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The
West Coast National Park is home to the Langebaan Lagoon,
which is a wetland if international importance.
The
Langebaan Lagoon was shaped by the rise and fall of sea
levels during pre-historic times. This is quite unlike
typical lagoons which form where fresh water rivers enter
the sea. The result is a purely salt water lagoon.
The
Langebaan ecosystem is a dynamic system that relies on the
interaction between the various atmospheric, aquatic and
terrestrial factors and elements. Their interaction is of an
interdependent nature and creates a self-sustaining
ecosystem.
The
basis of this ecosystem like all others are the abiotic
substances like water, oxygen, nitrogen, carbon dioxide,
salts etc. For the Langebaan Lagoon it all originates from
the depths of the ocean with the ocean currents, wind and
sun all working together. The cold benguela current that
flows up the west coast brings nutrient rich water from the
Antarctic. A combination of the ocean current and prevailing
southeasterly winds result in a phenomenon known as
upwelling. This upwelling forces nutrients consisting of
decomposed matter from the seabed to be brought up to the
surface along the shoreline. This decomposed matter is being
continually regenerated by the decomposition of dead sea
creatures. Further small plant matter is added to the water
by the destructive forces of the waves on the vast kelp beds
that line the west
coast. These nutrients provide food for the producer
organisms like floating plants and phytoplankton. The
suspended plant life utilise the available sunlight to grow
and multiply thereby further increasing the availability of
suspended nutrients.
It
is this plankton that provides food directly for the primary
macro consumer organisms. These are the shoreline filter
feeders like muscles as well as many of the fish like
pilchards and mammals like whales. The smaller fish then
also serve as a source of food for bigger fish and sea birds
that are referred to as secondary and tertiary consumers.
The
daily change in the tides provides the mechanism to bring
this nutrient rich water into the Lagoon. However not much
of it actually reaches down the 15km length of the lagoon.
This is largely due to the vast banks of clams under water
at the entrance to the lagoon that are supported by this
phytoplankton and filter it out. Harders also feed on a lot
of the floating nutrients. The rest is used up by the sea
grasses, salt marshes and reed beds on the edge of the
lagoon. The salt marshes in the southern end of the lagoon
utilise some of these nutrients and in turn release more
phosphate rich nutrients into the water. The plants living
in the shallows of the lagoon also provide a significant
amount of nutrients to the lagoon itself. These nutrients
come in the form of decayed plant matter from the seasonal
dead flowers that are recycled by bacteria. The bacteria
assisting with decomposition of plant and animal matter are
referred to as saprotrophic organisms and form a vital link
between living and non-living elements in an ecosystem.
In
the Lagoon the filter feeders like mud prawns and blood
worms rely on these nutrients from the decomposition cycle.
These organisms then in turn provide a food source for fish
and birds. The thousands of resident and migrant birds feed
off the nutrients generated from the salt marshes as well as
the sea creatures that live off these nutrients. The guano
from these birds is returned to the lagoon and in turn
provides vital nutrients back to the salt marshes.
This
ecosystem is sustained by the nutrient cycle which allows
nutrients to be recycled from the start of the process as
abiotic substances, through to end consumers and then
decomposed back to abiotic substances. The key energy source
in this process is the sun, which provides the necessary
energy to sustain the process. Since the ecosystem is like a
chain, if any of the links are removed then the entire
system can stop functioning. So no mater how small or
insignificant, every little piece of the ecosystem is there
for a reason and critical to its sustained existence.
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