Seagrass in Whāngārei Harbour.
Bruce Howse, of the Northland Regional Council, says artificially restoring even small areas of seagrass habitat would have a significant positive impact on the harbour’s ecological health.
Mr Howse, the Council’s Coastal Monitoring Team Leader, says seagrass offers a wide range of benefits, including serving as a ‘nursery’ for juvenile fish, a home to marine invertebrates and providing areas for birds to forage in.
“About 60 years ago, there were thriving undersea meadows of seagrass spread over about 1400 hectares of Whāngārei Harbour, including areas around Takahiwai, One Tree Point, Snake Bank, Parua Bay and McDonald Bank.”
But by the 1970s only small pockets of seagrass remained as the plant succumbed to increased sedimentation that effectively starved it of sufficient sunlight. Seagrass remains scarce to this day.
Mr Howse says 1960s work 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 almost three million cubic metres of sediment was estimated to have been discharged from the cement works at Portland between 1958 and 1971.
However, a recently completed joint study by the Council and the National Institute of Water and Atmospheric Research suggests harbour water quality has improved to the point where it could once again support the plant.
Funding for the two-year, $39,000 study was provided by Northport via the Whāngārei Harbour Health Improvement Fund and a Sustainable Management Fund grant awarded to the Council by the Ministry for the Environment.
Encouragingly, the study suggests harbour water clarity has improved in recent years under tougher environmental standards on sediment and other discharges.
Mr Howse says clean, clear water is vital to seagrass survival, as without it, photosynthesis cannot take place successfully. Similarly, nutrient levels in the seawater and seabed sediments must be enough to allow the plant to grow, but not so strong they encourage the growth of parasitic algae or poison the plant.
Mr Howse says the Council and NIWA are now keen to work with the community over the next 12 to 18 months to carry out initial trial plantings to see how best to replant seagrass.
“If successful, this will be groundbreaking work for New Zealand. While seagrasses have been replanted overseas, to my knowledge ours would be only the second formally-documented replanting trial in New Zealand’s history.”
Mr Howse says the earlier trial took place in the Auckland area about 10 years ago, however, after some initial success, the seagrass declined due to the large amount of sediment in the water.
He says lessons learned in Northland over the next year or so may eventually allow seagrass to recolonise large areas of Whāngārei Harbour.
People interested in learning more about the seagrass study can find more information by visiting the research and reports section of our website.