What is seagrass

Seagrass (rimurēhia), Zostera muelleri, is a marine perennial flowering plant - though not a true grass - that undergoes its entire life cycle under water. It has pollen adapted to dispersion by water and flowers that are reduced to a small spike for catching the pollen. It grows by budding stems and leaves from a creeping root system and by this means forms spreading beds of interconnected plants. It is the only species of Zostera in New Zealand but also occurs in southern Australia. It is one of many species of seagrass that occur throughout temperate and subtropical zones of the world.

Seagrass grows on tidal sand flats, channel banks and shallow subtidal sandy areas in estuaries throughout the country. Where it is abundant it forms a dense cover on the sand flat that can appropriately be called a seagrass meadow.

Pāuatahanui Inlet has the highest seagrass coverage of any harbour in the lower North Island.

Why is seagrass important?

Seagrass plays an important multipurpose role in the ecology of the Inlet.
·    The interconnecting rhizomes of the seagrass meadow help to bind together the sand particles and decrease erosion of the tidal flat.
·    When covered by the tide the upstanding leaves form a ‘forest’ that gives shelter to many small animals, including the young juvenile stages of several fish species.
·    The ‘forest’ slows down water currents, causing small particles to settle out on the leaves or the sand. Much of this is organic debris coming from the death and decay of plants in the salt marsh at the head of the Inlet. The particles provide food for numerous species of animals from microscopic crustaceans to worms, cockles and fishes.
·   This entrapment of fine particles directly improves water clarity in the Inlet and thus enhances the environment for other aquatic plants such as algae on the sea bed and diatoms in the plankton.
·   By absorbing nutrients from water and sand and releasing oxygen as they photosynthesize, seagrass plants help to offset the damaging effects of excess nutrient flow from the Inlet catchment. 
·   The small animals that live in the seagrass beds are an important food source for fish (e.g. yellow-eyed mullet, stargazer, and juveniles of flatfish, snapper, trevally, garfish, spotty) and wading birds (e.g. oystercatcher, pied stilt, spoonbill).
·   Seagrass itself is a major food species for black swans.

Is the seagrass population in the Inlet under threat?

Certainly. If there is too much fine sediment coming in from the catchment, the seagrass can be smothered and die. Very high levels of plant nutrients, mainly nitrogen and phosphorus from agricultural sources, can encourage the growth of plant parasitic fungi that cause ‘wasting disease’. Human activities on the foreshore can damage the habitat – the seagrass can be trampled by vehicles or boats being dragged over the shore, and even by horses being exercised.

Dense seagrass meadows were once abundant in the Inlet but a recent survey by NIWA shows that about 40% of its seagrass cover has been lost since 1980, mostly from the eastern end. This decline correlates in time with the doubling of nitrate concentration in the sea water in this area since the 1970s, possibly coupled with increased siltation. NIWA makes the obvious recommendation that these probable causes need to be eliminated before successful restoration is likely, but does suggest that some transplantation experiments could be done now to assess the current situation.

Further reading

‘New Zealand seagrass: General Information Guide.’ NIWA Information Series No. 72. (2009). You can download it from this link: https://www.niwa.co.nz/sites/niwa.co.nz/files/import/attachments/A4-Seagrass-Guide.pdf.

You can also read the Feature Article about Seagrass (Eel grass) in the December 2013 issue of our newsletter, The Inlet. (Note: This article was written prior to advice from NIWA that the accepted name of 'seagrass' is the more appropriate one for this species)