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Estuaries are probably one of the most
complex and interesting
ecosystems on earth. They are those unique areas
that form the interface between
marine and freshwater
environments. In the past people thought of
estuaries as simply a
combination of marine and
freshwater ecosystems, it is
only relatively recently that
ecologists have realized that
estuaries form a unique
ecosystem quite different from
either the marine or freshwater
systems they separate. |
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Estuaries have four main
characteristics that together
contribute to their uniqueness: |
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Estuaries are highly productive
systems; |
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Estuaries are multiple
producers; |
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Estuaries are nett exporters of
nutrients; and |
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Estuaries have a natural
tendency towards eutrophication. |
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High productivity |
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Estuaries rank with coral reefs
and tropical rain forests as the
most productive natural
ecosystems on earth. The
following table compares the
gross primary productivity of
several systems. There are
several ways of measuring gross
primary productivity, here we
use kcal/m²/year - this is a
measurement of the amount of
energy produced (kilocalories)
in a square meter of surface
area per
year. |
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Ecosystem |
kcal/m²/year |
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Estuaries: |
20 000 |
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Coral Reefs: |
20 000 |
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Tropical forests: |
20 000 |
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Moist temperate
forests: |
8 000 |
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Upwelling zones in
the sea: |
6 000 |
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Grasslands & natural
pastures: |
2 500 |
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Cultivated lands
with little or no
energy subsidy: |
3 000 |
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Fuel subsidised &
mechanised
agriculture: |
12 000 |
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The process of primary
production is, in very
simplified terms, the
conversion of elements like
oxygen, hydrogen, carbon,
nitrogen, etc into amino acids
(the basics of proteins,
vitamins, etc) and carbohydrates
(sugars, fats, etc) which can in
turn be utilized in the food
chain - in
other words nutrients. Plants
use energy from sunlight to
combine and convert these
elements, the nutrients thus
produced enter the food chain
through organisms feeding on the
plants, or in the process of
plant material decomposing
(rotting) which releases
nutrients into the soil or
surrounding water where it is
also taken up in a food chain.
Bacteria and a few other benthic
organisms can produce nutrients
through chemical reactions in
the absence of sunlight or
oxygen. |
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A typical estuary normally has a
meandering channel where the
major currents flow, this is
surrounded by sandbanks,
mudflats, saltmarshes and
reedbeds. Primary production
occurs in all these areas, but
it is minimal in the current
channel and on the sandbanks.
Most primary production in an
estuary takes place in the
following three areas: |
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Reedbeds are mostly situated in
the upper reaches of an estuary,
although they also occur lower
down in areas where seawater do
not penetrate too often.
Reedbeds are not
only important for filtering
water moving down into the
estuary, but the reeds are
primary producers like any other
plants. Nutrients from them are
released into the system, but
most will actually build up in
old plant material which is then
flushed down into the estuary
(and out to sea) releasing
nutrients as the plant material
decomposes. |
Spartina saltmarsh
Reedbeds & mudflat |
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Salt marshes are the most
productive areas and responsible
for a major portion of nutrients
produced in an estuary. The most
dominant plant in saltmarshes is
Spartina spp, a short
stubby grass with a very rapid
growth rate and turnover.
Spartina can survive in
freshwater and seawater, and as
it thrives in shallow areas
which is regularly flushed by
tides, nutrients produced by the
plants is rapidly released into
the rest of the system.
Spartina beds also host a
vast number of bacteria and
other organisms consuming plant
material and supporting small
invertebrates that form the
basis of the food chain. |
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The third production area are
mudflats, often adjacent to
Spartina beds and sometimes
forming part of salt marshes. In
the mud flats we find that
production is mostly through the
activity of benthic organisms
(see below). |
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Multiple producers |
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There are three types of
producers that power (ie provide
energy to) our natural world: |
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Macrophytes - mainly terrestrial
plants like trees, shrubs and
grasses that convert energy from
the sun into nutrients through
the process of photosynthesis.
Aquatic macrophytes in this
group include reeds, grasses and
a few other plant species like
Potamogeton growing under
water. |
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Phytoplankton - aquatic algal
forms free-floating in the
surface layers of oceans and
lakes where they photosynthesize
to convert the sun's energy into
nutrients; amongst these are
green and blue-green algae. |
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Benthic organisms - includes
types living in or on the bottom
sediments of oceans, lakes and
rivers. They mostly convert
energy by anaerobic means to
nutrients (ie not using oxygen)
and includes diatoms, benthic
algae, dinoflaggelates and
bacteria. |
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Macrophytes and phytoplankton
are only productive during the
day, many of them also cease (or
drastically decrease) production
during cold winter months.
Benthic organisms are not
dependent on light and can
produce throughout the year. |
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Most ecosystems have only one or
at most two of these producers -
terrestrial systems have
only macrophytes to produce
nutrients. In the open sea
phytoplankton is the most
important primary producer.
Estuaries, however, have all
three types of producers - this
is the main reason for the
very high annual productivity, the
processes of primary production
can go on day and night and
virtually throughout the year. |
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Nett exporters of nutrients |
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The high productivity, together
with the fact that they are
multiple producers, mean that
estuaries produce more nutrients
than the system itself can
utilize. The result is that: |
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The enormous amount of nutrients
produced in estuaries support a
vibrant food chain within the
estuary. Although there are not
many animal species living in
the relatively harsh conditions
in an estuary (see the
Estuarine Fish page), there
are large numbers of these
animals present. |
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However, even this food chain
cannot utilize all the nutrients
and a considerable percentage is
exported to the sea by tidal
action. |
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The exported nutrients often
form the basis of adjacent
inshore food chains,
particularly along sandy shores
where primary production is
lower than on rocky shores. In
many parts of the world coastal
fisheries are dependent on
nearby estuaries for the primary
source of nutrients to sustain
the fishery. |
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Natural tendency towards
eutrophication |
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The production of nutrients
occur in the estuary itself; it
is its own production system.
Contrary to common belief,
rivers in general do not
fertilize estuaries, and there
is not much contribution of
nutrients from the sea. |
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Although estuaries export
nutrients to the sea, they are
also nutrient traps in that
nutrients build up in tidal
pools, deep water areas, salt
marshes and adjacent wetlands.
Normal tides, even spring tidal
cycles, do not flush
estuaries adequately to remove all the
excess nutrients; it is up to
major floods to flush out the
estuary and remove built-up
nutrient material. |
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The result of nutrient levels
building up in an estuary is
eutrophication, in other words
an enrichment of the estuary
itself. Eutrophication is the
cause of algal blooms, as well
as the distinctive smells of
salt marshes and wetlands.
Eutrophic conditions in an
estuary or portions of an
estuary is normal, and is not a negative
factor. Without eutrophic
conditions, estuaries would not
be the highly productive systems
they are and would not have
excess nutrients to export. |
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While eutrophic conditions in an
estuary is a desirable state, it
does sometimes happen that this
develops into hypertrophic
conditions. Under such
conditions nutrient production
rises to such high levels that
the system virtually "chokes up"
- too much nutrients leading to
even greater algal blooms and
more over-production
resulting in oxygen depletion
and thus die-offs of oxygen-dependent organisms. This is
most noticeable when fish die
off in large numbers.
Read
about the fish die-off in
December 2006. |
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Hypertrophic conditions can also
be caused by pollution of the
sytem when high levels of
nitrogen and phosphates enter
the system. This can come from
stormwater drains, agricultural
drainage, and even just from
surface run-off in built-up
areas. Damming of rivers has
made the situation worse by
preventing floods which would
normally flush the estuary; now
global warming with its effects
of lower rainfall is also adding
to the problem in many parts of
the world. |
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Our
Water Quality section
contains specific data about
nutrient and pollutant levels in
Rietvlei and the Diep River
estuary. |
