Mangroves and Seagrasses (Phylum Tracheophyta, Class Angiospermae)
Mark Morrison and Taoho Patuawa
General Description
Mangroves and seagrasses are unique in being the only flowering plants occurring in New Zealand waters. Though they belong to different plant orders they are listed here for convenience.
Mangrove
New Zealand supports one species of mangrove, Avicennia marina, the most widespread mangrove species in the world, extending from East Africa through to Fiji, Australia, and New Zealand. This species reaches its southernmost extent in the world at Raglan Harbour on the west coast of the North Island (37°48' S), and at Ohiwa Harbour on the east coast (38°00' S), along with Victoria, southeastern Australia (38°27' S). Limited frost tolerances are thought to set this latitudinal limit. In 1996-1997 it was estimated that approximately 22,500 hectares of mangrove forests may be present in New Zealand.
A. marina occurs in sheltered harbours and tidal estuaries of the northern half of the North Island. There is a geographical cline of decreasing tree height from the far north (up to 12 m tall, with a metre or larger trunk diameter) to the south (less than 1 m tall). There is also considerable height variability within individual harbours, with taller trees tending to occur on the seaward edge of forests, especially those fronting on to tidal channels, while trees at the landward side of the forest are often very stunted, and less than 1 m tall. Mangrove trees are adapted for living in very muddy environments, using aerial roots (pneumatophores) that extend upwards from a buried root network to deal with the low oxygen content of the mud. Mangroves produce large seeds, which drop into the water on reaching a sufficient size, where they are dispersed by wind and tide, often over considerable distances. The germinating seeds anchor themselves into the sediment when a suitable site is reached. Subsequent mortality can be very high, both under well established adult mangrove stands, and in other habitats such as seagrass beds, where they appear to be killed off in the winter months by low surface temperatures. Nevertheless, these large seeding events are capable of colonising large areas of "new" habitat, as evidenced by substantial increases in forest extents in areas of high sedimentation. Adult trees may live for a long time; evidence from some locations suggests individual trees that are more than 50 years old.
Mangroves play an important role in trapping sediments, and protecting the shore-line from wave action. As with tropical mangroves, A. marina produces large amounts of organic detritus from the shedding of leaves, flowers and branches, which in large estuarine systems may add up to thousands of tonnes annually. The value of this productivity to the surrounding ecosystem is currently poorly understood. Benthic invertebrate species assemblages associated with mangroves are relatively modest in their diversity compared with other estuarine habitats (eg, sand-flats and seagrass beds), and dominated by small polychaete worms. Mangroves also support a terrestrial insect fauna, including scale insects, leaf-tying caterpillars, mealy bugs, slaters, and spiders. A number of native birds also utilise mangrove forests, including herons, bitterns, pukeko, kingfishers, banded rails, spotless crakes, and fantails. Much has been made of the value of mangroves as being important nurseries for juvenile fish, including commercially valued species. Recent extensive sampling of a range of estuaries and their mangrove forests in New Zealand has shown the diversity of small fish species in mangroves stands to be relatively low, although the abundances of a few species can be quite high. None of the species found can be considered to rely solely on mangrove forests as their habitat. On both the east and the west coasts, juvenile and sub-adult short-finned eels are widespread through mangrove forests. Juvenile parore occur in low but consistent numbers in east coast estuaries, and juvenile grey mullet in high numbers in west coast estuaries. Yellow-eyed mullet are ubiquitous in mangrove forests, but are also wide-spread through estuarine systems and sheltered coastal embayments in general.
Dense seagrass beds in Waikawa Harbour, Southland. Photographer M. Morrison.
Seagrass
As with mangroves, New Zealand supports only one species of seagrass, nominally Zostera capricorni, that occurs throughout the country across a range of environments, including estuaries, sheltered coastal beaches, intertidal rocky reef platforms, and in the bays of some coastal islands. There are an estimated 50-60 named species worldwide, spread across 12 genera in five families, with the greatest diversity being found in the tropics. They reach their most southerly limit at Stewart Island. Seagrasses are the only truly marine flowering plants, and are thought to have colonised the marine environment around 100 million years ago. They have a number of special adaptations, including extensive root and rhizome systems; for anchoring, nutrient and mineral extraction, and vegetative propagation, many veins and air channels in the leaves and stems to compensate for living in oxygen-deficient systems, thin leaves to maximise light penetration, nutrient adsorption, gas diffusion, and buoyancy; and flower pollination mechanisms suited to water pollination. New Zealand's seagrass is mainly an intertidal species, but can occur subtidally where water clarities permit, with maximum recorded depths of around 7 m.
Seagrass beds provide a number of important roles, including trapping and stabilising bottom sediments, nutrient recycling, the creation of high primary productivity, and the provision of habitat to a wide variety of plant and animal species, including invertebrates, fish and birds. Work on seagrass invertebrate assemblages in New Zealand has shown that while the presence of seagrass is often associated with a greater diversity and abundance of macro-fauna, there is considerable variability associated with how the seagrass bed is configured, its proximity to other habitats, and where it occurs within an estuary.
Seagrass beds are important juvenile fish nurseries in northern New Zealand, but their relative value varies strongly across landscape settings and latitude. In northern New Zealand, west coast estuarine seagrass beds (in particular those with subtidal elements) often support high abundances of juvenile snapper, trevally, grey mullet, and piper, while on the adjacent east coast grey mullets are absent, but additional species include juvenile parore and spotties. South of Cook Strait, only spotties and piper persist, while seagrass meadows in Southland support higher abundances of pipefish, and juvenile leatherjackets, as well as spotties
Status
While historically mangrove forests were reduced in abundance by extensive reclamations, and the building of causeways around many estuaries, in recent decades the extent of mangroves has been substantially increasing in some areas. This is thought to be a consequence of large increases in sedimentation into the coastal zone, as a result of changes in land catchment usage. There is now significant societal disagreement as to the most appropriate way of responding to these changes, ranging from doing nothing, to physically removing whole mangrove stands. Locations at which mangroves are increasing their spatial extent include the Whangape, Rangaunu, Mangawhai, Whangamata, Tauranga, and Ohiwa estuaries, as well as the upper Firth of Thames.
In contrast to mangroves, seagrass in New Zealand is thought to have declined significantly in its spatial extent and abundance, with a current total spatial extent of around 44 km2. For example, in Tauranga Harbour seagrass extent declined from 44.4 square kilometres in 1959 to 29.3 km2 in 1996 (34% over 40 years), including 90% of the subtidal seagrass in the harbour. Other large losses have been observed in the Whangarei, Manukau, Waitemata, and Avon-Heathcote harbours. Probable contributors to these declines include large increases in dissolved nutrients, with associated increased phytoplankton, macroalgae or algal epiphytes loads; and large increases in suspended sediment loads, with associated decreases in water clarity and light levels.
Key Locations
Mangroves occur across all of the tidal inlets and estuaries of the upper North Island, but there are some particularly large forests (or clusters of mangrove stands) that stand out. These include those of the large harbours of Rangaunu, Hokianga, Whangarei, Kaipara, Manukau, and the Firth of Thames.
Large seagrass meadows are found in the Parengarenga, Rangaunu, Kaipara, Tauranga, Whanganui, and Bluff/Awarua Harbours, as well as at Farewell Spit and Paterson Inlet, Stewart Island. Smaller but notable beds occur in the Kawhia, Aotea, and Whangapoua harbours. Strong variations in associated species assemblages occur across this latitudinal gradient, ranging from the presence of many subtropical snail species in the north, to large green isopods and wandering anemones in the south. Most of these seagrass meadows tend to occur in regions away from major centres of human settlement
Summary of Threats
The main threats to mangroves largely revolve around physical removal by humans, though whether this is perceived as a true threat depends on how mangroves are valued at different locations. At more local scales, human activities that negatively impact on mangroves include the dumping of rubbish, the restriction of tidal water flows through capital works such as culverts, and the use of some stunted mangrove areas for off-road four-wheel drive recreation. Increases in mangrove extents from increased sedimentation rates appear to be more than sufficient to offset such threats, at the overall scale of mangrove distribution within New Zealand.
For seagrasses, the most pervasive threats are increases in sedimentation rates, with associated increased water turbidity. These processes operate over relatively large spatial scales, and require management at the scale of catchments to mitigate. More localised threats include land reclamation, and propeller and anchoring scars. Concerns have also been raised over the possible negative impacts of grazing by the introduced black swan, Cygnus atratus. There is some evidence for natural recovery of seagrass meadows in the greater Auckland region, including recent increases in seagrass extent in the southern Kaipara Harbour, and at St Heliers and Snells Beach on the east coast.
Typical Habitats
Mangroves grow in the high intertidal margins of northern harbours and estuaries, in situations ranging from muddy sands, through to deep anaerobic (oxygen-deprived) mud. Stands range from a few metres wide, through to forests more than a kilometre deep. Seagrass meadows occur throughout the country, including estuaries, sheltered coastal beaches, intertidal rocky reef platforms, and in the bays of some coastal islands. In healthy northern estuarine ecosystems, seagrass beds abut directly onto mangrove forests, and extend down to the low-tide mark or beyond. Seagrass configurations range from small patches, to long ribbons, to extensive meadows several kilometres across. Subtidal fringes and/or beds occur in some harbours, notably Parengarenga, Rangaunu, and the Kaipara, as well as Great Mercury Island, while fully subtidal beds occur at the Cavalli, Urapukapuka, Slipper, and Ruapuke (Foveaux Strait) islands, as well as in some area/s of the Marlborough Sounds. The maximum depth recorded for seagrasses is around 7 m.
State of Information
Recent extensive knowledge reviews of mangrove and seagrass dynamics, and their respective roles in New Zealand marine ecosystems, have identified a number of important information gaps that need to be addressed. These include the need for quantitative resource surveys of both current and historical distributions of both species, with special attention being paid to subtidal seagrass. Associated with this is the desire for a better understanding of genetic diversity at both regional and national scales. The mechanisms by which seagrass changes its extent (recession, mortality, colonisation, and expansion), at scales ranging from patches up to entire beds, also require a better understanding, along with how anthropogenic activities affect these dynamics. This need also extends to mangrove forests, with a particular focus on how new management approaches might be used to mitigate negative effects.
Both species also provide important habitats to different life stages of fish and "shellfish', including commercial species. Much remains to be understood about how this role varies with different habitat landscapes, coasts, and latitudes, including work on the interrelationships between such species utilisation of these habitats, and their prey assemblages. Their indirect role as primary producers, underpinning trophic webs, also needs to be better understood. Finally, an assessment of the likely influence of climate change on the large-scale distribution of these plant habitats would also be useful.
Significance for Maori
Historically, seagrass leaves were occasionally used for adornment of clothing items, while mangrove forests continue to provide important habitats for customary fisheries of pupu (cat's-eyes), titiko (mud-snails), and tuna (short-finned eels) in the Hauraki region.
Key References
Inglis, G J. 2003. Chapter 12: Seagrasses of New Zealand. World Atlas of Seagrasses: present status and future conservation. 148-157.
Schwarz, A M and Sutherland, D. 2005. Seagrasses and mangroves. In: Rowden, A A, Brewin, P E, Dalen, J, Halliday, J, Hewitt, J E, Neill, K F, Nelson, W A, Oliver, M D, Probert, P K, Schwarz, A-M, Sui, P H, Sutherland, D, Thrush, S F and K Vopel. A review of the marine soft-sediment assemblages of New Zealand. New Zealand Aquatic and Environmental Biodiversity Report Series No X.
Turner, T and Schwarz A-M. 2006. Management and conservation of seagrass in New Zealand: an introduction. Science for Conservation 264. 90 pp.
Table 30: Mangroves and Seagrass (orders Lamiales and Alismateles in the class Angiospermae, phylum Tracheophyta) in New Zealand
| Phylum Class | Order | Family | Species | Endemic |
|---|---|---|---|---|
| Tracheophyta | ||||
| Angiospermae | ||||
| Lamiales | Acanthaceae | Avicennia marina | No | |
| Alismateles | Zosteraceae | Zostera capricorni | No | |
| Total | 2 |
Figure 86: Seagrass Zostera capricorni annual distribution