7 DISCUSSION OF RESULTS AND CONCLUSIONS
7.8 Overall Conclusions
From the results of the analyses undertaken (cf. Chapter 6) and the literature reviewed (cf.
Chapters 2 and 3), it may be concluded that the effects of climate change on estuarine ecosystems is location specific. However, the confidence in the projections of future climates varies considerably across South Africa (Schulze, 2011b). According to the literature reviewed in Sub-section 3.4.3, an increase in precipitation is projected to occur into the future over the eastern regions, while a decrease in precipitation is projected to occur over the western regions (cf. Results Chapter 6). Furthermore, non-uniform temperature increases are projected to occur over the entire country, with higher increases occurring over the interior of South Africa. In addition to the projected changes in climate, there are anthropogenic influences regarding the conversion of natural vegetation to agricultural land uses, which could affect downstream streamflows, especially if irrigation is practised upstream. When the joint changes in both climate and land cover conversion have been accounted for, the impacts of streamflow could, in some regions, be considerable, as summarised below.
From the comparisons of results from simulations in which natural vegetation is assumed, the following broad direct impacts of climate change on inflows into estuaries may be made:
• In the eastern regions the magnitude of streamflows entering estuaries is projected to increase into the intermediate and distant future. This increase, in some instances, may be considerable, e.g. in the Thukela system.
• In the southern regions high mean flows are projected to undergo a temporal shift into the summer period instead of occurring mainly in winter. Additionally, the magnitude of
166
flows entering some of these estuaries is projected to increase considerably in some catchments.
• In the western regions the magnitude of streamflows entering the Groen and Buffels estuaries is projected to increase marginally during the intermediate future. However, into the distant future flows are projected to decrease considerably. In addition, high mean flows are projected to undergo a temporal shift so as occur more into summer rather than in mid-winter.
In conjunction with the literature reviewed in Chapter 3, conclusions may be drawn regarding the possible effects of projected changes in inflows on estuarine ecosystems. Therefore, projected climate changes over South Africa may have the following indirect impacts on estuarine ecosystems:
• In the eastern regions, as a consequence of projected increases in freshwater inflows, estuaries could become hypo-saline and extremely turbid, thereby decreasing the overall productivity of these ecosystems. This is stated because the most productive systems are those containing both saline water and freshwater, as in a “normal” estuary, and implies considerable decreases in the number of organisms in these systems as a consequence of hypo-salinity.
• In the southern regions future projections show a more temporal rather than quantitative shift in flows entering estuaries, which could present a considerable problem for obligate species that rely on the opening of an estuary mouth at a specific time period in order to complete their life cycle stages.
• In the western regions the changes in inflows are small in most systems. However, the projected decreases in flow magnitudes in systems such as the Berg could have a negative effect on these estuaries, as nutrient inputs could decrease which, in turn, could reduce the overall productivity of these systems. Additionally, these systems could turn hyper-saline as a consequence of increased evaporation, in addition to decreased freshwater inflows and more frequent berm formation, which would prevent tidal intrusions. This would be extremely detrimental to many organisms found in estuaries, in regard to osmoregulation, nutrient and habitat availability.
Hence, if they are considerable, then the effects of climate change on estuarine ecosystems could be highly detrimental to the integrity of these systems. In addition to the effects of
167
climate change, there are those of land cover conversion which could affect streamflows entering estuaries. In this dissertation the Klein estuary was selected to demonstrate the possible effects of land cover conversion on streamflows.
Simulations were conducted utilising actual land use within the Klein catchment, both excluding and including effects of irrigation. The results of these simulations illustrate the effects of land cover conversion, along with those of climate change, on inflows into the Klein estuary and are summarised below:
• When results from simulations utilising actual land use (but excluding irrigation) are compared with those using natural vegetation during the present period, there is a slight increase in the magnitude of streamflows. However, when irrigation is included in these simulations there is a slight decrease in the magnitude of streamflows entering this system, especially during dry years when upstream irrigation abstractions may be high.
• Into the intermediate future the magnitude of streamflows is projected to increase, while into the distant future there is a considerable decrease projected in the magnitude of steamflows entering the Klein estuary.
• Sediment yields increase when results of simulations using agricultural land uses are compared with those assuming natural vegetation.
• The number of high flow pulses into the Klein estuary increases markedly when results of simulations using agricultural land uses are compared with those assuming natural vegetation. It should be re-iterated, however, that pulses reflect relative changes in flows from one day to the next, and not actual volumetric changes. The most marked projected increase in the number of pulses occurs when results of simulations using agricultural land use and including irrigation routines are compared with those from natural vegetation
• Results show a decrease in the number of breaching events when irrigation routines are included in simulations.
• A decrease in breaching frequency would affect nutrient availability, chemical constituents and mixing in this estuarine system.
In summary, the combination of possible effects of climate change with those of land cover conversion could have a more detrimental effect on this ecosystem than either climate or land
168
cover changes by themselves. This, in turn, could have a detrimental effect on people who rely directly on this ecosystem for their livelihoods, such as tour operators and fishermen.
Linking back to the introduction, a brief summary of the problem statement is that although recent hydrological studies have included ecological responses to hydrological shifts, there is still a need to increase the knowledge base with regard to climate change and estuarine ecosystems. This is due to a relatively limited number of comprehensive investigations of the effects of climate change on estuarine ecosystems, especially in a South African context.
From the problem statement in the introductory chapter, the objectives of this study were:
• to simulate the possible effects of climate change on freshwater inflows into 10 selected estuaries along the South African coastline,
• to illustrate the possible effects of land cover conversions in addition to those climate change on streamflows entering estuaries, and
• to show the broad possible effects of the above on the functioning of the estuarine ecosystems during the present and into the intermediate and distant future periods.
Therefore, it may be stated that this project has achieved its objectives, and has contributed to the knowledge of the possible effects of projected future climate and land cover changes on estuarine ecosystems in South Africa. However, there is scope for considerable additional research in this field, which is considered to be still in the developmental stages.
Recommendations for future research are discussed in Chapter 8.
169