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Important Porirua Harbour habitats at risk

Important Porirua Harbour habitats at risk

The results of a recent survey show sediment entering the harbour continues to be a problem in Porirua Harbour.

The latest five-yearly survey of Porirua Harbour intertidal habitats has shown that the area of estuary covered in freshly deposited mud has increased substantially, particularly near the Kakaho and Horokiri streams.

Soft mud reduces the water clarity, the amount of oxygen in the sediment, and the growth of vital seagrass. This results in a loss of habitat, reduces recreational and aesthetic values and the estuary’s ability to function effectively.

The density of seaweed growth near the mouths of the Porirua, Pauatahanui and Horokiri streams is evidence that nutrient inputs (nitrogen and phosphorus) remain high. Seaweed mats can smother sediment and seagrass. When the seaweed dies it starves the sediment of oxygen and causes black smelly mud.

Although there are large, healthy areas of saltmarsh in the eastern Pauatahanui Arm, saltmarsh is absent in the Onepoto Arm of the harbour. Saltmarsh areas are highly productive, provide valuable habitat for fish and birds, and offer flood and erosion protection.

“Though this is only the second habitat mapping survey in Porirua Harbour, the findings confirm that important habitats such as seagrass and saltmarsh remain vulnerable to the effects of human activities on land” says Greater Wellington Regional Council’s Coastal Scientist, Dr Megan Oliver.

“Sediment and nutrient inputs from the surrounding land and waterways are reducing sediment quality, biodiversity and healthy functioning of the estuary”.

The Regional Council commissioned the second broad scale habitat mapping survey of the intertidal area to assess the changes in key estuary habitats (eg, saltmarsh and seagrass) and substrate (e.g. areas of mud and sand).

The information has been gathered as part of the Regional Council’s State of the Environment Monitoring requirements under the Resource Management Act.
Information from the report will feed into the Porirua Harbour and Catchment Strategy and Action Plan’s sediment reduction and estuary restoration work and help focus policy and management decisions related to land-based activities.

The Strategy and Action Plan is a council and community project that has been prepared and adopted by four key stakeholders – Te Runanga o Toa Rangatira, Porirua City Council, Wellington City Council and Greater Wellington Regional Council - along with 10 other agencies. A new joint committee of the four organisations is now being set up to work more closely and to oversee the implementation of the strategy.

Ngati Toa Rangatira kaumatuaTaku Parai says it’s good that the regional council is closely scrutinising the condition of the harbour to ensure everyone is aware of these issues and helps to turn them around.

“Loss of significant habitat in the harbour is a problem that has affected Ngāti Toa for many generations. Having access to the information provided by these surveys is empowering for iwi and the whole community to work together for positive change,” says Mr Parai.

The issues arising from the survey are being addressed as part of the harbour strategy.

Porirua Mayor Nick Leggett and Regional Councillor Barbara Donaldson point out that a lot of work is happening including sewer and stormwater upgrades, community education, the development of an estuary restoration and catchment revegetation framework, research and a review of the region’s regulations
Work on a catchment erosion control plan will be completed in February followed by its implementation. This, along with improved earthworks control, will have an impact on reducing sediment entering the harbour. It’s also a reminder to the community that re-vegetation of the catchment is one of the most significant things we all can push to reduce sedimentation.

Over summer, the regional council will be carrying out routine annual measurements of sedimentation in the harbour. There are also plans to undertake habitat mapping of the subtidal areas of the harbour.

The report can be downloaded from the document library on GWRC’s website – search for “Porirua Harbour broad scale survey” or visit http://www.pcc.govt.nz/Publications/Porirua-Harbour-and-Catchment-Management-Programme


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