20 Apr 2023

Seeds of hope for seagrass meadows

From Our Changing World, 5:00 am on 20 April 2023

Nelson’s Haven is an 8-kilometre expanse of mudflats. If you don gumboots and squelch your way out at low tide, you’ll find it is also an expansive grassy meadow.  

The grass is seagrass, the only flowering plant that lives in the sea. It provides key habitat for many fish species, as well as performing a vital ecological role cleaning water, stabilising mud and sequestering carbon. 

Four people wearing gumboots stand on a puddle-pocked mudflat facing the camera. The sky is blue and there are hills in the background. One person is carrying a square frame.

The Cawthron Institute seagrass research team out on the seagrass meadows of the Haven Estuary in Nelson. From left: Dana Clark, Dan Crossett, Demi Fearn, Anna Berthelsen. Photo: Alison Ballance

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Marine ecologists from the Cawthron Institute have just completed the first summer of a three-year research project to develop new ways of restoring seagrass meadows – using seeds.  

Until recently, it was thought that seagrass in New Zealand, which belongs to the species Zostera muelleri, only rarely flowered and set seed. But the marine restoration team at the Cawthron Institute have discovered there are actually plenty of flowers, although team leader Dr Dana Clark admits the greenish flowers are cryptic and hard to find. 

A close-up of strands of seagrass showing the tiny flower structure that looks more like hairs than a quintessential flower.

Seagrass flower. Photo: Dana Clark / Cawthron Institute

The Haven is one of three study sites near Nelson, along with Waimea and Delaware / Wakapuaka estuaries, for the project Restoring Aotearoa New Zealand’s Seagrass Meadows. Over the past summer, the seagrass team has been surveying its field sites and monitoring flowering. The researchers have discovered that flowering begins in October and ends in February, with peak flowering in December. 

Looking down into a white bucket filled with dirty water and strands of seagrass.

A good day’s work: a bucket filled with seagrass flowers. Photo: Bruce Green / Cawthron Institute

Dana says the team collected about 3000 seagrass flowers, which were taken back to the lab. Here they were kept in two tanks of bubbling water which marine ecologist Dr Anna Berthelsen describes as a ‘seagrass spa.’ The bubbles keep the seagrass well oxygenated as they mature, allowing them to develop naturally and shed pollen into the water which fertilises female flowers that go on to develop seeds. 

A woman in gumboots kneels on a mudflat strewn with seagrass, holding a square frame.

Summer student Demi Fearn with a quadrat used to count seagrass flowers along a study site transect in the Haven Estuary. Photo: Alison Ballance

Dana says they were thrilled to collect nearly 600 seeds, which are being stored ready for germination trials. Armed with this summer’s findings, she hopes the project will produce even more seeds next summer. 

The aim of the project is to develop a method for seed-based seagrass restoration that iwi and community groups can use. Many hands make light work when it comes to picking seagrass flowers, and Dana says they are already involving a range of people in the field work, from project funders Nelson Port and the One Forty One forestry company, as well as interested locals from Friends of the Haven.  

A close-up of a tiny brown seed that has split open with a white sprout emerging.

A New Zealand first: successfully germinating a seagrass seed. Photo: Breanna Hindmarsh & Rachel Hooks / Cawthron Institute

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