The
mangrove environment
The term mangrove may be used to refer to both an
individual mangrove plant and to the habitat in which it lives. Worldwide there are 69
recognised species of mangrove plants belonging to 20 families - 34 mangrove species and
3 hybrids are known to occur in Queensland.
Mangrove forests are diverse communities growing
in the intertidal zone (between the average sea level and the high tide mark) of tropical to
subtropical coastal rivers, estuaries and bays. In north Queensland the coast is sheltered
from high energy wave action by the Great Barrier Reef, while the southern coast is protected
by an extensive chain of sand islands. The large amount of silt deposited by coastal rivers
along this protected shoreline has produced an environment suitable for the growth of extensive
mangrove forests. Mangrove plants can also be found growing on the carbonate sediments
deposited around reef associated islands.
Plants growing in the intertidal zone are subjected
to large fluctuations in salinity - they are inundated by seawater (high salinity) during high
tides, whilst at low tide, or during heavy rains or floods, they can be exposed to fresh water
(low salinity). Tidal ebb and flow, together with changes in river flow rate, can alter water
temperature and the supply of nutrients and oxygen to the plant's root systems. Soils of the
intertidal zone are usually soft and muddy and are often anaerobic (low in oxygen). Mangrove
plants are unique in their ability to grow in such a dynamic environment.
The occurrence of
individual mangrove species within the forest is reliant on environmental factors such as
salinity, nutrient availability, oxygen
level in the soil and wave energy. As mangrove species are variable in their tolerance of
these factors, a pattern of species distribution known as zonation occurs. For example, less
salt tolerant species often occur high in the intertidal zone (landward edge of the mangroves),
or only in places where there is some freshwater input. Those species that are most tolerant
of salty conditions generally occur low in the intertidal zone (seaward edge of the mangroves)
where they are exposed to seawater at high tide. Some of the most salt tolerant species are
found in environments where tidal influence is minimal but evaporation of water from the soil
surface is high. In these areas the concentration of salt in the soil rises to such an extent
that it becomes hypersaline (more salty than seawater).
Two general types
of mangrove environment are shown in the illustrations.
The riverine environment occurs
along freshwater rivers and creeks, such as the Daintree River
and Noah’s Creek near Cape Tribulation (there is a boardwalk
for viewing the mangroves at Noah’s Creek). Note how the
forest structure is dominated by mangrove plants near the
coast and how it gradually changes to a rainforest community
further upstream. The river provides freshwater and nutrient
inputs into the mangroves.
In the marine environment, most of
the water in the mangroves comes from the ocean, with only a
little fresh- water input from rainfall. Here the forest
structure is dominated by mangrove plants near the coast and
changes to woodland as you move inland, often passing through
a hypersaline area where few plants can grow. Mangroves like
this can be seen at the Mangrove Boardwalk near the Cairns
airport, at the Townsville Town Common, Hinchinbrook Island,
in the Gladstone region and in Moreton Bay.
Often, marine
environments can be found associated with riverine
environments, particularly in tributaries at river mouths. The
coloured bars below the illustrations show changes in soil
salinity. The soil is saltiest where the colour is darkest.
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