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Flora |
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Phytoplankton/Diatoms |
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Although
no specific data are available concerning this aspect of the estuarine
flora, phytoplankton blooms have been reported from the estuary when
water levels are low and/or the river mouth is closed. The
possible reasons for this are numerous. JR Grindley
(pers.comm.)reports that Potamogeton dies when the salinity of the
water rises with the closure of the mouth. The decay of this and
other plant material could enhance the nutrient levels of the
estuarine waters, thus causing excessive phytoplankton growth. (Ref 1) |
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Similarly,
if nutrients are enhanced by artificial means, e.g. seepage from
septic tanks, cattle excrement or sewage effluent (as from the
proposed sewage works), excessive phytoplankton growth, eutrophication
and eventually stagnation, could result. The tendency for
the mouth to close has already been discussed and an increase in the
river flow from the Hartebeeskuil Dam, could keep the river mouth open
for longer periods and thus offset some of the problems mentioned
above. (Ref 1) |
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Algae |
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Nearshore:
JR Grindley (pers.comm.) reports that the macrophytic algae on the
rocks near the mouth of the estuary include:
- Porphyra capensis
- Codium duthieae
- Zonaria subarticulata
- Pterosiphonia cloiophylla
- Corallina sp
- Lithothamnium sp
- Gelidium pristoides
- Caulerpa filiformis
- Plocamium cornatum
- Ulva sp
- Arthrocardia
sp
(Ref 1)
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From
this it would seem that the species composition of the rocky shore
algae of this area has a great affinity to that of the Western
Cape. However, Eastern Cape species are not uncommon e.g.
Caulerpa, Plocamium, Gelidium.
Few algal data are available for this
estuary, although the presence of Enteromorpha has been reported (ECRU
80-08-01) and algal problems do occur near the mouth when water levels
are low (JR Grindley, pers.
comm.). (Ref 1) |
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Aquatic
Vegetation |
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Although
Day (1981) reports that no submerged vegetation is found in the
Hartenbos estuary, Ruppia (ECRU 80-08-01) and Potamogeton (JR
Grindley, pers. comm.) have been reported from the lower and upper
parts of the estuary respectively. During the ECRU survey,
floating strands of Zostera were noted between the pipe and rail
bridges, but no attached plants were seen. (Ref 1) |
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Semi-aquatic
Vegetation |
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The salt marshes between the rail bridge
and the new freeway seem to be well developed and stratified from a
sparse Sarcocornia pillansii, through Sarcocornia cf
capensis to a dense Salicornia meyerana community.
Here Chenolea diffusa (soutbossie), Eragrostis sp and Spergularia
marginata have become established in the drier areas.
Landward of the new freeway and seaward
of the old rail bridge, similar Sarcocornia/Chenolea communities are
found. However, these communities show signs of disturbance,
possibly from grazing activities. Ehrharta delicatula, Bromus sp
(Parsons 334) and Plantago lanceolata (lamb's tongue) are
common. (Ref 1)
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Terrestrial
vegetation |
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Hummock
Dune Vegetation: The east facing dune to the south of the river
is well covered with Tetragonia decumbens (klappiesbrak)
and Cynodon dactylon (Bermuda quick grass) while Galenia
secunda (vanwyksbossie) and Chenolea diffusa are common.
The hummock dunes to the north of the
river have a sparse covering of T.decumbens and Senecio
elegans (strandblommetjies). Nearer the river, cover is poor
and Acacia cyclops (rooikrans) becomes dominant. (Ref 1)
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Zygophyllum
morgsana/Rhus crenata Dune Woodland: Although much of the
dunes to the north of the river are covered with A. cyclops,
remnants of the natural vegetation, in the form of Rhus
spp (kliptaaibos), Zygophyllum morgsana (leeubos), Ehrharta
villosa (pipe grass) and Pelargonium peltatum (kolsuring)
are to be found. (Ref 1) |
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Acacia
cyclops to Sideroxylon inerme/Rhus laevigata Dune
Woodland: The southern banks near the mouth of the river are
dominated by A.cyclops. However, natural trees and shrubs
such as Azima tetracantha (bee-sting bush), Sideroxylon
inerme (milkwood) and Rhus laevigata are
present.
Sporobolus viginicus (brakgras),
Tetragonia decumbens and Chenolea diffusa are found
toward the fringes with Galenia secunda and Penzia
pilulifera (beesbossie) in the more disturbed areas. (Ref 1)
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Semi-succulent
Riverine Scrub: The steep north-facing slope above the rail
bridge is covered with a relatively diverse riverine scrub.
Although true dominance is absent, common shrub species are Rhus
glauca (blinkblaartaaibos), Azima tetracantha, Diospiros
dichrophylla (bloubos) and Carissa bispinosa
(lemoenbessie). Grasses such as Eragrostis curvula
(blousoetgras), Ehrharta villosa, Phalaris arundinacea (reed
canary grass) and succulents such as Crassula spp, Aloe
spp and decumbent plants: Blepharis capensis (rankklits),
Senecio radicans (bobbejaantoontjies) are also found.
Although a number of alien plants such
as A. cyclops, Opuntia ficus-indica (prickly pear), Nerium
oleander, Lantana camara and Adromischus sp are present in
this area, their cover has not yet reached epidemic proportions.
(Ref 1)
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Grass
with A. cyclops intrusion: At the north-east, the dense Salicornia
meyerana salt marsh has become dominated by Ehrharta
erecta and Bromus sp, although salt marsh plants are still
present. A number of shrubs, Heteroptilis suffructicosum,
Limonium scabrum (sea lavender) and Drosanthemum sp are
also to be found.
Where A. cyclops dominates, Chironia
baccifera (aambeibossie), Solanum rigescens
(wildelemoentjie) and Gnidia squarosa are also present. |
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Acacia
karroo/Rhus laevigata Riverine Bush: This vegetation unit is
dominated by A. karroo and R. laevegata with Carissa
bispinosa, Aloe ferox, Bromus sp, Carpobrotus edulis
(Hottentots fig) and Maytenus heterophylla (common spike
thorn). This bush-type community is, however, endangered by the
enroachment of adjacent A. cyclops. (Ref 1) |
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Relhania
garnotii/Cynodon dactylon Heath: This community has a number of
elements similar to coastal renosterveld. It has, however, been
extensively disturbed in areas and is endangered by encroaching A.cyclops.
Although the vegetation around this
river is described as 'Valley Bushveld; by Acocks (1975), this veld
type, in a relatively undisturbed state, occurs only in the upper
reaches of the river while renosterveld and fynbos are also found in
some areas in the catchment. (Ref 1) |
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Fauna |
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Zooplankton |
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No
available information. (Ref 1) |
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Aquatic
Invertebrates |
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During
the ECRU survey 20 intertidal invertebrate species were recorded as
living on the rocks of the shoreline adjacent to the mouth of the
Hartenbos. Two species of mussel and a snail were found to be living
on the adjacent sandy beach. |
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As part of a pollution survey in July
1978, the meiofauna of the beach at Voorbaai, 3km to the south of the
mouth of the Hartenbos, was sampled. The populations were made
up chiefly of nematodes and hapacticoid copepods, with smaller numbers
of flatworms, polychaetes, oligochaetes, archiannelids, mystacocarids
and others (Eagle et al, 1979). After a follow-up survey in
November 1979, the numbers of the three dominant groups, nematodes,
harpacticoid copepods and flatworms, were found to be significantly
lower (Fricke et al, 1981). However this was attributed by
Fricke et al (1981) to be the result of seasonal differences in the
activity of the canning factory at Voorbaai. (Ref 1) |
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In the estuary itself, colonies of tube
worms Ficopomatus enigmatica were found on the suppports of the
pipe causeway during the ECRU survey. The amphipod Grandidierella
sp was found in association with these colonies.
Adult prawns appeared to be most
concentrated on the sand banks on the northern side of the estuary a
few hundred metres upstream of the mouth. They occurred on both
banks up to the pipe causeway. (Ref 1) |
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C. kraussi became sparse upstream
of the pipe causeway on the northern bank, but continued up to the
eroded northern drift on the northern tributary. On the southern
bank, C. kraussi concentrations were dense up to the new
national road bridge but sparse above that. The data indicate
extensive distribution of C. kraussi throughout the estuary,
particularly where suitable substrates occurred. The
distribution of juvenile burrows suggested that there was a breeding
population at the mouth (CM Gaigher, pers.comm.). (Ref 1) |
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The observation of a fresh moult of the
mud prawn Upogebia africana and shells of pencil bait Solen
capensis, suggested the occurrence of these bait organisms in the
estuary although no holes of the former were seen (CM Gaigher, pers.
comm. and ECRU survey). Sandprawns are collected for bait on the
northern bank near the mouth, at times (GJ Lamprecht, BP Kellerman,
and GL de Lange, pers. comm.).
An eight -minute "D" net haul
in the middle reaches of the estuary yielded 7 crown crabs, Hymenosoma
orbiculare and more that 100 sandshrimps Palaemon pacificus,
during the ECRU survey. (Ref 1) |
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Insects |
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Day
(1981) reports that a species of water beetle was collected at the
Hartenbos in May 1950. (Ref 1) |
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Fish |
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According
tp Messrs Lamprecht, Kellerman and De Lange, a 16 kg kob Argyrosomus
hololepidotus was caught in front of the Riviera hotel in earlier
days. They also mentioned that in the past, mullet mortalities
have occurred when the mouth has been closed and that Tilapia have
occurred in the lower estuary with freshwater conditions. Day
(1981) records the collection of the flathead mullet (Mugil cephalus)
from the Hartenbos in May 1950.
During the ECRU survey, fish were
sampled using a "D" net and gill net in the middle reaches
of the estuary and a scoop net in the shallower upper reaches. (Ref 1)
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All
twelve fish species recorded during the ECRU survey are considered by
Day (1981) as being common in estuaries. Of the twelve species,
two breed only in estuaries, two breed in both estuaries and the sea,
and seven are marine migrants using estuaries as nursery areas
only. The breeding biology of the outstanding species is unknown
(Day, 1981). (Ref 1) |
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Fish
were generally abundant in the estuary during the ECRU survey.
This may be attributed to the fact that there was good tidal exchange
at the time and that the mouth had been open for most of 1981.
It is notable that specimens of virtually all the species collected
were juvenile or immature, indicating recent recruitment to the
estuary. With closure of the mouth and the onset of salinities
of the order of 50 parts per thousan (as recorded in August 1980) few
of the recorded fish species would be able to survive. According
to the salinity ranges given by Whitfield, et al (1981), of the fish
species recorded in the Hartenbos, only R. holubi, A. hololepidotus,
M. cephalus and L. dumerili would be able to survive salinities of 50
parts per thousand. This further emphasizes the serious
ecological consequences of reduced run-off due to the Hartebeeskuil
Dam. (Ref 1) |
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Amphibians
and Reptiles |
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According
to Messrs Lamprecht, Kellerman and De Lange, puff adders Bitis
arietans have been seen on the banks of the estuary.
No specimens of reptiles or amphibians
were collected during the ECRU survey.
Two species of tortoise, eight snake
species and eight frog species have been recorded in the area.
Lizards have been excluded in these records as lizard taxonomy is
presently under review (AL de Villiers, pers. comm.). (Ref 1)
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Birds |
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Bird
counts for the Hartenbos amount to those in Underhill and Cooper
(1982, unpublished) done on 16 January 1981 and casual observations
made during the ECRU survey. As such, the latter was not a fully
representative bird count.
In January 1981 a total of 91
water-associated birds consisting of 16 species was counted. Of
these, 11 species were recognized waders making up a total of 73
birds. The waders consisted of 4 resident species (10 birds) and
7 migrant species (4 birds).
The Hartenbos does not seem to be a
particularly important estuary for waders. This is borne out by
the relatively low counts mentioned above. (Ref 1)
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Mammals |
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According
to Messrs Lamprecht, Kellerman and De Lange, Grysbok Raphicerus
melanotis and Bushbuck Tragelaphis scriptus are found in
the area. During the ECRU survey, water mongoose Atilax
paludinosus tracks were seen on the banks of the estuary.
Of the total of mammal species
recorded, 3 have been listed in South African Red Data Books as being
rare or threatened. These are the Honey badger and Cape greater
gerbil (Meester, 1976) and Leopard (Skinner et al, 1977).
Ref 1)
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