7 DISCUSSION OF RESULTS AND CONCLUSIONS
7.5 Discussion on Berm Formation
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The decreased scouring action of estuarine outflows, which prevent the deposition of marine sediments in the mouth of the estuary, promotes the formation of a berm across the estuarine mouth (cf. Literature Review Sub-section 3.1.2). Hence, restricting the interaction, and with that the transfer of nutrients, between the marine and freshwater systems could result in the decreased overall productivity of the estuary (cf. Literature Review Sub-sections 3.1.2 and 3.1.3). In the case study of the Klein estuary, the conversion of natural vegetation to agricultural land use, especially upstream irrigation, has been shown in simulations to promote berm formation (cf. Results Sub-section 6.2).
One of the main reasons for the projected increase in the frequency of berm formation is the decrease, resulting from upstream irrigation abstractions, in inflows entering the Klein estuary which, in turn, would result in decreased outflows from the estuarine mouth.
Decreases in outflows from the estuary mouth could be greater than decreases in inflows entering the estuary. This would be due to losses from the estuary itself, which would promote berm formation (cf. Literature Review Chapter 3; Methodology Sub-section 5.8).
In the case of the Klein estuary, irrigation is the major cause of decreased outflows (cf.
Results Sub-section 6.2) which, if coupled with marine deposition, could result in an increase in the frequency of berm formation across the estuary mouth (cf. Literature Review Chapter 3). During low rainfall and thus low flow years, when irrigation abstractions are high, it is likely that berms of significant size could form as a result of little or no scouring action during these periods. The implication of this would be that breaching events may not occur for a period of time longer than the normal interval. The resulting isolation of the Klein estuary from marine influences could result in the system turning hypo-saline (cf. Literature Review Chapter 3; Results Sub-section 6.2). However, as berms generally form as a consequence of decreased outflows, which result from decreased inflows and evaporative losses, it is far more likely that this estuary could turn hyper-saline during low flow years. In addition to the effects of freshwater inflows on salinity concentrations of estuary water, are the effects that this may have on the chemistry within the estuarine eco-system (cf. Literature Review Chapter 3). This is described below.
162 7.6 Discussion on Estuarine Chemistry
The water in estuaries is not simply diluted sea water, but consists of numerous chemicals and trace metals that aid in sustaining life, with only a small number of these being toxic (cf.
Literature Review Chapter 3). All chemicals in estuaries are obtained from either the oceanic or the river systems. Hence, in order to facilitate the proper functioning of estuaries, a connection with both the freshwater and marine systems must be maintained (cf. Literature Review Chapter 3). An increase in inflows entering an estuary, as is projected to occur along the eastern and southern coastal regions under future climatic conditions, could result in an excess in ions, such as calcium and sulphate, which are supplied via the river system (cf.
Literature Review Chapter 3). This would affect certain organisms such as molluscs and bacteria, which aid in the decomposition processes occurring in estuaries. In contrast, a reduction in certain chemicals entering via the marine system may occur as a result of increases in freshwater inflows, which would force a reduction in tidal influences and could affect some organisms that require certain levels of salinity for their survival (cf. Literature Review Chapter 3). However, the impact of a reduction in marine inputs may not be as significant as a reduction in freshwater inflows, as rivers carry the majority of nutrients and trace metals required for survival of estuarine ecosystems.
Conversely, a decrease in flows, as is projected to occur under future climates in the western coastal regions in systems such as the Berg and Olifants catchments, especially during future winter rainfall periods (cf. Results Sub-section 6.1.1), could result in a decrease in all chemicals and trace metals entering these estuaries. The overall decrease in chemicals could occur as a consequence of the formation of a berm across the mouth of the estuary, which would prevent the entry of marine based chemicals, such as magnesium (cf. Literature Review Chapter 3). Additionally, the formation of a berm across an estuary mouth may cause an accumulation of toxic trace metals, such as arsenic, lead and mercury which, in combination with hyper-saline conditions, could destroy most estuarine ecosystems if tolerance thresholds are exceeded.
Owing to the complete control that the abiotic factors exercise on organisms in estuaries, any changes in streamflows, as a consequence of climate change and/or anthropogenic
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interferences upstream of the estuary, could have a considerable effect on organisms in estuarine ecosystems. Increased inflows into estuaries would decrease salinity concentrations which could affect the number and distribution of organisms, as many primary producers are vulnerable to fluctuations in salinity, i.e. these organisms are incapable of the necessary osmoregulation, and so cannot control their internal equilibrium (cf. Literature Review Chapter 3). Hence these organisms are unable to tolerate wider salinity ranges, which may occur into future periods as a result of higher fluctuations in streamflows and marine inputs entering estuaries (cf. Literature Review Chapter 3; Results Sub-section 6.1.1). Therefore, if a marked increase in flows entering an estuary occurs, as is projected to occur in the Thukela estuary, then the number and diversity of organisms could decrease as a result of variations in the sediment yields, freshwater and marine inputs (cf. Literature Review Chapter 3; Results Sub-section 6.1.1).
Conversely, in systems along the west coast the projections indicate a decrease in streamflows into the distant future, e.g. in the Berg, Groen and Buffels catchments (cf.
Results Sub-section 6.1.1). A decrease in flows entering these estuaries could have a more detrimental effect than an increase in flows, as primary production would increase when turbidity levels decrease, which is due to decreased sediment yields. This results in an increase in photosynthesis and acceleration in the metabolic processes, thereby resulting in a higher rate of consumption of the available nutrients. Once all available nutrients have been consumed, then primary production would decrease very sharply and this would eventually affect all trophic levels within the system. Additionally, prior to the formation of a berm, the estuary would be subjected to greater tidal intrusions as a result of decreased freshwater inflows, which could then increase the salinity concentration considerably and may cause the dehydration of primary producers and their consumers (cf. Literature Review Chapter 3;
Results Sub-section 6.1.1).
Hence, in the event of droughts, berms could form as a consequence of decreased scouring action through decreased freshwater inflows. In some instances freshwater inflows may cease, as is projected to occur in the Klein estuary when irrigation routines are included (cf.
Results Sub-section 6.2). This would lead to the system becoming hyper-saline as a result of little or no marine interaction due to the berm across the mouth, and evaporation losses. In such systems very few organisms can survive as a result of salinity concentrations exceeding
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tolerance thresholds (cf. Literature Review Chapter 3). Similarly in such systems, mortality rates may be extremely high and the resulting decomposition of organisms could cause a reduction in dissolved oxygen, potentially turning the entire estuary anaerobic (cf. Literature Review Chapter 3).
7.7 Discussion on Estuarine Ecology and Economic Contributions of Estuarine