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Flora |
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Phytoplankton/Diatoms |
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The
phytoplankton biomass has not been investigated but the clarity of the
water suggests that it is low. A total of 39 species were
identified by Korringa in phytoplankton samples taken for Thesen's
Wharf, The Heads, Featherbed Bay and 2,5 miles offshore.
The phytoplankton has not been studied
in detail but a number of species of diatoms have been recorded.
These include Skeletonema costatum, species of Chaetoceros,
Coscinodiscus, Navicula and Pleurosigma. Among the
dinoflagellata Noctiluca miliaris is abundant at times and
species of Ceratium and Peridinium are common. (Ref
1) |
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Algae |
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Attached
algal vegeation is rich. The rocky banks at The Heads are
colonised by an unusually wide variety of algal macrophytes.
The species of algae present include Gelidium
pristoides, Ulva lactuca, Enteropmorpha sp. and Zonaria
tournefortii. Algae present within the lagoon include Ulva
spp. Enteromorpha spp. and Chaetomorpha spp.
The alga Zonaria tournefortii is common in Zostera
beds. Algae collected at The Heads, include Splachnidium
rugsosum, Gelidium pristoides, Codium duthiae, Ulva rigida, Bryopsis sp.
The dry biomass of Ulva east of
Thesen's Causeway was 61,5 g/m2. While the ash-free
dry biomass of Ulva east of Thesen's Causeway was 241 g/m2.
Patches of Ulva, Entermorpha and Chaetomorpha occur along the banks of
tidal channels and on the surface of the mud on the lower shore.
The alga Bostrychia vaga forms tufts entangling the bases of
intertidal saltmarsh plants. (Ref
1)
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Aquatic
Vegetation |
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From
mid-tide down to the lowest level of spring-tide and even below, the
dominant plant is the sea-grass Zostera capensis. It grows all
over the estuary up to the Old Drift. It is most luxuriant at
the low water of spring-tide level on shelving mud banks where the
salinity is about 30 parts per thousand. It is sometimes mixed
with Halophila ovalis. The mean dry biomass of Zostera is 67,5
g/m2 at Ashmead and 238,4 g/m2 north of Leisure
Island. The mean dry biomass of Zostera near the sewage outfall
is 135 g/m2, the ash free dry biomass of Zoster in this
area was 125 g/m2. From Westford Bridge upstream
Ruppia maritima becomes common eventually largely replacing Zostera at
the Old Drift. (Ref 1) |
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Semi-aquatic
Vegetation |
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While Zostera beds predominate near the
low-tide mark, borad saltmarshes comprised of various other species
occur at higher levels. Spartina maritima is abundant at mid
levels and species of Sacocornia, Triglochin, Chenolea, Limonium and
Plantage carpet the upper levels of the saltmarshes. Near the
high spring-tide mark, Juncus kraussii appers and it covers extensive
areas of mud banks in the upper reaches of the estuary. Biomass
values for these macrophytes are high but primary production has not
been measured.
The highest intertidal areas are
carpeted with a dense growth of Limonium scabrum or Chenolea
diffusa. These species tend to form large patches covering areas
of several hundred square metres each rather than a continuous
band. Limonium pathches extend down to 15 cm below high water of spring tide.
Below this it forms mixed communities with Chenolea, Sarcocornia and
Triglochin but with Limonium still dominant in patches down to 20 cm high water of spring tide. Slightly lower, occasional isolated plants of Limonium are
each enmeshed in tufts of the alga Bostrychia vaga. Sarcocornia
decumbens tends to border the patches of Limonium but not spreading as
widely in area. The densest patches are in the region of 15 -20cm
below high water of spring tide. Below this there are mixed patches of Chenolea and
Limonium with Sarcoconia. The lowest isolated specimens of
Sacrcoconia were observed at 50 cm below high water of spring tide in the Triglochin
zone. Chenolea diffusa covers patches up to several hundred
square metres in area between about 10-20 cm below high water of spring tide. Isolated
plants merge into Triglochin zone about 25cm below high water of spring tide.
Chenolea appears to tend to favour the margins of channels and the
slopes of humps perhaps favouring better drainage. The lowest
specimens occur in humps in the Triglochin zone at levels between
20-30cm below high water of spring tide. (Ref 1)
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Terrestrial
vegetation |
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On
the banks of the lagoon above high water of spring tide a large number of species of plants
appear which are tolerant of marginal conditions including occasional
salt spray. Away from the immediate shore there is dense bush or
scrub forest on the slopes. The coastal bush on the Eastern Head
(Foundation Point) and Western Head (Brenton Head) includes melkbos Sideroxylon
inerme, camphor bush Tarconanthus camphoratus, taaibos Rhus
glauca, assegaihout, Curtisia dentata, aloe Aloe ferox,
num-num Carissa bispinosa, bietou Chrysanthomoides
monilifera, blombos Metalasia muricata, polygala Poilygala
sp., asparagus Asparagus sp. and buchu Agathosma
sp. (Ref 1)
Acock's map of the Veld types of South
Africa indicates that the whole of the catchment and the surrounding
of Knysna river and estuary fall in the Knysna Forest. However,
the crest of the hills stretching west from the western or Brenton
Head support plant communities which appear to be representative of
Coastal Fynbos although in Acock's map this is only represented west
of Mossel Bay. The vegetation of the Knysna Forest has been
exhaustingly described by Phillips in this nmonograph on forest
succession and ecology in the Knysna region. Acocks points out
that it is probable that the importance of Fynbos in the forest
succession may be due to a large extent to carlesss
exploitation. He considers that sub-tropical grassveld might
have been more important in the past in this region with high,
well-distributed rainfall, sour sandy soils and vigorous growth. (Ref 1)
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The
composition of the forests is described by Von Breitenbach
(1974). There are about 125 tree and major shrub species of
which about 100 are trees. Throughout the Knysna area invasive
alien vegetation has become a major problem. Acacia saligna,
Acacia cyclops, Acacia longifolia and Acacia melanoxylon
have invaded large areas, sometimes eliminating the natural vegetation
in the coastal belt. In the Outeniqua Mountains Pinus
pinaster and Hakea sericea now seriously threaten the
survival of the rich Cape flora. (Ref 1) |
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Fauna |
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Zooplankton |
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The
mouth of the Knysna Estuary is inhabited by neritic species while
further up estuarine species dominate the zooplankton. Among the
copepod genera Paracalanus is abundant in the lower reaches while
Pseudodiaptomus and Acartia species become abundant futher up.
Day has described the zooplankton as rather poor and suggested that
plankton is not a major source of food in the estuary. He
pointed out that there are relatively few plankton feeders.
The most common species of zooplankton
recorded were Oithona nana, Paracalanus parvus, Harpacticus gracilis
and Euterpina acutifrons, while Oithona oculata and Oncaea subtilis
also appeared in sample from a cold current. (Ref 1)
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The
dominant copepods through most of the estuary are Acartiella
natalensis, Paracartia longipatella, Paracalanus crassirostris,
Paracalanus parvus, Oithona nana, Pseudodiaptomus nudus and
Harpacticus gracilis. These species occur in most estuaries on
the south and east coasts of South Africa. In addition various
neritic marine forms such as Clausoclanus furcatus, Centropages
brachiatus, Pleuromamma abdominalis, Euterpina acutifrons and various
species of Corycaues penetrate some distance into the estuary.
The Knysna estuary is unusual among South African estuaries in being a
deep open estuary in which clear blue open water penetrates on each
high-tide. The blue water during spring-tides penetrates to well
beyong the railway bridge mixing with the more turbid green water in
the shallows. This facilitates the penetration of neritic marine
species and limits the survival of purely estuarine species such as
Pseudodiaptomus hessei and varous Hapacticoids. These latter
species are abundant in the upper reaches from the Belvedere area
right up to Charlesford Rapids. The true estuarine plankton
species can only survive in areas where the residence time of the
water is sufficiently long for them to be able to complete their life
cycles before being swept out to sea by the tides.
At times, very cold upwelled water
apppears at Knysna and penetrates into the lagoon. At such times
various deep ocean species appear such as Oncaea subtilis, Oithona
oculata and Microsetella rosea. These are oceanic pelagic
species which do not usually form part of the estuarine or neritic
marine plankton community. (Ref 1)
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The
Chaetognatha are represented by a species of Sagitta. Large
numbers of larvae of benthic invertebrates appear in the plankton
particularly during the spring and summer months. These include
polychaete larvae, larvae and post larval stages of gastropods and
lamellibranchs, ophioplutues and other echinoderm larvae, copepod
nauplii, cirripede nauplii, cirripede cyprids, crab zoea and megalopa
larvae and mysis larvae of decapods. Several species of
Ostracoda have been recorded. Among the Mysidacea Gastrosaccus
brevifissurs and Leptomysis tattersali occur in the lower estuary
while Mesopodopsis africana and Rhophalopthalamus egregius
occur in the upper estuary. Cumacea include Iphinoe truncata.
Amphipoda include Paramoera capensis, Lysianassa ceratina and
Corophium triangonyx. Isopoda include,Eurydice
longicornis, Janira capensis, Paridotea ungulata and Exosphaeroma
spp. Various fish eggs and larvae appear and occasionally small
fish such as the pipefish Syngnathus acus. (Ref 1) |
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As
might be expected in a clear open estuary penetrated by ocean water at
every tide there are more neritic marine species of phytoplankton and
zooplankton that in most South African estuaries. Organic
detritus particles are abundant in most plankton samples sometimes
even exceeding the plankton in abundance. It is clear that the
detritus derived from the attached plants of the saltmarshes plays an
important role in this estuary. Nanoplankton is particularly
abundant in the upper reaches of the estuary and it appears to be the
major food source of the dominant copepod in that area Pseudodiaptomus
hessei. (Ref 1) |
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Results of numerous
studies indicate that total phytoplankton biomass within the estuary
is low, generally < 2 mg chl-a
m-3, which can be linked to the low macronutrient availability within
the system. Statistical analyses
reveal no significant spatial differences in the total phytoplankton
biomass along the length of the estuary
(Grindley, 1985; Allanson et al., 2000a). Wind generated
coastal upwelling in the marine
environment appears to play an important role in determining the
phytoplankton biomass within the Knysna
Estuary. Cold, nutrient rich water penetratong into the estuary
is associated with a dramatic increase in the total phytoplankton
biomass within the estuary with
levels of up to 18.4 mg chl-a m-3, being recorded (Allanson et al.,
2000). The freshwater inflow from
the Knysna River is also associated with increases in the
phytoplankton biomass as riverine
inflow represents the most importance source of macronutrients
necessary to sustain the growth of
the phytoplankton. These results highlight the importance of catchment
management strategies in maintaining the
health of the Knysna Estuary. A
comprehensive description of the zooplankton community within the
Knysna Estuary is given byGrindley (1985). The zooplankton community
structure within the Knysna Estuary demonstrates strong
horizontal patterns reflecting the hydrodynamics of the system.
Neritic (oceanic) species dominate
in the lower and middle reaches of the estuary while true estuarine
species predominate only in the
upper reaches (Grindley, 1985). The zooplankton community throughout
the estuary is numerically
dominated by copepods. Among the copepods, species of the genera Oithona,
Paracalanus,
Oncaece and Pseudodiaptomus dominate in the lower and
middle reaches while in the upper
reaches the estuarine copepods on the genus Psuedodiaptomus and
Acartia are most numerous.
In addition to the copepods, several additional neritic species of
chaetognath, mysid and gelatinous
zooplankton have also been recorded within the system. (Ref
5) |
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Aquatic
Invertebrates |
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The
benthic macrofauna includes 310 species with rich beds of Upogebia
africana and numbers of Arenicola loveni, Solen corneus, Atrina
squamifer and other bivalves. The gastropods Asiminea
globulus, Hydrobia sp. and Nassarius kraussianus are
abundant. There are numerous amphipods and isopods and the
saltmarsh vegetation harbours Sesarme catenta, Cleistostama spp.
and other crabs. |
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Corresponding to physical changes along
the length of the estuary there are a series of faunsitic
divisions. Day et al. (1952) pointed out that salinity
changes, current strength, pH and temperature may all be important,
but that the environment must be considered as a whole. Any one
factor only becomes important when the limits of tolerance for that
factor are reached. Four basic divisions of the Knysna Estuary
were distinguished:
- Knysna Heads
- the Lagoon from Leisure Island to
beyond The Point
- the Westford channels to the Old
Drift and
- Charlesford Rapids.
It was shown that the diversity of
species declines progressively as one passes upstream from The
Heads. While the number of species present at Thesen's Island is
roughly similar to that at The Heads, the component species are
notably different. Thus over half the species at The Heads do
not extend to The Heads. A characteristic lagoon fauna was
recorded from Leisure Island to The Point. The percentage of
typical seashore species decrease progressively up the lagoon and none
reaches Charlesford Rapids. (Ref 1) |
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The Pansy Shell, Echinodiscus
bisperforatus occurs and bredds on particular sand banks in the
main channel of the lagoon. They are popular with visitors but
their numbers appear to have declined alarmingly in recent years,
dejpsite conservation legislation.
Knysna is a major supplier of
oysters. Oyster larvae settling on collectors placed inside the
Knysna Lagoon come from parent stocks in the sea. They are
carried by the counter-current flowing very close inshore, and are
washed in to the lagoon with the flood tide. Therefore settling
takes place close to the entrance of the lagoon and at about
high-tide. Conditions in the central part of the lagoon, above
the Rail Bridge, are not suitable for oyster larvae, but spat and
older oysters may thrive there because of the high food content of the
water.
Three species of indigenous oysters
appear to occur at Knysna. Of these the common South African
oyster, Crassostrea margaritacea, an oviparous species, is
considered the most promising of oyster cultivation. The
small Ostrea algoensis should be considered as a
'weed-oyster' because of its limited dimensions and slow growth.
By means of proper timing of the placing of collectors it is possible
to avoid this species settling in appreciable numbers. The red
oyster, Ostrea atherstonei, another larvipqarous species, is
very interesting because of its rapid growth and perfect shape.
Previously this species was only known from fossil deposits. It
occurs at many places along the coast of South Africa, including the
west coast, but the numbers settling at Knysna are not of commercial
significance, so that it will be extremely difficult to build up a
stock of this oyster. In addition to the indigenous oysters, the
Pacific oyster Crassostrea gigas was introduced for cultivation
in 1973. (Ref 1) |
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The Pacific oyster C.gigas was
introduced in 1973. Present production is two million per annum
from Knysna Lagoon. This represents approximately 200 tonne or more
per hactare per year. Cultured seed oysters are procuded at the
Fisheries Development Corporation hactchery and supplied to the oyster
farming industry. An intertidal tray system is currently
employed for further growth until harvesting. The Portuguese
oyster, Crassostrea angulata, grows and fattens satisfactorily
in the Knysna Lagoon. (Ref 1) |
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The Knysna Estuary is home
to a number of critically endangered invertebrate
species including the Knysna seahorse (Hippocampus capensis),
the pulmonate impet, Siphonaria
compressa, and the Pansy shell (Echinodiscus bisperforatus)
(Grindley, 1985; Angel et al.,
2006). As a consequence, the Knysna Estuary ranks very highly in terms
of its conservation importance and
the preservation of its fauna and flora would ensure that nearly 43%
of South Africa estuarine biodiversity
would be conserved (Turpie, 2000). (Ref
5) |
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Insects |
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Insects
have not received detailed study but those collected included
chironomid midges and their larvae, dragonflies, mayflies, kelp flies,
staphylinid beetles and other beetles, house flies and water boatmen. (Ref 1) |
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Fish |
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More
than 200 species of fish are reputed to be found in the lagoon which
is, inter alia, the home of a rare seahorse, Hippocampus capensis.
During the University of Cape Town
ecological studies, over 50 species of fish were identified.
Most fo these were juveniles of marine species but anglers also take
large kob, leervis, spotted grunter, white steenbras, elf and even
stenohaline marine fish such as Sparodon durbanensis on
occasion. Forage fish such as Gilchristella, Hepsetia breviceps
and three or four species of mullet are plentiful. (Ref 1)
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The
white stumpnose is one of the commonest fishes and feeds on mussels
and small crustaceans particularly such as amphipods. The white
steenbras grubs in the sand for polychaetes such as Orbinia and
Nephthys spp. and small crustaceans particularly Pontogeloides
and Urothoe. Large specimens develop teh cpaacity of
blowing holes in the sand to feed on Upogebia and Arenicola.
The kabeljou appear when they are about 15cm long. The feed ona
variety of small crustceans including mysids, shrimp, Upogebia
and Hymenosoma by they do not seem fo feed on the infauna of
muddy bottoms. Large specimens predatory and feed on Sepia
and Mugil spp. Adult kob extend the whole length of the
estuary including the upper reaches. The leervis Lichia amia is
a vicious predator even when small, feeding on shoals of post-larval
mullet in the shallows. They extend into the upper reaches of
the estuary and as adults feed on Mugil spp. Hyporhamphys and juvenile
Lithognathus and Rhabdosargus. They are swift predators and form
the final link in the food web illustrated in a diagram of the trophic
relations within Knysna Estuary. (Ref 1) |
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Because
of the permanently open wide rocky entrance, the area near The Heads
includes typical marine fishes. The width of the mouth is such
that there is no barrier to shoals of pilchards, anchovies, etc. and
their attendant predators, so they enter freely. As a result
there have been many records of fishes unusual for estuaries. At
times, the upwellign on the south coast drives numbers of species into
the lagoon. Some of these are thrown ashore numbed or
dead. (Ref 1) |
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Amphibians
and Reptiles |
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Snakes
and lizards that have been observed around the estuary include the
russet snake, the night adder, the puff adder, a skink, and a
gecko. There are also a number of toads and frogs that occur in
the Knysna area. (Ref 1) |
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Birds |
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A
great diversity of birds frequent the Knysna area and the Knysna
Estuary and its saltmarshes in particular. The Knysna loerie,
Tauro corythaix, is well known.(Ref 1) |
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Mammals |
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The
largest mammal found in the Knysna area is the African elephant
Loxodonta africana africana. Mammals that are seen from time to time
around the estuary include the Cape dassie, the Cape Grysbok, Cape
Grey mongoose, the Cape dune mole and the Cape fruit bat. (Ref 1) |
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Ian
Flemming Photography |
