A saltmarsh is a type of wetland that occupies the intertidal zone between land and sea. It is an area of low level, flat, poorly drained ground subject to regular tidal flooding by salt or brackish water. Saltmarshes are common along low sea coasts, inside barrier bars and beaches, in estuaries and on deltas.
The plant communities found in saltmarsh habitats are dominated by rushes and sedges. These plants are specialised for living in various concentrations of salinity and various periods of submersion. They are called halophytes and as well as the rushes, there are glasswort and sea primrose, found at the low tide zone, withstanding the longest periods of submersion.
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 saltmarsh plants provides enough food for creatures at the base of food chains to support and sustain major inshore fisheries.
The importance of saltmarsh
The importance of the Pāuatahanui Inlet saltmarsh with its associated wetland bird population was recognized, and its protection assured, with the designation of the Pāuatahanui Inlet Wildlife Reserve in the 1980s.
- 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 sedimentation of 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 fresh water 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 plants of the saltmarsh
The dominant plants of the saltmarsh are the sea rush (Juncus krausii [formerly Juncus maritimus]) and the jointed wire rush or oioi (Apodasmia similis). Rushes are among the few plants that are adapted to grow in the harsh conditions of a saltmarsh. 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.
Glasswort (Salicornia quinqueflora) is a species of succulent halophyte (highly salt tolerant plant) found growing on flat areas of the shoreline. It’s a small shrubby plant that can withstand submersion but also grows above high tide levels.
Sea primrose (Samolus repens) is a low growing spreading rhizomatous plant with white flowers. It is found in the upper reaches of estuaries, saltmarshes and muddy inlets.
How rushes contribute to the Inlet
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 to the saltmarsh floor. 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 wire 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.
Birds of the saltmarsh
Many species of birds find food within the saltmarsh. 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 banks and similar sites. Mallard ducks breed on the banks of freshwater ponds and herons in tall trees on the landward side of the marsh.