There are some 70 species from two dozen families - most in Southeast Asia, where they are thought to have originated. Most live within 30 degrees of the Equator.
Some Aborigines in northern Australia believe one mangrove species resembles their primal ancestor, Giyapara, who walked across the mudflats and brought the tree into existence.
Ecology
The intertidal zone is characterised by broad ranges in salinity, temparature & moisture:- high tide brings in salt water, and when the tide recedes, solar evaporation of the seawater in the soil leads to further increases in salinity
- the return of tide can flush out these soils, bringing them back to salinity levels comparable to that of seawater
- at low tide, organisms are also exposed to increases in temperature and desiccation, and are then cooled and flooded by the tide
Ecological benefits
Mangroves benefit the overall health of the sea and provide a nursery for young organisms; the plants' interlocking roots stop riverborne sediments from coursing out to sea:
- mangrove roots provide an oyster habitat and slow water flow, thereby enhancing sediment deposition in areas where it is already occurring.
- this sediment (fine & anoxic) in turn act as sinks for a variety of trace heavy metals.
- mangrove removal disturbs these underlying sediments, often creating problems of trace metal contamination of seawater and biota.
- the root system hosts algae, barnacles, oysters, sponges, and bryozoans, which all require a hard surface for anchoring while they filter feed, plus shrimps and mud lobsters; mangrove crabs mulch the mangrove leaves, adding nutritients to the mangal muds for other bottom feeders.
- in at least some cases, export of carbon fixed in mangroves is important in coastal food webs.
They can protect coastal areas from erosion, storm surge and tsunamis:
- their massive root systems are efficient at dissipating wave energy plus slow tidal water enough to allow sediment to be deposited as the tide comes in, leaving all except fine particles when the tide ebbs. However, mangrove swamps' protective value is sometimes overstated ie. wave energy is typically low in areas where mangroves grow.
NZ Mangroves
Avicennia marina, or the grey or white mangrove; Māori name = mānawaMangroves have been in New Zealand for about 19 million years; they are an indigenous New Zealand plant, and are a protected plant in Northland (where the most mangrove forests are).
They are the southernmost mangrove in the world. New Zealand mangroves grow in an area from near Cape Reinga in the north southwards to Bay of Plenty’s Ohiwa Harbour (near Opotiki) on the east coast and in the Kawhia Harbour on the west coast.
The mangrove can tolerate very saline water conditions (they excrete salts through their leaves), but grow to full height only in waters where both salt and fresh water are present.
Adaptation - too much salt
- Osmosis - the concentration of salt in New Zealand mangrove’s cell sap is higher than land-only plants. The mangrove’s higher salt sap concentration helps it stop osmotic water loss from its plant tissues into the sea around it, helping it to survive in such a salty environment.
- The leaves of the mangrove secrete salt - hundreds of tiny salt-secreting glands on the mangrove leaf’s upper surface. They get rid of extra salt by exuding a salty brine that is more concentrated than full-strength seawater.
- Tough leathery upper leaf surfaces - easier to wash off salty sea splash and spray residues.
- Leaf shedding - mangroves lose about 60 percent of their leaves in a year. The drop is mainly of older leaves which contain the greatest salt concentrations (annual litter drop can be as high as about five to six tonnes per hectare) Leaf drop increases during summer which also helps the mangrove reduce the impact of losing water through evaporation.
Adaptation - not enough oxygen
Pneumatophores - the many snorkel-like breathing roots that stick up above the mud. These grow along the mangrove’s shallow radiating roots.- Each pneumatophore has small port holes or lenticles in its outer surface which let air into the root when the tide is out.
- these aerial roots can grow to a height of about 20cm. They allow the plant to absorb oxygen, which is deficient in its habitat. They also help to anchor the plant during the frequent rush of tide water
Adaptation - 'prepackaged seedlings'
Mangrove propagules grow on their parent tree for up to five months. They grow into a furry, 'packaged seedling' ready to quickly take root after they are dropped from the tree into the surrounding water. Air inside the pericarp allows the propagule to float for up to three days.- These propagules are not seeds. They are ‘mangroves to go’, ready-made future plants that have already done almost half a year’s growing on their parent tree.
Mangrove ecosystem
As many as 30 different species of fish can be found in mangroves at high tide (including yellow-eyed mullet, grey mullet, parore, pilchards and anchovies).Birds found there can include the heron, pukeko, banded rail, harriers, kingfishers, grey warblers, fantails, shining cuckoos, bitterns, royal spoonbills pied and black shags.
Maori use
Māori traditionally gathered food from the mangrove forests. This included mullet (kanae), oyster (parore tio), sea snail (karahu) and eel (tuna).The black earth formed by rotted mangrove leaves was also used to dye flaxes for making kits and skirts. A green dye was made from the lichen on mangrove trees.
To Encourage Growth - 10-50% seawater
Seedling development is affected by salinity of the substrate in which they grow. A study by (Downton 1982) of development of A. marina seedlings at different salinities revealed that optimum growth occurred between 10 and 50% seawater. Plants growing in higher salinities were comparatively slow to develop and had lower biomass.Pros & Cons
Mangroves can be beneficial:- Home for many marine and land animals
- Nursery habitat for short-finned eels, grey mullet and parore
- Erosion control & shoreline protection
- Organic material for nearby habitats
- Reducing urban stormwater pollution
- Bird roosts
- Storm surge protection
- Major ecosystem food web component
But also have some ecological adverse effects:
- Colonising the habitat of other indigenous coastal flora and fauna e.g. bird roosting sites
- Accelerating estuary infilling
- Restricting usability of coastal structures e.g. wharves
- Loss of beach areas and open harbour spaces
- Restriction of public access to the harbour
- Loss of feeding areas for wading and shorebirds
- Restriction of tidal flushing
- Obstruction of natural and manmade land drainage channels
Distribution in Mangere Inlet - 2006 (almost none prior to 1970)
Manukau harbour mangrove removal 2 (http://www.stuff.co.nz/auckland/local-news/manukau-courier/7231724/Managing-mangroves)
Auckland Regional Council - Mangrove info (http://www.aucklandcouncil.govt.nz/EN/planspoliciesprojects/plansstrategies/DistrictRegionalPlans/regionalplans/regionalplancoastal/Documents/newzealandsmangrovesarc2007.pdf)
Auckland Regional Councial Plan - Control of Mangroves (http://www.aucklandcouncil.govt.nz/EN/planspoliciesprojects/plansstrategies/DistrictRegionalPlans/regionalplans/regionalplancoastal/Pages/regionalplancoastalmangroves.aspx)
Research paper on temparate mangroves 1 (http://www.aucklandcity.govt.nz/council/documents/regionalplans/coastal/Morrisey%20et%20al%202010%20Temperate%20Mangroves%20OMBAR.pdf)
Research paper on temparate mangroves 2 (http://www.aucklandcity.govt.nz/council/documents/technicalpublications/ARCTP%20325%20Mangrove_review_(web).pdf)
Distrubution in the BOP region (http://www.boprc.govt.nz/environment/coast/tauranga-harbour/mangroves/)
Whangamata ratepayers association comment (http://www.whangamata-ratepayers.org.nz/mangroves.html)





















