Uilkraals Situation Assessment - Anchor Environmental

iophysical variables. The current state is then scored for each of these variables on a scale of0 (no resemblance to original state) to 100 (same as natural state). The health scores andoverall score are summarised in Table 6. Although the estuary currently receives some 80% ofits natural MAR, an important part of the hydrology and natural functioning of the estuary hasbeen removed (winter and summer base flows) which affects the mouth conditionsignificantly. The similarity score given for the hydrology of system is 50% of the naturalcondition, which is lower than the percentage in natural MAR (80% of natural). This is becausean important component of the natural flow regime has been modified to a large extent, thehydrodynamics and mouth condition (0% of natural condition) are severely altered. Thereduction in flow has also had an impact on the water quality of the system, both due to thereduced ability to dilute pollution and due to the increase in polluted return flows as a result ofwater use for irrigation. The reduced flows may have also altered the physical habitat of theestuary in that the depth and profile have changed. There has been a recorded 81%transformation in the 1 km buffer zone of the estuary (van Niekerk, unpub. data), most likely aconsequence of increased alien vegetation and reduced flows.The reduction in flows has also resulted in considerable changes to the biota of the estuary.Primary productivity by microalgae is thought to have increased due to the nutrient input andreduction in flushing of the estuary. Being a blackwater system, the Uilkraals is naturallyoligotrophic and because the water is being retained in the estuary for an extended period oftime, the primary productivity has increased substantially. Plants have also been significantlyaffected. Mouth closure for such extended periods can lead to a significant reduction insaltmarsh vegetation. Saltmarsh cannot survive inundation which is caused by the permanentrise of the water level in the estuary due to a closed mouth. A reduced cueing effect toestuarine dependent invertebrate and fish species could result in a reduction in nurseryfunction, abundance and diversity of species. Birds have also been significantly affected by theclosure of the estuary. The large intertidal feeding habitat which covered the entire sandflatregion below and the above the causeway has been lost, resulting in a severe decrease inwader numbers. The estuary has also become less suitable as a tern roost. The score of 80%allocated to birds is most likely an over estimate and has become even lower as the estuarycontinues to remain closed off from the sea.Table 6. The Estuarine Health Index scores allocated to the Uilkraals Estuary (Present State)VARIABLESCORE (% resemblance to natural condition)Hydrology 50Hydrodynamics and mouth condition 0Water quality 70Salinity 50Total Water Quality Score 62Physical habitat 70Habitat health score 45.5Microalgae 35Plants (macrophytes) 70Invertebrates 70Fish 75Birds 80Biological health score 64OVERALL EHI SCORE 55Uilkraals Estuary Situation Assessment32Anchor Environmental