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What are Mangroves?
Mangroves are a diverse group of unrelated trees,
palms, shrubs, vines and ferns that share a common
ability to live in waterlogged saline soils subjected to
regular flooding. They are highly specialised plants
that have developed unusual adaptations to the
unique environmental conditions in which they are
found.
There are around 80 species of mangroves found throughout the
world (Saenger et al., 1983). Most commonly they occur within
tropical and subtropical sheltered coastal areas subjected to tidal
influences. An area influenced by tide can be interpreted to mean a
shoreline inundated by the extremes of tides, or it can more widely
refer to river-bank communities where tides cause some fluctuation
in water level but no change in salinity (Tomlinson, 1986).
Therefore, mangroves can be found not only inhabiting extensive
tidal mud flats but also along freshwater riverbanks.
Mangroves can be divided into two distinct groups: exclusive and
non-exclusive. Exclusive mangroves are the largest group,
comprising around 60 species (Saenger et al., 1983). These
mangroves are confined to intertidal areas and have not been found
to exist within any other type of vegetation community.
The remaining 20 plant species considered to be mangroves are
referred to as non-exclusive. These plants are not restricted to the
typical mangrove environment and are often found within drier,
more terrestrial areas. Examples include Hibiscus tiliaceus and
Barringtonia acutangula.
2.2 Mangrove Distribution and Requirements
Mangroves are commonly found throughout the world between
latitudes 32°N and 38°S. The upper and lower limits of this range
are determined by temperature (Tomlinson, 1986), while rainfall and
the level of protection from wind and wave energy effect forest
extent and diversity.
Temperature
Mangrove communities most commonly occur in areas where the
average temperature of the coldest month is higher than 20°C and
where the seasonal range does not exceed 10°C. Temperatures of
around 5°C and frosts also limit mangrove distributions (Tomlinson,
1986).
2.2.2 Rainfall
Areas, which have a great variety of mangrove species, are found
along coasts that receive high rainfall, heavy run off and seepage
into the intertidal zone from the hinterland. Such areas are
commonly subject to extensive sedimentation, which provides a
diverse range of substrate types and nutrient levels, which in turn
are favourable for mangrove growth (Tomlinson, 1986).
2.2.3 Protection
Mangrove establishment requires protection from strong winds and
wind generated waves, as wave action prevents seedling
establishment. As a consequence, mangrove communities tend to
be located within sheltered coastal areas, surrounding highly
indented estuaries, embayments and offshore islands protected by
reefs and shoals.
Mangroves of the Northern Territory
Over 4,000 km2
of mangroves are found along the 10,953 km NT
coastline. This represents around 35% of the 11,600 km2
of
mangroves found throughout Australia (Saenger et al., 1983).
These mangrove stands vary significantly in size and species
diversity. Isolated pockets can be found within any number of small
embayments or surrounding small coastal estuaries, whilst more
extensive stands are located along the northern coasts of Arnhem
Land, in Darwin Harbour and on Bathurst and Melville Islands (refer
to Figure 1).
Estimates of mangrove diversity vary and depend on the definition
used for classification. Studies conducted by Wightman (1989) have
identified 48 plants recognised as being regular inhabitants of NT
mangrove communities.
A mangrove regionalisation study was conducted in order to focus
mangrove mapping projects (see 3.1.1). This study identified seven
key mangrove regions along the NT coastline. Because most of the
existing coastal development is currently focussed on the greater
Darwin region, additional mapping and surveying was conducted
within this region in order to gather baseline information on
mangrove community structure, composition and content.
Darwin Harbour, for example, contains approximately 20,400
hectares of mangroves which constitutes around 5% of the
Northern Territory’s entire mangrove area. In addition to being one
of the largest discrete blocks, Darwin Harbour is recognised for its
diversity, containing 36 mangrove species, 6 of which are most
common, Rhizophora stylosa, Ceriops tagal, Sonneratia alba,
Bruguiera exaristata, Avicennia marina and Camptostemon schultzii
(Brocklehurst and Edmeades, 1996a).
In response to tidal elevation and geomorphology, the mangrove
species of Darwin Harbour can be grouped into 10 distinct
communities (refer to Figure 2):
• shoreline forest;
• tidal creek;
• transition;
• mid tidal flat;
• high tidal flat;
• hinterland;
• low woodland;
• woodland;
• rocky shores, and
• tidal mudflat.
Essential Functions of Mangroves: Why are Mangroves
Important?
While scientists have placed a high value on the ecological function
of mangrove ecosystems for some time, only recently has the
broader community come to recognise the multi-faceted role that
mangroves play in the environment.
When assessing the importance of mangrove ecosystems from a
scientific perspective, it is necessary to attempt to identify and
measure these values. Notably, these values can be divided into
ecological, community and economic values.
2.4.1 Ecological Values
From an ecological perspective, mangroves are a unique and
significant ecosystem. They support a diverse range of plants
including palms, trees, shrubs and even ferns, which have
developed unusual adaptations to the prevailing environmental
conditions. In fact these plants have been so successful in their
development that mangroves are among the most productive
natural systems found throughout the world.
Mangroves are used by a vast array of organisms as breeding,
nursery and feeding areas. They also play a valuable role in
foreshore protection, reducing erosion by cyclones and lessening
the impact of storm surge.
Productivity
Productivity is a concept used to describe the ecological value or
function of a vegetation community. Notably, productivity can be
estimated by gaining measurements of the amount of living material
(ie. leaves, branches, stems and roots) that is produced by a
mangrove community over a specified time.
Mangrove productivity is important because it has direct impact on
the health and function of the marine food chain (Saenger et al.,
1983). Like other plants, mangroves convert energy from the sun
into organic matter through the process of photosynthesis.
When the leaves and branches of a mangrove fall to the ground
they provide a wide variety of aquatic animals such as molluscs,
crabs and worms with a primary source of food. These primary
level consumers in turn support an array of secondary consumers,
including small fish and juvenile predators such as barramundi
which, when mature, become third level consumers.
In general, high levels of organic matter, or high productivity, means
that a larger number and more diverse array of animals can be
supported within a particular ecosystem.
Measuring mangrove productivity is not easy, and will probably
never be achieved in absolute terms. It is, however, possible to
measure changes in a particular element of a mangrove
community, which can then be used as a guide to productivity.
Leaf Litter
Many studies have used the collection and measurement of leaf
litter fallen from a mangrove area over a particular period of time as
one such guide.
These studies have shown that mangrove ecosystems have the
ability to produce large quantities of litter, ranging from 10,000 to
14,000 kg dry weight ha-1 year-1 (Hamilton and Snedaker, 1984).
Specific work in this area has been undertaken in the NT with
research indicating that Darwin Harbour mangroves have an
average rate of leaf litter production estimated to be 7,680 kg dry
weight ha-1 year-1 (Metcalfe, 1999). Further information and the
most recent results with regard to leaf litter studies are described in
section 3.2.1
Biomass
Other research has concentrated on the use of standing biomass as
a guide to productivity. Biomass is the amount of living matter that
is present within an ecosystem at a given time. Studies have
shown that the above-ground biomass of mangrove communities
can range from 5.4 to 18.4 kg m-2 (Hutchings and Saenger, 1987).
The higher values within this range have been found to be
equivalent to both temperate and tropical forests (Tomlinson, 1986).
While most studies have concentrated on obtaining estimates of
above-ground biomass, few have investigated the below-ground
portion of tree biomass. Results suggest that the standing biomass
of Darwin Harbour mangrove communities is similar to estimates
reported for similar mangrove communities in other parts of
Australia. Shoreline and Tidal Creek mangrove forests were found to contain the highest standing biomass, with estimates of 262.29
and 224.76 t/ha respectively. In comparison, mangrove
communities dominated by low Ceriops tagal recorded lowest
standing biomass estimates - Mid Tidal Flat 89.28 t/ha and High
Tidal Flat 69.63 t/ha. The total standing biomass of the mangrove
communities of Darwin Harbour has been estimated at 2,727,638
tonnes (Comley, 2002).
By calculating consecutive measures of standing biomass it is
possible to estimate the rate of biomass accumulation within an
ecosystem. Biomass accumulation within the mangrove
communities of Darwin Harbour has been estimated to range from
4,156 to 11,346 kg/ha. Total Harbour biomass accumulation has
been calculated to be approximately 5 % of standing biomass and
is expected to be in the vicinity of 126,085 tonnes per year.
(Comley, 2002). These studies are described in more detail in
3.2.2.
Habitat
Mangroves also provide important permanent and temporary
habitats for a large number and range of marine and terrestrial
fauna. Marine fauna commonly found in mangroves includes
molluscs, crustaceans (such as crabs and prawns), a wide range of
fish and of course, the saltwater crocodile.
Research has shown that the abundance and diversity of marine
fauna found throughout mangrove areas is quite high. Studies in
Darwin Harbour, for example, have identified around 36 species of
crustaceans and 31 species of molluscs (Northern Territory
Government, 1997b).
Other research conducted in eastern Australia, has estimated that
67% of the entire commercial fish catch is composed of species
dependent upon mangrove estuarine areas (Hamilton and
Snedaker, 1984).
A wide range of terrestrial fauna is also found in mangroves and
includes insects, snakes, frogs, and mammals such as possums
and flying foxes. Nursery areas for flying foxes in the Darwin
Harbour catchment have been identified in Rapid Creek and
Sadgroves Creeks.