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Physio-chemical
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The
major characteristic of Swartvlei is that it is usually stratified
into water layers of different density, caused by vertical variations
in salt concentration. Such lakes are known as meromictic lakes.
During the tidal phase, when water enters the Swartvlei from the upper
estuary, the salinity of this water is generally higher than that of
the surface waters of the lake. This tidal flow of dense estuarine
water, runs down the sandy slope in the vicinity of the rail bridge
and accumulates in the Swartvlei basin to form a deep layer some 5m
thick. If a pulse of even higher salinity enters a Swartvlei, this
layer will be displaced upwards and a new more saline layer will form
across the bottom of the lake. The end result is that surface
salinities vary between 1 and 12 parts per thousand during periods of
stratification with bottom salinities rising to 20 parts per thousand.
The amount of wind stress required to break down this layered
structure is considerable and as long as the mouth is open, allowing a
continual input of estuary water into Swartvlei, this layering
persists. The most important aspect of this stratification is the
effect it has on the dissolved gases, oxygen (O2), carbon
dioxide (CO2) and hydrogen sulphide (H2S).
Because stratification prevents wind from mixing the oxygenated
surface waters of the lake with those below, decomposition of the
abundant organic matter in the bottom sediments rapidly uses up the
oxygen and increases the concentrations of carbon dioxide. The
decomposition of organic matter with the proteinaceous sulphur, and
the reduction of sulphate under the deoxygenated conditions, result in
the accumulation of hydrogen sulphide in these bottom waters. The high
carbon dioxide and hydrogen sulphide levels, together with the
anaerobic conditions, therefore make the bottom half of Swartvlei a
very toxic environment for animal life (Howard-Williams and Allanson,
1979). (Ref 1)
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The
closing of the estuary mouth has a marked effect on the maintenance of
stratification because it prevents the inflows of saline water. During
the lagoon phase is when the mouth is closed wind mixing of the surface
waters gradually disturbs the salinity layering. The longer the mouth is
closed the more likely that it is that the stratification will break
down and oxygenation of the bottom waters will occur (Allanson and
Howard-Williams, 1983). The level to which the water rises in the lake
has little influence on mixing since is it the length of time for which
the mouth is closed, coupled with wind stress, which is important. (Ref
1)
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Stratification
has important consequences for the concentration of plant nutrients in
the water column. Soluble reactive phosphate (SRP) concentration is
very low in Swartvlei and is often present in undetectable quantities
(1ug per l SRP). This nutrient ion was thought by Roberts (1973) to
limit algal growth in the lake. Total dissolved phosphorous (TDP) was
up to 30ug per L. According to Howard-Williams (1977) the SRP, TDP and
TP concentrations in stratified bottom waters all exceed 100ug per L
but are unavailable for plant growth. Howard-Williams and Allanson
(1978b) and more recently Silberbaur (1982) determined that phosphate
is released from the bottom sediments of Swartvlei and maintained in
solution, provided the water remains anaerobic. Oxygenation causes the
soluble phosphate to precipitate out so that it is then lost to the
sediments. Values for Ammonia nitrogen and Nitrate ammonia obtained by
Robarts (1973) were very variable with no distinct patterns except
that the concentration of Ammonia nitrogen was higher in deoxygenated
conditions and that of Nitrate ammonia higher in oxygenated surface
waters. (Ref 1)
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Swartvlei
is thus richer in nutrients such as phosphorous during a stratified
meromictic condition but these are not directly available to the
plants. (Ref
1)
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Details
of physical and chemical processes which occur in the estuary are
described by Liptrot and Allanson (1978). Typical salinity profiles of
the three tidal regimes (neap, mean and spring tides) show that:
a)
Salinity decreases from the mouth to the rail bridge and from spring
to neap tides. Transgression of seawater, with a salinity of 35 parts
per thousand, into Swartvlei occurs infrequently and is usually
associated with spring tides and abnormally low barometric pressures.
b) Vertical mixing of the water column is greatest during spring
tides. During neap tides the salinity of the surface waters draining
through the estuary varies from 10 to 19 parts per thousand which
results in slight salinity stratification in the deeper portions of
the estuary. As more sea water enters the estuary towards spring tide,
this stratification is broken down.
c) During a tidal cycle, a substantial volume of water is actually
retained in the estuary, which means that over a period of several
tides the water in the middle section of the estuary will be moved
back and forth with the sea water front acting as a piston. The effect
of this is that the water in the lower reaches is replaced by seawater
every tide, while in the upper reaches, estuary and lake water are
continually exchanged.
d) Minimum salinities occur when the rivers are flooding and the mouth
is open.
e) After the mouth has closed, salinities in the estuary gradually
fall as the estuary becomes diluted with low-salinity (8 to 12 parts
per thousand) water from Swartvlei. Wind-induced mixing breaks down
vertical stratification during this period. (Ref 1)
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Dissolved
oxygen values vary mainly in response to biological activity.
Horizontal variations depend on the distribution of the aquatic plants
in the estuary, higher oxygen values being associated with the
presence of aquatic plants. Generally, the dissolved oxygen
concentrations are higher nearer the mouth than further up the estuary
during the tidal phase and the lagoon phase showed that the closing of
the mouth has no effect on the mean dissolved oxygen values in the
channel areas. However, no deoxygenation does occur in localized areas
towards the end of the lagoon phase. These areas are at the sides of
the channel, where large mats of the floating algal Enteromorpha is
also responsible for a marked rise in total carbon dioxide,
bicarbonates and carbonates in the water at the end of winter during
the lagoon phase. (Ref 1)
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Temperature
variations in the estuary follow a seasonal pattern with the
temperature ranging between 10ºC in winter to 29ºC in summer.
Closure of the mouth has no effect on water temperatures, but during
summer when the mouth is open, the waters near the mouth are generally
1ºC cooler than further up the estuary. (Ref
1)
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The
rivers flowing into the Swartvlei system are stained dark brown by
dissolved organic matter leached from the vegetation of the catchment
area. This humic matter (60 to 80 mg per L) precipitates out in the
estuary at salinities above 17,5 parts per thousand provided the pH
value is above 8,0. Such conditions are almost always present during
the tidal phase, and this humic precipitate probably represents a
significant import of organic matter into the estuary. (Ref 1)
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Of
the minor icons, nitrates and phosphate, most work has been done on
the latter. Surface nitrate values were studies in 1976 by Coetzee
(1979) who found values ranging from not detectable to 21 ug per L.
Howard-Williams and Allanson (1979) recorded values of NO3-N
ranging from 3 to 70ug per L. NH4 – N is low (Robarts
ranging from not detectable to 4ug per L). (Ref 1)
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Total
dissolved phosphorous remains fairly constant in the main channel of
the estuary at about 24ug per L, except at times during the early
lagoon phase when isolated foci of deoxygenated saline water occur at
the deeper points. Over these points dissolved phosphorous can rise up
to 260ug per L. The main source of phosphorous for the estuary is the
sea. During the tidal phase there is a net import and accumulation of
phosphorous in the estuary, whereas during the lagoon phase
phosphorous is cycled between water plants and sediments. The strong
outflow phase immediately after opening of the mouth is characterized
by an extensive net outflow of phosphorous, mostly in particulate
form. (Ref 1)
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Trace
metal concentrations in the waters and sediments of the estuary are
thought to be low, but the concentrations of the major cations Na+,
K+, Ca2+and Mg2+ are all much higher
than those in Swartvlei (Watling, 1977). (Ref
1) |
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Pollution |
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