Date: 08 April, 2011

Mystery Marine ‘Pest’ Is Beneficial Seagrass

A local resident’s fears over the rapid spread of a grass-like marine plant at Taurikura have been allayed with news the ‘pest’ is actually a beneficial native seagrass.

The woman contacted the Northland Regional Council after becoming alarmed at the pace the plant appeared to be colonising the intertidal area around Taurikura Bay, about 30km south-east of Whangarei.

Council Estuary Management Advisor Jon Hampson says the council was able to identify the plant as the native seagrass ‘Zostera muelleri’, known to Maori as rimurehia.

“The good news is that this species seems to be thriving in Taurikura Bay at a time when seagrasses internationally are being lost at a rate of about two to five percent annually.”

Mr Hampson says seagrass can be found all around New Zealand’s coast, inhabiting the intertidal and sub-tidal areas of relatively sheltered waters such as estuaries.

“It’s a hugely important habitat acting as a ‘living filter’ for the sediment and nutrients running into our coastal waters, as well as being a nursery for some of our favourite fish such as young snapper.”

He says only a few years ago, Whangarei Harbour’s seagrass beds had almost entirely disappeared, surviving in only small isolated pockets.

However, a change in the cement manufacturing process at Portland and cleaner wastewater discharges had significantly reduced sediment levels coming into the harbour allowing seagrass to recolonise increasingly large areas. Plants had also been successfully transplanted from recovering areas into the previously bare parts of Takahiwai.

“The original decline was linked to poor water quality in Whangarei Harbour, including high levels of sediment from rural, urban and industrial activity. That in turn reduced water clarity, both blocking the sunlight seagrass needs for photosynthesis and also having a smothering effect.”

Mr Hampson says excessive nutrient levels in the water can also encourage algal growth which then competes with seagrass for the available light.

Despite the improvement in seagrass populations, he says reducing sediment run-off from the land remains one of the biggest physical environmental challenges for Northland and the reduction in seagrass levels in Whangarei Harbour is by no means unique.

“There have probably been similar losses across the region during the last century.”

A study of the Eastern Bay of Islands found a marked reduction in seagrass coverage in the mainland bays around Rawhiti between 1961 and 2006.

“Overall the reduction in seagrass coverage in the Eastern Bay of Islands during this period was around 73%.”

Mr Hampson says for most of Northland the land itself is the vehicle for sedimentation, with human uses of the land, rural and urban, the accelerator.

“The heavily-weathered clay soils covering much of the region are easily eroded by wind and rain producing the fine sediments that reach our coastal waters. Inappropriate land use exacerbates the rate of erosion so we need to manage the land for rural and urban uses in a way that minimises sediment losses.”

Mr Hampson says the regional council works with a wide variety of Northlanders to try to manage activities that can significantly increase sedimentation and may adversely affect our marine environment.

“We do this through statutory measures (regional plans and resource consents) and voluntary means such as farm visits, workshops and funding initiatives that prevent soil and riverbank erosion. This can include tree planting and fencing stock out of waterways.”

This year’s NRC Environment Fund alone contributed more than $400,000 towards environmental improvement initiatives with thousands of metres of stock exclusion fencing erected across the region

Mr Hampson says managing erosion requires collective action and council’s Land Management Advisors are happy to provide advice. They can be contacted on 0800 002 004.

ENDS

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