Assessment of the ecohydrological functioning of Otakairangi Wetland, Northland

Otakairangi is the largest wetland remnant on the floodplain of Northland’s Wairua River near Hikurangi. Extensive peat deposits many metres deep accumulated over thousands of years on the valley floors of the Otakairangi and Riponui valleys, as they did elsewhere within the former Hikurangi “Swamp”. The present‐day Otakairangi Wetland, with an area of 2.6 km², represents around 20% of its former extent.

This report provides an assessment of the ecohydrological functioning of Otakairangi Wetland in its current state, based on information gathered during a field survey, discussions with landowners, and other sources. The wetland, which contains representative communities of rare peatland plants, is a good candidate for restoration, including re‐introduction of the now locally extinct climax peatland plant species Sporadanthus ferrugineus.

Hydrological modification to Otakairangi over the past century has included a deep central drain that has largely eliminated the natural diffuse flows of the former Otakairangi Stream as it traversed the wetland, and lowered water tables. Deep drains bordering the wetland’s south western and south eastern edges have also contributed to a modified hydrological regime. Over much of the past century water table lowering and frequent fires led to dominance by tall manuka, reduced peatland biodiversity, and severe peat degradation.

There is good evidence of fairly recent changes to Otakairangi’s vegetation towards greater dominance, especially in the east, of the key peat‐forming plant and ecosystem engineer Empodisma robustum, including vigorous growth of raw fibrous peat. As the dense manuka cover has declined, Empodisma and allied plants have been able to naturally colonise and spread.

Despite encouraging signs of some natural vegetation recovery within Otakairangi Wetland, there remain many unanswered questions about the trajectory of this recovery and the ongoing effects of drainage, as well as inundation by flood flows from the highly modified upstream catchment. Also, fire remains a constant threat that could reverse this trajectory. A number of key research questions are provided in this report that will guide the development of a research plan in support of ongoing restoration and monitoring efforts.

A number of restoration options are presented that should be considered at Otakairangi. These include minor to more major hydrological manipulations, which will require careful investigation and flow modelling in addition to working closely with affected landowners. As a first step, detailed surveys are required of drain invert levels and wetland and farmland surface levels. Monitoring of drain and wetland water tables should be initiated as soon as possible, to support research into wetland hydrological regimes and to further understand the spatial effect of water table lowering adjacent to drains. Minimum water level controls should be established along the central drain to prevent further negative effects within the wetland.