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Flora |
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Phytoplankton/Diatoms |
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The
structure of the biological community in the open water of the
Swartvlei was first studied by Robarts (1973) who examined
phytoplankton, bacterial numbers and with Grindley and Wooldridge
(1973) also made some zooplankton counts. More recently Coetzee (1981)
carried out an extensive analysis of the zooplankton of Swartvlei and
found two zooplankton communities in the lake during day time. The
aerobic community was dominated by the copepod Acartica natalensis,
Pseudodiaptoms hessei, Musculus virgilae, veligers and calanoid
copepod nauplii. A community dominated by cyclopoid copepods
(mainly Halicycolps species), cyclopoid nauplii and polychaete larvae
and juveniles was found to occur in water of very low oxygen and in
some instances in water where the level of dissolved oxygen had fallen
to zero. In how far this represents a rea division of the plankton
community or as suggested by Coetzee (1981) was the result of sampling
technique is uncertain. (Ref 1)
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Grindley
(1981) showed that Acartica natalensis was the dominant species
over the deep deoxygenated areas while the Pseudodiaptomus hessei
was the dominant species in the marginal area. In these shallow
marginal areas there was no deoxygenated zone. Pseudodiaptomus
normally migrates to the bottom during the day so they avoided the
areas over deep water. The lowest plankton biomass was recorded over
the deep water in the area dominated by Acartica natalensis.
(Ref 1)
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Robarts
(1976a) found three major categories of phytoplankton in Swartvlei
viz. diatoms, dinoflagellates and flagellates. The dominant organism
was the centric diatom Coscinodiscus lineatus, a species common
in estuarine and marine habitats in
South Africa
. A list of common diatoms in Swartvlei is given by Robarts (1973).
These primary producers were mostly nanoplanktonic (60um in diameter)
and population density never exceeded 3,5 x 10 cells per L.
Chlorophyllous
flagellates and dinoflagellates generally formed a relatively minor
part of the plankton, but occasional short-lived blooms of flagellates
and dinoflagellates occurred during periods when diatom numbers were
low (Robarts, 1973).
Chlorophyllous
flagellates and dinoflagellates generally formed a relatively minor
part of the plankton, but occasional short-lived blooms of flagellates
and dinoflagellates occurred during periods when diatom numbers were
low (Robarts, 1973).
Chlorophyll
– a determinations in Swartvlei during 1976 (Coetzee, 1979) and in
1977-78 (Howard-Williams and Allanson, 1981a) support the initial
observations of Robarts (1973) that phytoplankton populations are low.
The waters of Swartvlei can be classified as oligotrophic and Robarts
(1976a) concluded that the rates of primary production in the pelagic
zone were the lowest recorded for an African water body. (Ref 1) |
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Algae |
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Aquatic
Vegetation |
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In
contrast, the Swartvlei littoral has for many years been characterized
by extensive beds of submerged aquatic plants which produce up to 2500
g per m2of dry matter per year, and which have been
intensively studied by Howard-Williams (1978, 1981), Howard Williams
and Allanson (1978; 1981a; 1981b), Howard Williams and Davies (1979)
and Howard-Williams and Liptrot (1980). This submerged vegetation
occurs in fairly distinct zones except in the vicinity of the railway
bridge where, due to high salinities Ruppia cirrhosa
predominates. Potamogen pectinatus grows to a water depth of
approximately 2m while Charophyta are found in the shallower
water behind the Potamogeon beds. Emergent vegetation grows along the
edge of the vlei and consists predominantly of Scirpus littoralis
and Phragmites australis. Other emergent sedges and rushes
include Typha capensis and Scirpus
maritimus. (Ref 1) |
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It
is immediately clear that by far the largest amount of food for the
ecosystem is produced by the aquatic macrophytes, which together with
their associated epiphytic algae are responsible for 86 percent of the
primary production of the lake. (Ref 1) |
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Semi-aquatic
Vegetation |
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Terrestrial
vegetation |
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The
terrestrial vegetation around the Swartvlei system consists mainly of
an Ehrharta-dominated foredune vegetation, though Rhus dominated dune
scrub to thorny riverine scrub. There are also numerous patches of
fynbos on the dunes and further inland. Most of the vegetation in this
area has been severely disturbed by grazing, trampling (recreation),
injudicious development and invasion of numerous Acacia and Pinus
species, especially in the more sensitive marsh, dune and riverine
areas (O’Callaghan, pers.comm.) (Ref 1)
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The
most striking of the estuary is the presence of extensive tracts of
two seagrasses, Zostera capensis and Ruppia cirrhosa,
which occur in both mixed beds and pure stands. The zone between high
spring tide water level and low neap tide water level is a zone of
tidal sand flats, with little or no submerged macrophyte vegetation.
In slight depressions Ruppia is fairly dense but on the main
sand flats the growth of this plant is very sparse, with only small
individual leaves occurring above the sand surfaces. These develop
rapidly when the estuary mouth is closed and the sand flats are
continuously inundated for long periods. During the past two years
(1981/82) the estuary has been permanently open and this has resulted
in a reduction in the size of the area covered by Ruppia.
(Ref 1)
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In
that part of the estuary, which is permanently submerged, dense
macrophyte beds occur.
Ruppia occurs to a depth of 0,4m below low water neap
tide level, where Zostera replaces the Ruppia
(Howard-Williams and Liptrot, 1980). Zostera is especially
prevalent on the sides of the estuary channels. The bottom of the main
channel is normally bare of macrophytes but where the channel broadens
dense Zostera beds may occur. Apart from the distinct vertical
zonation, there is a horizontal zonation of species with Zostera
predominating nearer the sea and Ruppia predominating upstream
(Howard-Williams and Liptrot, 1980). (Ref 1)
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The
macrophytes, particularly Zostera, support a large epiphytic algal
population. The principal genera include Enteromorpha, Lyngbya,
Cladophora, Percursaria, Cocconeis, Ectocarpus, Polysiphnia, Chondria
and Hypnea (Howard-Williams and Liptrot, 1980). The biomass of
periphyton in the Swartvlei estuary is large, usually about 25 percent
of the supporting leaf, although it occasionally exceeds the mass of
the supporting leaf. This is particularly true of the green algae
Enteromorpha which occurs initially as an attached epiphyte but which
may form dense mats of entangled filaments over the Zostera
beds in winter. These extensive growths then rot during the summer
often causing sulphide odors and unpleasant conditions in the
recreational areas of the estuary.
The
marginal salt marshes of the estuary occupy about 65ha and are flooded
at high spring tide and during periods when the mouth is closed. These
marshes are made up of the salt marsh plants Sarcocornia natalensis
and Salicornia meyerana, amongst a dense sward of the grasses Paspalum
vaginatum and Sporobolus virginicus. In heavily grazed
areas the latter two species are severely depleted. Juncus kraussi
occurs in clumps throughout the salt marsh (Howard-Williams and
Liptrot, 1980). (Ref 1)
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The
macrophytes, particularly Zostera, support a large epiphytic
algal population. The principal genera include Enteromorpha,
Lyngbya, Cladophora, Percursarica, Cocconeis, Ectocarpus,
Polysiphonia, Chondria and Hypena (Howard-Williams and Liptrot,
1980). The biomass of periphyton in the Swartvlei estuary is large,
usually about 25 percent of the supporting leaf, although it
occasionally exceeds the mass of the supporting leaf. This is
particularly true of the green algae Enteromorpha which occurs
initially as an attached epiphyte but which may form dense mats of
entangled filaments over the Zostera beds in winter. These
extensive growths then rot during the summer often causing sulphide
odors and unpleasant conditions in the recreational areas of the
estuary. (Ref 1)
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Fauna |
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Zooplankton |
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Aquatic
Invertebrates |
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Associated
with the littoral plants is a large invertebrate population dominated
by five species; the bivalve Musculus virgilae, the isopod Exosphaeroma
hylecoetes and amphipods Corophium triaenonyx, Grandiderella
Apseudes digitalis, the isopod Cyathura estuaria, the crab Hymenosoma
orbiculare, Chironomidae and Ostracoda (Davies,
1982). (Ref 1)
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The
distribution of M. virgiliae in the Swartvlei littoral varies
according to the surface area of plant available for attachment. With
increasing distance away from the shoreline the numbers of M.virgiliae
increase from approximately 1x103 individuals per m2
in the Scirpus/Phragmites belt to 1x104 individuals
per m2 in the outer Potamogetan zone (Davies, 1982).
(Ref 1)
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The rich
littoral invertebrate community attract many fish species to the area
and are indirectly responsible for the abundance of the popular
angling species such as the leervis Lichia amia which consumes
fishes associated with the plant beds. The muller species do not feed
on invertebrates but instead utilize the detritus from decaying
plants. The fish community is normally dominated by two species, the
Cape
Moony
Monodactylus falciformis and the
Cape
stumpnose
Rhabdosargus holubi, the former feeding on invertebrates
associated with the macrophyte beds and the latter on both
invertebrates and growing plants (Whitfield, 1982). (Ref 1)
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There
are large populations of invertebrates in the estuary, both in the
sediments (eg Sand prawns: Callianassa Kraussi) and among
submerged macrophytes (eg Sand shrimps: Palaemon pacificus). A
detailed investigation of the invertebrates and fishes of the
Swartvlei estuary commenced in January 1983 as part of the SANCOR
National Programme. (Ref 1)
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During
January 1976, Coetzee and Palmer of the Cape Department of Nature and
Environmental Conservation carried out a survey of bait organisms in
Swartvlei and the Swartvlei Estuary. Only the sand prawn (Callianassa
kraussi) was found in the Swartvlei proper, while the mud prawn (Upogebia
Africana), pencil bait (Solen capensis) and the sand prawn
were all found in the estuary ie seaward of the railway bridge.
Sand
prawns were distributed throughout the estuarine area, but mud prawns
were confined to an area near the mouth where a muddy substrate occurs
in association with the Zostera beds. At the time of the survey it
appeared as if the mud prawn was the most heavily exploited of the
three main bait organisms.
(Ref 1)
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Insects |
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Fish |
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Kok
(1981) monitored the changes in the pattern of recruitment of juvenile
fishes into the Swartvlei Estuary between July 1978 and December 1980.
He found a strong summer recruitment for most fish species, with the
muller Liza richardsoni and Liza dumerili showing
protracted immigration. Kok (1981) also recorded a lag in the
recruitment of juvenile steenbras Lithognathus lithognathus and
stumpnose Rhabdosargus holubi into the Swartvlei Estuary when
the mouth opened after a prolonged closed period. He concluded that
the timing and the period for which the estuary is open to the sea is
crucial to juvenile fish recruitment. (Ref 1)
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Coetzee
(1982a; 1982b) investigated the diet of three fish species from the
Swartvlei Estuary namely, estuarine round-herring Gilchristella
aestuarius,
Cape
silverside Hepestia breviceps and leervis Lichia amia.
The leervis is an important angling sand shrimp Palaemon pacificus
(Coetzee, 1982b) (Ref 1)
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Amphibians
and Reptiles |
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Birds |
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Boshoff
and Palmer (pers.com) are in the final year of a four year study of
the species diversity, seasonality and habitat preferences of the
birds of the Wilderness lakes, including Swartvlei. Altogether 57
species have been recorded at Swartvlei of which 5 species have been
listed in the South African Red Data book as species requiring
conservation. The numbers of birds on the lake range from 100 to 5000
with an average of approximately 1 000 (Palmer, pers.comm.) (Ref
1) |
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Altogether
61 bird species have been recorded at the Swartvlei Estuary, three of
these have been listed in the South African Red Data Book as species
requiring conservation (Siegfried et al, 1976). The number of birds on
the estuary range between 700 to 3200 with a mean of approximately 1
400 (Palmer, pers.comm.). The Swartvlei Estuary is particularly
important to Palaearctic migrants which make extensive use of the
tidal flats for foraging and roosting. (Ref
1)
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Mammals |
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