Grindley and Snow (1983) give values ranging from 7,6 (at the sewage outfall) to 8,9. Le Roi Le Riche and Hey (1947) give the pH of the river as 5. (Ref 1)
Water temperatures with the estuary demonstrate a strong seasonal pattern with maximum temperatures (up to 29°C) recorded during summer and minimum values during winter (12°C) (Grindley, 1985). The temperature regime within the estuary may, however, be modified by the intrusion of colder marine water derived from coastal upwelling into the system. Salinities within the estuary range from freshwater to hypersalinity (> 35 practical salinity units) within the upper reaches of the estuary during periods of drought (Grindley, 1985). A distinct horizontal pattern in salinity is evident with marine waters dominating in the lower and middle reaches of the estuary and freshwater in the upper reaches. (Ref 5)
Salinities decrease gradually from the mouth (approximately 35,3 parts per thousand) falling below 30 parts per thousand near Westford Bridge and to 0 parts per thousand above Charlesford Rapids. Table of values presented by Day indicate considerable seasonal variation. (Ref 1)
In the Knysna River 6 parts per million was recorded. (Ref 1)
Overall, the Knysna Estuary can be considered as a nutrient poor (oligotrophic) system largely due to the large volumes of nutrient poor marine waters that pass through the heads twice daily (Allanson et al., 2000). Localised regions of high nutrient impute are derived from storm water and sewage plant inflows although the contribution of these sources to the total nutrient budget within the system is generally considered low (Allanson et al., 2000). It is worth noting that inflow of freshwater from the Knysna River contributes to increased nutrient loads within the system. The observed pattern appears to relate to agricultural activities within the catchment area, which contribute to increased nutrient loads within the Knysna River (Allanson et al., 2000). Associated with the oligotrophic status of the system is high water clarity. However, the water clarity does demonstrate a strong spatial pattern with the highest water clarity values recorded within the embayment and lowest, in the upper reaches of the estuary (Grindley, 1985). The observed pattern can be ascribed to the hydrology of the estuary. Furthermore, a distinct seasonal pattern in water clarity has been observed with lowest water clarity recorded during the rainy season in summer. The observed pattern can be related to river and stream inflows which are high in total suspended solids (Allanson et al., 2000a). (Ref 5)
Model tests on sewage pollution levels in Knysna Lagoon based on introducing a fluorescein solution into the NR10 model at the sewage outlet position have been carried out. Samples were taken at various positions in the model and the concetration of pollutant determined. Grindley and Eagle (1978) reported on the environmetnal effects of th discharge of sewage effluent into Knysna Estuary. While a further study concluded that the position of the sewage outfall is unfortunat, in that it opens into blind ending arm of the estuary, in the midst of a series of public recreation facilities. It was noted that while the effects of the present discharge of effluent on water chemistry, substratum, flora and fauna in the area around the outfall did not appear to be serious, increased discharges would have greater effects. The high recorded coliform and in particular Escherichia coli counts obtained in this area. However, Ninham Shand and Partners Inc. have stated that the chlorination unit will "ensure a zero E. coli discharge." The aesthetic implications of this discharge are nonetheless questionable.
It would seem that the limited tidal exchange in the area east of Thesen's Causeway causes stagnation in that area and that the effects of pollution are worse there that near the sewage outfall where there is a better tidal circulation. A number of effects in the area east of Thesen's Causeway gave cause for concern. The fluctuations in dissolved oxygen are evidence of high organic production. The high percentage of sub-sieve particles and the high total Kjehldahl nitrogen values indicate that this area is accumulating fine sediments with a high organic content in contrast to the unaffected area west of the causeway. The abundance of Ulva and Enteromorpha in this area is apparently impoverished at the lowest intertidal level as the biomass is lowest here at the level where it is normally highest. It would seem that the limited tidal exchange in this area causes the effects of pollution to be worse here than near the sewage outfall. Studies of tidal movement have shown that the exchange of water in the channel between Thesen's Island and the mainland is poor. Studies using a hydraulic model showed that the situation could be improved b opeing a section of Thesen's Causeway. The removal of this section of the causeway caused a 98 percent reduction in pollution levels with spring-tides and a 71 percent improvement on neap-tides. A further study carried out by Grindleyy and Snow again stressed that the limited tidal exchange in the area of Thesen's Causeway caused the effects of pollution to be worse there than near the sewage outfall. It is hoped that the opening of the causeway will alleviate this problem. The value of the reed marsh below the sewage works as a nutrient uptake mechanism must not be neglected and probably deserves further study. (Ref 1)
A number of studies have been made of ways and means to prevent oil pollution of the Knysna Estuary in the event of an oil spill at sea. The circumstances under which oil could enter through the Knysna Heads if no preventive measures were taken, along with various means of solving the problem, are reviewed by Retief et al.(1979). Attention is focussed on the possible use of oil barriers (booms) and their probable effectiveness at various locations is evaluated.
The conclusion drawn are that the probability of oil entering Knysna Lagoon from a spill at sea is remote. If oil did enter between The Heads, commercially available oil booms would only be effective in keeping oil out at that point during favourable conditions such as neap-tides. More promising solutions could, however, probably be achieved within the estuary.
Following the collision between the super-tankers Venpet and Venoil south of Knysna in December 1977, oil pollution in the form of tar-balls did enter the lagoon in small quantities. The sinking of a fishing vessel at the Thesen's Island wharf caused local oil pollution in 1983. The widespread use of outboard engines on Knysna Lagoon is responsible for some pollution but no data are available. (Ref 1)
Trace-metal concentrations are low. Knysna Estuary is one of the most, if not the most, biologically productive estuaries in South Africa, and as such is listed as an area of primary importance in the National Marine Pollution Monitoring Programme.
There are, however, indications that transitory anomalies do occur and represent point sources of input. Of particular interest in this respect is the 1976 mercury anomaly near The Point. In addition certain of the town drains are responsible for the input of zinc, copper, nickel, cobalt and mercury. The anomalous areas associated with these drains are small and concequently their ecological impact is insignificant.
Analysis of cores taken during the 1976 and 1978 surveys indicate the possibility of transitory anomalies forming along the Leisure Island shore. Four sites of metal accumulation of a more permanent nature have been identified from the results of the 1978 survey. These are Thesen's Island Point, Thesen's Island Jetty, the Rail Bridge and the National Road Bridge. However, concentrations at these sites are only slightly elevated above background levels and do not represent significant metal inputs.
Comparison of the trace-metal concentrations in three species of oysters grown at Knysna with those reported for many other locations, immediately showed that there is no major trace-metal source in the estuary. It is particularly fortunate that so many other species of molluscs also grow in the estuary because we can assume that trace-metal concentrations in these will also represent background levels. It will therefore be possible to compare these results with those obtained for the same species from other estuaries, many of which do not have naturally occurring oyster populations.
Metal concentrations in Crassostrea gigas and Ostrea edulis from Knysna Estuary are much lower than many of the reported values for these species; this indicates that Knysna Estuary is unpolluted with respect to zinc, cadmium, copper, iron, manganese and nickel. Metal concentrations in other molluscs grwoing in or near the Knysna Estuary are generally low and it is assumed that these values represent near-natural levels for indigenous species.
The fact that the estuary is relatively unpolluted makes Knysna an excellent marine-pollution monitoring station for the southern coast of South Africa. (Ref 1)