instead, it suggested that shading the river channel by planting trees on the riverbanks would be a more effective way of reducing the amount of periphyton in them.

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Periphyton and macrophytes in seven Hauraki-Coromandel rivers [PDF, 3.8 MB]