Eel grass

What is eel grass?

Eel grass plants Eel grass (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. It also occurs in southern Australia. It is one of many species of seagrass that occur throughout temperate and subtropical zones of the world.

Eel grass meadow

Eel grass 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.

Why is eel grass important?

Eel grass plays an important multipurpose role in the ecology of the Inlet.

· The interconnecting rhizomes of the eel grass meadow help to bind together the sand and decreases 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, eel grass plants help to offset the damaging effects of excess nutrient flow from the Inlet catchment.

· The small animals that live in the eel grass 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).

· Eel grass itself is a major food species for black swans.

Is the eel grass population in the Inlet under threat?

Certainly. If there is too much fine sediment coming in from the catchment, the eel grass 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 eel grass can be trampled by vehicles or boats being dragged over the shore, and even by horses being exercised.

Dense eel grass meadows were once abundant in the Inlet. It appears from anecdotal evidence that over the last 30-40 years these meadows have declined dramatically in size and density of individual plants. Equally anecdotal is the belief by some that the decline has been reversed in the last 5-6 years. A programme for monitoring eel grass in the Inlet is long overdue. GOPI hopes that one will be included in the Porirua Harbour and Catchment Management Programme now being developed by Porirua City Council.

Eel grass

Eel grass

What is eel grass?

Why is eel grass important?

Is the eel grass population in the Inlet under threat?

Further reading