What is a salt marsh?

A salt marsh is a type of marsh occupying the intertidal area between land and salty water. Marshes and wetlands everywhere used to be thought of as wasteland that could be filled in to provide land for agriculture or industrial development. Fortunately, they are no longer seen as unimportant – in fact they are now recognized as one of the most biologically productive habitats on the planet. In some parts of the world (on the southeast coast of the USA, for example) the organic detritus resulting from the decay of cast-off leaves of salt marsh plants provides enough food for creatures at the base of food chains that they are able to sustain major inshore fisheries.

The importance of the Inlet salt marsh was recognized and its protection assured with the designation of the Pāuatahanui Inlet Wildlife Management Reserve in the 1980s.

Why is the Inlet salt marsh important?

·   It traps sediment that is washed into the Inlet by streams and stormwater drains. While this natural reclamation process slowly advances the marsh into the Inlet it slows down the sedimentationof the main body of the Inlet.

·  It reduces erosion of the seabed and of the shoreline by wave action.

·  It absorbs excess nutrients from both the freshwater and sea water that enters the marsh. This reduces eutrophication and the excessive growth of undesirable species of algae.

·   It slowly transfers nutrients from the sediment to the water, enabling the growth of phytoplankton, an important food source for filter-feeding animals in the Inlet.

·   It produces most of the food required by the Inlet's herbivores and detritus-feeders at the start of the food chain.

·  It provides shelter, breeding sites and food for wildlife.

The viability of the Inlet depends to a large extent on the continued presence of a sizeable salt marsh area.

The photo above shows the outer salt marsh.

The plants are sea rushes - see 'The plants of the salt marsh' below. (Image by Keith Calder)

The plants of the salt marsh

The dominant plants of the salt marsh are the searush (Juncus maritimus) and the jointed rush (Apodasmia similis). Rushes are among the few plants that are adapted to grow in the harsh conditions of a salt marsh. They overcome the lack of air in the water-logged soil by obtaining the oxygen that their roots need from air trapped in their hollow stems.

The rushes

Sea rush can tolerate its roots being covered by sea water for up to four hours a day. Its stems will gradually absorb salt and eventually stop functioning, at which point they die off. At any one time up to half of the stems of plants near the shoreline will be dead. These dead stems, far from being useless, break away and crumble into small pieces which fall on the mud. This detritus is then carried out to the mudflats where it provides 60%-70% of the food eaten by the herbivores such as mud crabs, mud snails and cockles.

Jointed rush is less tolerant of salinity – it cannot grow if its roots are immersed in sea water for more than two hours a day. It plays an important role in trapping silt and protecting the banks of tidal streams from erosion. The rusty colour often seen in late summer and autumn is a result of the salt the plants have absorbed.

The birds of the salt marsh

Many species of birds find food within the salt marsh. Others use it for shelter when the tide is in and at least five species breed in the marsh. Pukekos build nests in the rush beds. Pied stilts nest on shell banks and other suitable sites that are raised just above the tide level. Kingfishers nest in holes in stream and other higher banks. Mallard ducks breed on the banks of freshwater ponds and herons in tall trees on the landward side of the marsh.