Seagrass restoration trials 'promising'

Initial attempts to restore a beneficial undersea grass in Whangarei Harbour are being hailed a success by increasingly optimistic researchers.

Jacquie Reed, the Northland Regional Council’s Monitoring Programme Manager – Coastal, says seagrass offers a host of environmental benefits, including serving as a nursery for young fish, home to marine invertebrates and a bird-foraging area.

About 60 years ago flourishing undersea meadows of seagrass covered about 14 square kilometres of Whangarei Harbour, including areas around Takahiwai, One Tree Point, Snake Bank, Parua Bay and McDonald Bank.

But by the 1970s only small pockets remained, the grass falling victim to increased sedimentation and discharges.

Jacquie Reed says work in the 1960s to deepen the harbour’s shipping channel saw more than a million cubic metres of sediment excavated and dumped in places like Snake Bank, Takahiwai and the entrance to Parua Bay. Another nearly three million cubic metres of sediment was estimated to have been discharged from the cement works at Portland between 1958 and 1971.

Subsequent improvements in water quality – largely attributable to tougher environmental standards - saw researchers and local tangata whenua begin investigating attempts to restore the grass several years ago.

She says seagrass relies on clean, clear water as without it, photosynthesis - the way a plant processes light to feed itself - can’t take place successfully.

“Similarly, nutrient levels in both seawater and seabed sediments need to be high enough to allow growth, but not so high they encourage the growth of parasitic algae or poison the plant.”

Jacquie Reed says as part of the trial, seagrass was taken from One Tree Point in April last year and transplanted to an area at Takahiwai where it had once grown.

The $50,000 trials have been funded by Northport and carried out by the NRC, the National Institute of Water and Atmospheric Research (NIWA) and Whangarei Terenga Paraoa Kaitiaki Roopu.

The process trialled three methods of transplant using sprigs of seagrass, sods of seafloor with seagrass in it and artificial seagrass mats with sprigs.

Nine one-metre square transplantation plots were monitored at Takahiwai every three months with use of the sprigs and the transplanted sods proving the most successful.

Seagrass now covers more than three-quarters of the trial plots and encouragingly, after just nine months it, had also completely recolonised the One Tree Pt areas it was originally taken from.

Jacquie Reed says although it is relatively early days, researchers are cautiously optimistic at what they regard as a highly successful trial to date. Researchers now hope to use knowledge gained during the trials to attempt larger scale seagrass restoration in Whangarei Harbour.

She says NIWA staff also plan to present initial findings from the Whangarei study to a national coastal conference at Auckland in September.