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The Seashore

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The Mangrove Environment

 

Mud Skipper - Note the eyes on top of the head so that it can see above the muddy water when it is submerged

Monitor Lizard - these are common predators in the tropics, feeding on fish and invertebrates as well as scavenging

At low tide mud skippers climb out of the water. Males have territorial disputes chasing each other over the mud surface. At other times hugemonitor lizards can be seen swimming through the murky waters and hauling themselves up in the sun. Mangrove is very different to saltmarsh!

Figure 1 The mangrove environment

  Introduction.

The mangrove environment is an extremely harsh one which includes abiotic

factors such as high (variable) salinity in soil solution; essential nutrient ions present as a low proportion of the total present in soil solution; anaerobic soil and sulphide toxicity; temperature shock on immersion; changes to photoperiod; scour and drag (that famous music hall double act) and burial.

Consequently mangroves are generally species poor but biodiversity increases with height.

Plants that complete their life cycle in saline environments are referred to as halophytes, there are approximately 60-70 mangrove species in 18 genera across 13 angiosperm families. Between 60-70% of tropical coasts are lined with mangroves, this habitat type is believed to have been in existence approximately 60 000 years.

Summary of factors affecting mangrove species.

 

A. Salinity.

Salinity varies with the ebb and flow of the tide and with input from freshwater sources. Actual amounts of salt vary daily, seasonally and with the exact nature of the site. The relationship of salt concentration in the soil with height up the mangrove is not a simple one. Although the bottom of the mangrove receives sea water more often salt will be flushed through the system fairly regularly, in the middle although salt input is less frequent the ground dries out (especially in drier months) between floodings and the salt concentration rises accordingly. At the top of the mangrove flooding with saltwater is rare so even though the ground is dry salt concentration still drops off until eventually reaching normal terrestrial levels at around or just above extreme high water of spring tides (EHWS). Salinity readings from within a mangrove have been found to vary between 0.5-35 parts per thousand.

Salt in the environment is a problem for organisms for a number of reasons

direct toxicity of Na & Cl

If plants are to avoid water loss to the environment by osmosis they must increase solutes in plant cells to compensate by

synthesis of organic solutes e.g. proline and other amino-acids

The second option seems to be the favoured one as it is least metabolically expensive; plants do it anyway just more so. In general halophytes’ low water potentials are generated by ion accumulation but the ions are probably sequestered in vacuoles and are balanced by organic solutes in the cytoplasm. Many mechanisms exist but they all seem to separate enzyme systems from areas of high salt concentration.

BUT salt levels in shoots still need to be regulated and many cunning strategies are used such as

salt secretion by special glands in the leaf e.g. Avicennia, Aegiceras and Acanthus ilicifolius but most mangrove plants do not excrete this way.

 

Figure 2 . Avicennia underside

Figure 3 . Acanthus ilicifolius

Deposition of salt in the bark e.g. Avicennia, Rhizophora, and Sonneratia.

B. Flooding.

Under this heading there are a number of related problems as flooding affects the oxygen concentration in the substrate (with associated effects of sulphide toxicity), the quantity and quality of light reaching the plants and also causes scour and drag.

Lack of oxygen.

Roots need to respire so anaerobic substrate conditions can be fatal. Many mangrove plants have air filled tissue (aerenchyma) in the roots, which receive air from:

Pneumatophores (Avicennia, Sonneratia)

Knee roots (Bruguiera)

Wavy, plank-like buttress roots (Xylocarpus)

Proproots (Rhizophora).

Figure 4 Sonneratia pneunatophores

 

Figure 3 . Bruguiera knee roots

 Air and stilt roots have perforations (lenticels), which connect to the air. During immersion oxygen levels in the roots decrease and carbon dioxide is dissolved so gas pressure decreases, lenticels are too small to allow ingress of water but during emersion gas supplies are replenished.

 

Toxicity.

Many products of anaerobic microbial metabolism are toxic, particularly sulphides. Toxicity can be direct or indirect (as it reduces the availability of sulphate which can lead to sulphur deficiency) and can also reduce the availability of Fe, Mn, Cu, and Zn, which are all essential trace elements for enzyme production. Some plants can incorporate toxic chemicals into harmless compounds and thereby achieve sulphide tolerance. Soil condition is one of the factors which contributes to observed zonation patterns, Avicennia and Sonneratia do well in sandy areas, Rhizophora copes better with soft, humus-rich mud and Bruguiera favours stiff, nutrient-poor clays.

Instability of substrate.

 

The specialised air and stilt roots mentioned in the section above also help to stabilise mangrove trees, many of which can be quite substantial, in the soft, muddy substrate.

 

Figure 5 Rhizophora proproots

 

D. Dispersal.

 

Many mangrove species have large propagules and show vivipary. Vivipary in this context refers to precocious germination of the seedling while the fruit is still attached to the plant. In this way the seedlings receive nourishment from the adult plant and salt transfer is regulated (and increased) during development. Bruguiera, Rhizophora, and Ceriops all show vivipary. Aegicera, Avicennia and Nypa show cryptovivipary (this is where the embryo breaks through the seed coat but not the fruit wall, before it splits open).

 

The seedlings in some genera (Rhizophora and Bruguiera) develop a pointed hypocotyl, which facilitates mud penetration on release from the parent plant.

 

Figure 6 Rhizophora vivipary

In contrast Sonneratia is the only mangle with fleshy fruits, which are often dispersed by animals including bats and fish!

Vivipary has arisen in independent groups of mangrove plants and sea-grasses and so must convey a selective advantage to inter-tidal/ shallow sea dwellers.

Basic zonation Patterns.

In relatively undisturbed mangrove forest zonation patterns exist in the vegetation reflecting the change in environmental conditions as one moves back (up in height) from the most seaward extent of the habitat.

Typically the most seaward genera are Avicennia and Sonneratia, especially on sandy areas. Rhizophora is found next in the sequence, often associated with muddy substrates and finally Bruguiera, Ceriops, Xylocarpus and Heritiera at the back of the forest.

In Singapore zonation patterns are not clear as there has been so much disturbance to the habitat but the following sequence might be regarded as typical.

To seaward the first zone is composed of Sonneratia ovata and Avicennia alba. Then in the next zone A.rumphiana, A. officinalis, Bruguiera cylindrical, B.gymnorhiza and Rhizophora apiculata, will dominate the canopy. Finally there is the mud lobster (Thalassina anomala) mound and pool complex, bearing mainly Excoecaria agallocha, B.cylindrical, B.gymnorhiza and R.apiculata.

 Xylocarpus, Aegiceras, Osbornia, Excoecaria and Nypa are favoured if there is fresh water input into the system.

Figure 7 Xylocarpus wavy, plank-like butress roots

 

 


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