The estuary is a hostile environment for most plants because salt dominates. A few grow further back on the shore, where they live in a fluctuating environment of sea water and fresh water. These plants must cope with:
- varying salinity levels
- strong currents and storm waves
- varying exposure to sunlight and wind
- low oxygen levels in muddy soils.
Seagrass (Zostera capricorni) is the only flowering plant in New Zealand capable of living submerged in sea water. Small and dark green, with ribbon-like leaves, this plant takes root in sandy silts on tidal flats throughout the country. It forms extensive meadows that spread from just above the low-tide level to full submersion in 1 to 2 metres of sea water.
Stems of seagrass creep a few centimetres beneath the mud and become so interwoven with those of adjacent plants that a firm mat develops. This anchors the plants and helps stabilise shifting sediments during the tidal cycle. Over time, sediments build up within and behind seagrass beds, and other flowering plants colonise the higher ground.
Seagrass is in decline around New Zealand. In the Avon–Heathcote estuary near Christchurch, seagrass beds started disappearing in 1920, and by 1952 few patches were left. Extensive loss was noted in the Auckland area in the 1930s, and seagrass cover reduced by 25% between 1945 and 1996 in the Tauranga and Ōhiwa harbours, in the Bay of Plenty. There is no general consensus on the reason: it could be fungal disease, or due to extra sediments entering the estuaries and smothering the plants.
New Zealand mangrove or mānawa (Avicennia marina) forms intertidal forests in the estuaries of the far north. At its southern limits (Kāwhia and Ōhiwa Harbour) the species grows in stunted shrublands. In 1996, mangroves covered 22,500 hectares – less than 0.1% of New Zealand’s land area. Like seagrass, mangroves can withstand daily inundation from sea water, but they need to be exposed for at least half the tidal cycle for their roots to absorb oxygen.
The mangrove is a small tree or shrub with a grey twisted trunk, leathery olive leaves and tiny yellow flowers. It produces special vertical breathing roots (pneumatophores) that take in air during low tide. Fruits germinate early and a young plantlet drops from the tree. Equipped with a well-developed root and two fleshy seed leaves, the young plant quickly starts growing if it strikes fresh mud. Those that fall into the tide float away and may establish new colonies.
Salt marsh plants
Salt marsh areas are found at the head of estuaries, landward of the seagrass and mangrove. The dominant plants on salt marsh are sea rush (Juncus kraussii) and jointed rush or oioi (Apodasmia similis), which form dense rushlands up to 1.5 metres tall. Often present is the marsh ribbonwood shrub or makamaka (Plagianthus divaricatus), especially along the banks of tidal streams. Oioi was used by Māori for thatching the outsides of their houses.
Beyond salt marsh, where the land becomes drier, turf-forming plants are favoured and a salt meadow develops. Saltwort (Sarcocornia quinqueflora), a native succulent, forms distinctive red, grey or green colonies alongside or sometimes intermixed with mats of creeping herbs like remuremu (Selliera radicans) and shore primrose or mākoako (Samolus repens). Saltmarsh vegetation gives way to coastal scrub on dry land, or to freshwater swamp in wetlands beyond the influence of salty water.
In an attempt to reclaim estuary land for farming, cordgrass (Spartina x townsendii) was introduced from England and planted at the Manawatū river mouth in 1913. It formed intertidal meadows which trapped sediment with such efficiency that the estuary bed quickly built up, changing the environment from bare tidal flat to pasture. This plant and two related cordgrasses subsequently spread into many estuaries around New Zealand, changing conditions so markedly that native plants and animals failed to thrive. Cordgrasses are now classified as noxious plants, and attempts are made to remove them.