Diatom and geochemical indicators of acidification in a tropical forest stream, singapore 7

Chapter Seven CONCLUSION 7.1 Overview 125 7.2 Summary of Study 125 7.3 Contribution of Study to Wider Acidification Research 127 7.4 Extensions of Research 129 124 7.1 Overview Chapter seven provides a summary of this study and begins by discussing its primary aim, which was to look at the potential acidification of Jungle Falls stream. Following this, an evaluation of the use of paleolimnological indicators for acidification research in Asia is provided, which is the secondary aim of this thesis. As this study is a first attempt at paleolimnological research on acidification in Singapore and possibly the region, the chapter concludes by looking at several extensions that would either expand or supplement the data obtained. 7.2 Summary of Study The primary aim of this study was to investigate the potential acidification of Jungle Falls stream in BTNR. This reserve has much ecological value and faces concerns of acidification, particularly being located in a region where atmospheric pollution is increasing rapidly. While the Singapore government has been successful in controlling domestic pollution, pollution levels, particularly SO2, are increasing in recent years. It is therefore vital to examine whether acidification is currently affecting streams within the nature reserve to aid management of it. While there is a lack of suitable study sites in Singapore, the damming of Jungle Falls stream provides an excellent location for paleolimnological studies. It is believed that this dam was constructed during late 1930s. In this study, diatom along with geochemical analysis was adopted to investigate the potential acidification of Jungle Falls stream. Results indicate that the Jungle Falls stream sediments contain a record of atmospheric contamination into the area. As there were no abrupt changes within the data, such as sharp drops in %LOI or trace metal concentrations, this 125 sedimentary record is likely to be continuous, with no hiatus in deposition or significant erosion occurring. The stream was dammed at a time when atmospheric pollution levels were also expected to being increasing and hence, there is a need to separate one signal (changing hydrological conditions due to the damming of the stream) from another (atmospheric contamination). At the base of the record, trace metal concentrations are low. The diatom assemblage at this depth points to an acidic environment, but with a higher abundance of taxa present in less acidic waters (Eunotia vanheurckii, Eunotia curvata and Fragilaria bicapitata), and a lower abundance of taxa present in more acidic waters (Eunotia incisa and Eunotia flexuosa). Trace metal concentration begin increasing up-core and peak at 15cm. This could be due to changing catchment conditions caused by the impoundment compounded by a rise in atmospheric pollution. This peak at 15cm likely corresponds to the mid to late 1960s when Singapore was undergoing rapid industrialisation and pollution controls were not yet in place, resulting in a high level of atmospheric pollution and contamination. This interpretation is complimented by the fact that the diatom assemblage also changes at this depth, with levels of Eunotia vanheurckii, Eunotia curvata and Fragilaria bicapitata dropping and levels of Eunotia incisa and Eunotia flexuosa rising. This implies that the stream is becoming more acidic. During the 1970s, with the implementation of the Clean Air Act and effective management of industrialisation and urbanisation, pollution levels, and thus, atmospheric contamination into the stream, began to drop. This is in line with the sedimentary record whereby, moving up past 15cm, lead and zinc concentrations once again begin to decrease even as sodium and potassium levels appear to stabilise. 126 At the top of the sedimentary record, while lead levels remain low, zinc and sulphur levels appear to rise once again. This increase in sulphur concentration can be linked to an increase in sulphur dioxide levels in Singapore (figure 4-2) and an increase in acidity measured in Singapore (figure 6-9). Based on comparing sulphur concentration levels within the core to acidity levels measured in Singapore (figure 6-9), there is a possibility that sedimentation within the impoundment may have ceased by the mid-1990s due to the dam reaching full capacity. This increase in atmospheric contamination within the core, implying an increase in atmospheric pollution to the catchment, may be due to transboundary pollution. While the diatom assemblage does not show a similar change as the trace metal concentrations at the top of the core, this could be because the freshwater ecosystem is currently not degraded by atmospheric pollution into the catchment. Ultimately, there does appear to be a viable record of anthropogenic acidification of Jungle Falls stream contained within the sedimentary record. 7.3 Contribution of Study to Wider Acidification Research The secondary aim of this study was to evaluate the potential of paleolimnological indicators in anthropogenic acidification studies within the region and in tropical environments in general. Recall from chapter two that in the late 1990s, following the completion of large-scale acidification research projects in Europe and North America, along with a focus by developing countries on economic development at the expense of environmental health, acidification studies dropped sharply. However, the problem of acid deposition in Asia persists. As the majority of paleolimnological acidification studies were conducted in temperate countries, where environmental processes differ, it is vital to 127 investigate whether the techniques employed in Europe and North America are transferrable to a tropical environment. This is the first time diatom and geochemical analysis has been conducted in a freshwater acidification study in Singapore, and literature on such paleolimnological acidification studies in Asia, particularly SEA, are exceedingly rare. Thus, the methodologies employed in this study had to be devised from scratch based on a literature review of similar studies conducted in the temperate countries. As such, an evaluation of the effectiveness of the selected techniques employed is necessary. While diatom preservation would be an issue in tropical environments, particularly with high temperatures, results obtained from this study are exciting. A change in diatom assemblage can be seen moving from pre-anthropogenic acidification conditions to modern day. This is in spite of the fact that air pollution in Singapore has been carefully controlled and thus, Jungle Falls stream would not be as heavily impacted by acidification as the freshwater ecosystems seen in temperate studies. Geochemical analysis has proved similarly useful in tracking the acidification of freshwater ecosystems in the tropics - lead, zinc and sulphur concentrations change with varying atmospheric contamination into the catchment. The fact that lead levels remain low at the top of the core, even as zinc and sulphur levels are increasing, demonstrates the accuracy of this signal. This is because measurements of atmospheric pollution levels in Singapore show that while lead pollution has decreased from 1981 to present (figure 4-3), sulphur dioxide levels have increased from the late 1980s (figure 4-2). It is crucial to measure other trace metals in a geochemical study to better interpret down-core variations. Thus, for this study, because the series of dams built at Jungle Falls stream would affect sediment supply levels into the stream, 128 sodium and potassium levels were measured. Iron and manganese concentrations were also determined to check if redox driven cycles affected trace metal concentrations. It can be seen that changes in lead, zinc and sulphur appear to vary above that of background environmental levels, a sign of the anthropogenic pollution into the catchment. The diatom and geochemical data complement well and reinforce each other, emphasising the value of using multiple proxies in paleolimnological analyses. 7.4 Extensions of Research This study has provided a good foundation for further acidification research in Singapore and the region. It has shown the value of paleolimnological indicators in tracking acidification, and this approach should be pursued further. Firstly, more coring sites need to be found. An issue with the Jungle Falls coring site is that the atmospheric contamination signal is intertwined with changing hydrological processes due to the damming of the stream. More coring sites would aid in verifying the Jungle Falls results, along with increasing the knowledge about the extent of anthropogenic acidification in Singapore. The use of caesium-137 to analyse the Jungle Falls core will also provide further validation of the data interpretation in this study. Secondly, a modern diatom dataset for Singapore and the region is necessary. Most diatom studies conducted in Asia still refer to diatom keys from Europe and North America. As mentioned in section 6.6.1, diatoms would have different ecological parameters in different environments and in order to accurately interpret the assemblages, a regional diatom database is paramount. This would also aid in the development of a transfer function for the area, “a mathematical formula that estimates environmental variables from diatom species’ composition data” (Moser et al, 1996: 28), enabling quantitative analysis which provides values for changes in environmental variables such as pH. 129 Thirdly, concentrations of SCPs and PAHs could be analysed within the Jungle Falls sediment core. As these particles are produced due to industrial processes, it would give a clear indication of whether anthropogenic pollution is being recorded within the sediment core. Lastly, the use of sulphur isotope analysis in the Jungle Falls sediment core would also provide more data on potential anthopogenic influences within the core. By looking at variations in the 34 S and 32 S stable isotopes of sulphur within a core, decreases in 34 S concentrations accompany anthropogenic acidification, providing independent information on the impact of anthropogenic pollution on a freshwater ecosystem (Fry, 1990). 130  ... and Fragilaria bicapitata), and a lower abundance of taxa present in more acidic waters (Eunotia incisa and Eunotia flexuosa) Trace metal concentration begin increasing up-core and peak at 15cm... the diatom assemblage also changes at this depth, with levels of Eunotia vanheurckii, Eunotia curvata and Fragilaria bicapitata dropping and levels of Eunotia incisa and Eunotia flexuosa rising... time diatom and geochemical analysis has been conducted in a freshwater acidification study in Singapore, and literature on such paleolimnological acidification studies in Asia, particularly SEA,