URBAN NATURE – Seagrass plays a vital role


Seagrass meadows are a feature of shallow-water coastal ecosystems around the world. There are about 50,660 named species of seagrass worldwide. The greatest diversity is found in tropical regions.
New Zealand has just one species, Zostera muelleri subsp. novazelandica, which grows mainly in the intertidal zone. Seagrass grows throughout New Zealand, from the far north down to Stewart Island. It is found from sheltered estuaries and coastal beaches to intertidal coastal reef platforms, to subtidal bays around coastal islands.
Seagrasses are true flowering plants, with stems, leaves, roots and flowers, which have become specialised to grow rooted and submersed in estuarine and coastal environments. Seagrasses are the only flowering plants that grow in the sea, and are thought to be derived from terrestrial plants that colonised the marine environment 100 million years ago.
Seagrasses are known to play important ecological roles in estuarine and shallowwater coastal ecosystems. They enhance primary production and nutrient cycling, stabilise sediments, elevate biodiversity and provide nursery and feeding grounds for a range of invertebrates and fish.
As in many regions around the world, New Zealand’s seagrass meadows are at risk from human activities, especially from increased sedimentation and reduced water clarity associated with runoff from land-use. Over the last 100 years, extensive areas of seagrass have been lost from many locations, including Whangarei, Manukau, Waitemata, Tauranga and Avon-Heathcote estuaries. Subtidal seagrass has been most affected.
Encouragingly, there has been some resurgence of seagrass in the Auckland region and there are still extensive seagrass meadows in a number of New Zealand harbours, including Otago Harbour.
While a lot is known about seagrasses in other regions of the world, the role that these plants play in estuarine and coastal ecosystems in New Zealand is less well understood.