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Zostera muelleri is New Zealand’s only native seagrass. Photo, Ewen Cameron.
Snells Beach residents concerned by the swathes of seagrass inundating the coast of Kawau Bay held a meeting with Auckland Council marine experts this month.
Dr Megan Carbines and marine ecologist Tarn Drylie from Auckland Council’s research and evaluation unit (RIMU) met with the Snells Beach Ratepayers and Residents Association (SBRRA).
The cause of the sudden surge in seagrass is not entirely known, however it has been noted in various locations across the Auckland region.
Ms Drylie and Dr Carbines theorised that a return of seagrass to Kawau Bay may be a result of a slight improvement of water quality recorded by RIMU.
But, they said it could equally be recovery from disease caused by marine slime mould or an extended lifecycle.
“But we definitely would not expect to see an increase in seagrass if the environment was degrading,” Ms Drylie said.
She said that seagrass used to be prevalent but was believed to have been smothered by sedimentation caused by development from the 1930s to the 1970s.
However, seagrass thrives in an environment with a moderate amount of silt and better siltation management may have improved conditions for seagrass.
She said seagrass tends to anchor some sediment making the sea floor muddier rather than sandy.
The question on residents’ lips was whether the silty seafloor might cause Snells Beach to be “taken over” by mangroves, similar to Kawau Island.
Ms Drylie said it was unlikely as coastal currents still dispersed sediments, unlike estuaries where mangroves had taken hold.
Zostera muelleri is New Zealand’s only native seagrass and it changes its pigment from green to brown depending on how much sunlight is available.
Ms Drylie said healthy seagrass beds slowed down currents, removed carbon dioxide from the water column and stabilised the seabed. They are also an important snapper and trevally nursery.
