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GROOT BRAK ESTUARY: Ecology
  Flora
Phytoplankton/Diatoms
No available information
Algae
Caulerpa has been reported  from the deeper water below  The Island bridge (Heydorn, 1979), possibly in association with Ruppia and Zostera. (Ref 2)
Aquatic Vegetation
Day (1981) reports that well developed Zostera capensis beds are to be found in this estuary.  Heydorn (1979) reports that Zostera and Ruppia beds were common in the deeper water between The Island and the new freeway, but these beds might be endangered by unconsolidated embankments.  During the ECRU survey (12 October 1981), only small patches of Zostera capensis were found in the deeper channels on the north-eastern side of  The Island bridge.  (Ref 2)
Semi-aquatic Vegetation
The semi-aquatic vegetation was divided into four main mapping units, namely:

  • Sarcocornia cf capensis/Sarconcornia pillansiae Salt Marsh:  The mud flats to the north-west of The Island bridge have a sparse covering of Sarcocornia cf capensis, S.pillansiae, Sporobolus virginicus (brakgras) and  Cotula coronopifolia (gansgras).

  • Cotula coronopifolia/Triglochin bulbosa Salt Marsh:  In the bend of the river, the tidally covered muddy banks have a sparse covering of Triglochin bulbosa (arrow grass) and Cotula coronopifolia with Sarcocornia cf capensis and Cynodon dactylon ( Bermuda quick grass).  On the western bank adjacent to the sawmill C. coronopifolia and C. dactylon are predominant.

  • Stenotaphrum secundatum/Sarcocornia pillansiae Dry Salt Marsh:  Relatively large areas are covered by  Stenotaphrum secudatum (buffalo grass) and Sarcocornia pillansiae with Disphyma crassifolia and Cotula coronopifolia.  The area south of the saw mill does, however, have a number of elements from the nearby Drosanthemum sp., (Parsons 406)/ Salicornia meyerana and Acacia dominated communities.

  • Phragmites australis/Stenotaphrum secundatum Reed Swamps:  This unit covers relatively small areas approximately 3 km inland. (Ref 2)
Terrestrial vegetation
Six main terrestrial vegetation mapping units were identified:                                                                                                                        
  • Hummock Dune Vegetation:  The dunes to the east and west of the river mouth have similar vegetation consisting of Ammophila arenaria (marram grass), Arctotheca populifolia (sea pumpkin) and Passerina paludosa (gonna), while Trachyandra divaricata and Galenia secunda (vanwyksbossie) are found to the east.  The rocky outcrops on the western banks of the mouth have a different vegetation with Myrica cordifolia (wax berry), Carpobrotus edulis (Hottentots fig) and Limonium scabrum (sea lavender).  However, large areas of the older dunes are dominated by Acacia cyclops (rooikrans).

 

  • Sideroxylon inerme/Rhus lucida Dune Forest:  Apart from the Acacia cyclops intrusions, there are well developed and aesthetically appealing dune forests on the western and eastern banks of the river.  These forests include Sideroxylon inerme (milkwood), Rhus lucida (wild currant), Carissa bispinosa (lemoenbessie) and Zygophyllum morgsana (leeubos).  On the werstern side of the river, the dune forest seems to be more stunted and includes Capparis citrifolia (wild lemon) and Rhoicissus digitata (bobbedruif).  Towards the top of the eastern dune, the vegetation becomes sparse and is dominated by Sutherlandia frutescens (eendjiesblom) and Ammophila arenaria with Chrysanthemoides monolifera (bietou) and Carpobrotus edulis.  The removal of vegetation from these dunes might result in severe sand dune slumping such as can be noted near the cottages at the northern end of the western dunes.

 

  • Drosanthemun sp./Salicornia meyerana Littoral Scrub:  As Salicornia meyerana is common in this vegetation type, it might be regarded as being semi-aquatic.  However, many terrestrial elements are present, such as Rhus lucida, Lycium afrum (kraaldoring), Drosanthemum sp, Pseudognaphalium undulatum and Elytropappus rhinocerotis (renosterbos).  In areas near the new freeway where water is allowed to accummulate, Typha capensis (bulrush), Cyperaceae and Cotula coronopifolia are found.

 

  • Heath and Shrublands:  These vegetation units have a number of species in common and no single species attains true dominance.  The vegetation unit described as 'heathland' is found on the north-west facing sandstone-clay conglomerate and contains Erica sp., Phylica purpurea, Eriocephalus cf africanus (kapokbossie), Bobartia orientalis Subsp. orientalis (geelblombiesie), Aloe ferox (ware alwyn) and a number of grasses and restrioids e.g. Hyparrhenia hirta (blougras), Ficinia cf gracilis and Restio cf fruticosus (kanett).   The shrubland is found on the south-facing gneissic granite slope and contains Helichrysum sp., Erica glumaeflora, E. imbricata (raasheath), numerous restioids (Restio cf patens, Thamnochortus cf pluristachys) and herbaceous plants (Hypoxis cf villosa (inkbol) Satyrium sp. Ornithogalum dubium (chincherinchee)).  Hakea sericea is a common alien in both these communities and the encroachment of pine trees from adjacent plantations should also be controlled.

 

  • Maytenus heterophylla/Diospyros dichrophylla Woodland:  In this vegetation unit, the canopy species, such as Maytenus heterophylla (common spike thorn), Diospyros dichrophylla (poison peach), Commelina africana and Rhus lucida, are relatively sparse while most of the ground cover consisits of shrubs (Passerina rubra, Elytropappus rhinocerotis), herbs (Crassula subulata, Stachys gracilifolia, Sebaea aurea) and grasses (Eustachys paspaloides, Eragrostis curvula, Themeda triandra), while ferns are not uncommon.  If the adjacent pine plantations are not economically viable, a gradual and controlled removal of these aliens could result in an interesting transgresson between this community and the nearby heath- and shrublands.

 

  • Acacia Dominated Communities:  Acacia cyclops (rooikrans) and A. mearnsii (black wattle) successfully outshade competing plants, thereby decreasing ground cover.  This can be seen clearly immediately below the village bridge where they compete with Acacia karroo and on the eastern bank of the river competing with Lysium afrum, Rhus glauca and Salicornia meyerana.  However, some 'weed' species (Oxalis pes-caprae, Briza minor, Sonchus spp.) may form a sparse undergrowth.  To the south of the saw mill A.cyclops and A.mearnsii dominate in the salt marsh and Salicornia/Drosanthemum communities.  As the soil binding ability of the natural vegetation is creased by the presence of these invasive species, these areas may be severely eroded during times of flood.  (Ref 2) 
Fauna
Zooplankton
Grindley sampled zooplankton at seven stations at night on 29 January 1969 in the estuary from the marsh to above the bridge at Groot Brak Village.  23 species were recorded with a mean zooplankton biomass of 14,09 mg/m3 (Unpublished information).  (Ref 1)
Meiofauna

Eagle et al. (1979) collected meiofaunal samples from Mossel Bay and noted that the populations consisted chiefly of nematodes and harpacticoid copepods with smaller numbers of flatworms, polychaetes, oligochaetes, archiannelids, mystacocarids and other organisms.  The relative numbers of these organisms and the depths at which they occur in the substrate are used as indicators of the degree of pollution of the aquatic environment.  They concluded that the beaches and rivers around Mossel Bay are largely unpolluted although point sources of pollution do exist.  (Ref 1)
Invertebrates
Day (1981) states that the aquatic fauna is rich.  Prior to the ECRU survey of November 1981 the only studies carried out were those by a University of Cape Town team in May 1950 (UCT Zoology Department field notes) and a survey following the pollution of the coast as a result of the Venpet/Venoil oil tanker collision (Moldan et al., 1979).  The UCT team noted that the san prawn, Callianassa, and the mud prawn, Upogebia, were abundant, the bivalve, Loripes clausus, was "suprisingly common" and at the head of the estuary the fresh water shrimp, Palaemon capensis, was recorded in an estuary for the first time.  When oil from the Venpet/Venoil disaster entered the estuary most of the crabs and prawns were smothered in their burrows by "chocolate mousse" (emuslified crude oil) (Moldan et al., 1979;Day, 1981).  (Ref 1)
Fish
The Spot damsel fish Abudefduf sordidus was found by the UCT survey team in 1950.  This is much further south than its normal range (Day, 1981).  (Ref 1)
Amphibians and Reptiles
No attempt to collect amphibians and reptiles was made during the ECRU survey.  However, eight species of frog, two of tortoise and eight of snake have been recorded from the area covered (AL de Villiers, Dept Nature and Environmental conservation in litt.)    (Ref 1)
Birds
The Great Brak estuary does not appear to be particularly important for water birds; few ducks, cormorants, gulls and migrant Palaearctic waders occur there.  Most notable are the 40 Ringed Plovers Charadrius hiaticula recorded by Summers et al.  (Ref 1)
Mammals
12 Species of mammals likely to occur in the area were listed.  The larger predators such as leopard and the honey badger are probably confined to the upper catchment areas and are unlikely to occur in the vicinity of the estuary itself.  (Ref 1)

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