Diatom and geochemical indicators of acidification in a tropical forest stream, singapore 3

Chapter Three ACID DEPOSITION RESEARCH 3.1 Overview 32 3.2 Investigating Acid Deposition 32 3.3 Diatom Analysis 35 3.4 Geochemical Analysis – Total Sulphur Content 41 3.5 Geochemical Analysis – Other Analysis 48 3.6 Other Techniques 3.6.1 Spheroidal carbonaceous particles and polycyclic aromatic hydrocarbons 3.6.2 Biological indicators 3.6.3 Magnetics 52 52 3.7 Summary 56 53 55 31 3.1 Overview With the causes of and impacts from acid deposition examined in chapter two, chapter three looks at how the possible acidification of freshwater ecosystems is investigated. The chapter begins by looking at the difficulties faced by researchers investigating acid deposition on a freshwater ecosystem, namely that there is a lack of direct data monitoring the acidity of a water body and there is also a lack of suitable study sites. Researchers therefore use sedimentary records from suitable water bodies to trace potential acidification. The chapter then looks at the methodology employed in this study to examine the potential acidification of Jungle Falls stream – diatoms and geochemical analysis. The majority of paleolimnological studies into lake acidification involve the use of diatom analysis. These microscopic algae are often preserved well in freshwater ecosystems and, as they are highly sensitive to changing environmental conditions, they make an excellent proxy for environmental conditions within an ecosystem. Other techniques involve examining the geochemistry of the record, specifically the variation in sulphur, lead, zinc, potassium, sodium, iron and manganese levels within the sediment. This will help track the levels of atmospheric pollution and contamination going into a water body. Lastly, chapter three briefly elaborates on alternative paleolimnological methods used to study the acidification of freshwater ecosystems that are not employed in this study. These include the use of spheroidal carbonaceous particles (SCPs) and polycyclic aromatic hydrocarbons (PAHs), other biological evidence, and mineral magnetic analysis. 3.2 Investigating acid deposition Investigators studying acid deposition and its effects on freshwater ecosystems face two significant hurdles. Firstly, as acid deposition spreads over 32 a broad area and can cover entire regions, there are a lack of control lakes for comparison (Mitchell et al, 1985). In the tropical environments, this is compounded by another related issue – there are significantly less lakes than in temperate areas, as lakes of glacial origin are extremely rare in the tropics (Lewis, 1996). By studying available maps, Lewis (1996) estimates that no more than 10% of lakes worldwide are tropical, demonstrating the importance of glaciation in the formation of lakes at temperature latitudes. In tropical latitudes, most lakes have a riverine origin, with other lakes having volcanic, coastal, manmade or aeolain origins (Lewis, 1996). Another hurdle faced when studying acid deposition is that historical data monitoring the changes in water chemistry in a lake or river are often unavailable or imprecise (Pienitz et al, 2006). This is because many environmental changes and impacts are rarely foreseen and consequently are not monitored during the period of change from pristine to present day conditions (Renberg and Battarbee, 1990). Thus, baseline data of pre-acidification conditions are often nonexistent and yet are exceedingly vital (Mannion, 1999). In this absence of long-term monitoring, lake sediments offer one of the few reliable and effective ways of identifying the onset, rate and variation of environmental contamination in a freshwater ecosystem (Charles et al, 1987; Rose and Rippey, 2002). Unlike other environmental archives, such as documentary historical sources, the paleolimnological approach, involving studies based on the biological, chemical, and physical information preserved in lake sediments, often provides a record that is continuous, can cover both short and long timescales, and usually accumulates rapidly enough to provide a high resolution record (Renberg and Battarbee, 1990). This approach is effective in reconstructing past changes in water chemistry variables because biota and geochemical processes 33 respond in predictable ways to changes in lake water chemistry (Antoniades, 2007). Since acidification is a dynamic process, “effective management will require analyses of trends over different time scales, including estimates of preacidification conditions” (Smol, 2008: 92). Prior to intervention and management, scientists need to prove conclusively that a lake had been acidified through anthropogenic pollution and that it was not naturally acidic or acidified through natural processes (Antoniades, 2007). Thus, “without the historical perspective that paleolimnology can provide, many naturally acidic lakes may be unjustifiably limed, resulting in massive alterations to specialised ecosystems and food webs that have persisted for thousands of years in a naturally low pH state” (Smol, 2008: 105). During the 1980s, due to rising concerns about the effects of acid deposition on freshwater ecosystems, two major paleolimnological projects were started – the Surface Waters Acidification Programme (SWAP) in Europe and the Paleoecological Investigation of Recent Lake Acidification (PIRLA) project in North America. The SWAP project focussed on tracing the recent (post-1800) history of a number of carefully chosen lakes in Norway, Sweden and the UK in order to assess the causes of acidification rather than focussing on the evidence of acidification per se (Battarbee and Charles, 1987; Renberg and Battarbee, 1990). The PIRLA project looked into the history and effects of acid deposition, spatially and temporally, on lakes in eastern North America in order to determine the relative role anthropogenically induced atmospheric acid deposition played in causing recent acidification (Battarbee and Charles, 1987; Moser et al, 1996). One of the main focuses of these paleolimnology programmes was therefore to test alternative or additional causes for lake acidification (Renberg 34 and Battarbee, 1990). They have been instrumental in identifying the major cause of acid deposition as fossil fuel combustion (Mannion, 1992). The main methods employed in these paleolimnological investigations of acidification are biological analyses (diatom, pollen, scaled chrysophytes, caldoceran and chironomid analysis), geochemical analysis (such as sulphur concentrations and heavy metal concentrations), examining SCPs and PAHs along with dating techniques (Renberg and Battarbee, 1990; Mannion, 1992; Smol, 2008). Overall, the SWAP and PIRLA projects found that at individual sites, recent acidification always postdates the beginning of major industrialisation in the late 18th and early 19th century. Diatoms are often the first indicator to respond to atmospheric contamination and, when comparing diatom-inferred pH trends with regional patterns of sulphur deposition, “recently acidified sites are found in areas of high S deposition and no recently acidified sites have been reported from areas of very low S deposition“ (Renberg and Battarbee, 1990: 296). 3.3 Diatom Analysis Biological evidence, in particular diatoms, has been key in reconstructing past environments and is based on the principle of uniformitarianism, “namely that a knowledge of factors that influence the abundance and distribution of contemporary organisms enables inferences to be made about environmental controls on plant and animal populations in the past” (Lowe and Walker, 1997: 162). Three criteria should be considered for biological proxies to be useful for environmental reconstruction – “the material must withstand decomposition, exhibit sufficient morphological differences to be of taxonomic significance and provide sufficient quantities to reflect the nature of the entire assemblage from which it is derived” (Rovner, 1971: 343-4). Often fulfilling the three criteria above, diatoms have thus proved valuable in paleolimnological acidification research. 35 Diatoms are microscopic algae found in almost all aquatic environments (Battarbee et al, 2001) and are the dominant algal group in freshwater systems (Smol, 2008). They have a resistant siliceous outer shell and are thus a popular biological proxy in paleolimnological reconstructions (Mannion, 1982; Korhola, 2007). Diatoms have six characteristics that make them particularly useful: 1. Large number of species: There are thousands of diatom species (Smol, 2008). This makes their assemblages taxon-rich, increasing the ecological information obtained and strengthening the confidence of the environmental reconstructions (Korhola, 2007). 2. Easily identified: As diatoms have been comprehensively documented and classified, scientists are able to identify them down to species or even subspecies levels (Korhola, 2007). When well preserved, diatoms are also readily identified and counted (O’Hara, 2000). 3. Sensitive indicators: Diatoms cover a wide range of environmental conditions, yet, different taxa have different environmental optima and tolerances (Mannion, 1982). Since this optima and tolerance is usually well defined and narrow, diatoms are very responsive to changing environmental conditions (Moser et al, 1996). 4. Short lag time: Diatoms have short life cycles of approximately two weeks (Korhola, 2007). They also migrate rapidly and are able to colonise a habitat quickly (Smol, 2008). This means that they will respond to any changes in the environment swiftly; a contrast to pollen analysis whereupon vegetation may take as long as centuries to be in equilibrium with climate (Tibby and Haberle, 2007). 5. Good preservation rates: Because silica is resistant to degradation, diatoms are often well preserved in various sedimentary environments (Smol, 2008). 36 6. Reflects local changes: While pollen analysis will provide scientists with a regional picture of the environment, diatom analysis “generally relate to the lake being studied, providing a more detailed view of change on a local scale” (O’Hara, 2000: 135). Diatoms have been studied for approximately two centuries and began with a focus on systemic and taxonomic studies. This was later supplemented with ecological data concerning habitat and environmental conditions of specific species before their paleoecological significance was recognised in the 1920s (Mannion, 1982). Their size ranges from 2µm to 1-2mm and their shape varies from round (Centrales) to needle-like (Pennales) (Crosta and Koç, 2007). The distribution of diatoms is related to a number of variables such as temperature, turbulence, light availability, pH levels, nutrient availability and salinity (Jones, 2007). Diatoms are particularly good indicators of changing acidity levels in ecosystems and is the most widely employed technique to investigate the acidification history of a lake (Battarbee, 1984). This is because their distribution in freshwater habitats have been shown in numerous studies, conducted since the 1930s, to be strongly correlated to pH or to factors that co-vary with pH, like alkalinity and concentration of aluminium (Battarbee and Charles, 1987; Moser et al, 1996). While freshwater diatom assemblages are also influenced by other physical and chemical factors, in particular salinity and nutrient availability (Lowe and Walker, 1997), pH reconstructions have provided the most convincing results (Battarbee and Charles, 1987). Diatoms are also well preserved in acid conditions and have a high concentration in acid lake sediments (Battarbee, 1984). Thus, changes between the assemblage of old diatom samples and modern diatom samples can be used to examine whether acidification has occurred in an area (Battarbee, 1984). 37 The use of diatoms in paleolimnological investigations of lake acidification is most often based on Hustedt’s classical study on the diatom flora of Java, Bali and Sumatra, conducted in the late 1930s (Battarbee and Charles, 1987). He divided diatoms into five categories based on their individual pH preferences (Lowe and Walker, 1997): 1. Alkalibiontic diatoms: occur at pH values >7. 2. Alkaliphilous diatoms: occur at pH values of about 7 but with widest distributions at pH >7. 3. Indifferent (circumneutral) diatoms: occur equally above and below a pH of 7. 4. Acidophilous diatoms: occur at pH values of about 7 but with widest distributions at pH [...]... Plate 3- 1: Images and line drawings of various paleolimnological biological indicators (A) A fossil chrysophyte scale (from Smol, 2008); (B) A line drawing of a cladocera (from Güntzel et al, 2004); (C) A line drawing of a chironomid (from Dias et al, 2007) Cladocera are freshwater crustaceans whose skeletal fragments are often abundant in lake sediments (Lowe and Walker, 1997) These invertebrates are... Jones et al (19 93) also reported enhanced lead and zinc concentrations in lake sediments from four sites in Scotland, with the timing and nature of the changes consistent with acid deposition Charles et al (1987), Mannion (1992) and Rose and Rippey (2002) all note increases in zinc and lead concentrations in lake sediments relating to atmospheric contamination and lake acidification In a study of Lake Kholodnoye,... paleolimnological studies (Lowe and Walker, 1997) They are mainly used as paleotemperature indicators, but have been used as indicators of acidification as well Henrikson et al (1982) studied the effect of acidification on the chironomidae population of Lake Gårdsjön and Lake Härsevatten, finding that acidification results in a decrease in the total abundance of the population, along with a decrease in the... are mainly used to investigate the change in trophic status of a lake but can also be used to reconstruct acidification When examining lakes in Norway, Nilssen and Sandøy (1990) found that the changes in the caldoceran assemblage occurred simultaneously with pH changes, in agreeance with diatom data and other paleolimnological evidence Unfortunately, when pH changes in a lake are small, the changes in. .. corroborate any 48 findings In particular, lead, zinc, sodium, potassium, iron and manganese concentrations are often examined in geochemical investigations of paleolimnological acidification With regard to lead and zinc, in remote lakes that are not directly affected by anthropogenic activity, the most likely cause of contamination of these two trace metals is deposition from the atmosphere (Jones et al,... For instance, in a study of acidification in Lilla Öresjön, Sweden, along with diatoms and total sulphur concentrations in the sediment, lead and zinc levels were also measured It was found that the trace metal concentrations of lead and zinc began to increase after 1800, reaching their highest concentrations in the 1960s and 1970s, during the period of greatest inferred acidification (Renberg et al,... (Lower and Walker, 1997) These factors need to be taken into account in any environmental interpretations Ultimately, lake sediments contain a vast pool of information on the extent, rate, and causes of lake acidification Diatom analysis is likely to remain the most widely used and powerful paleolimnological technique for pH reconstruction The analysis of other biological parameters and geochemical analysis... late 1880s and peaking between 1950-1970 (Charles et al, 1987) While not a direct cause of lake acidification, as SCPs and PAHs are the products of fossil fuel combustion, they complement studies of anthropogenic lake acidification well 3. 6.2 Biological indicators Aside from diatoms analysis, other biological indicators which supplement data on lake acidification include chrysophytes, caldocerans and. .. followed by a steep rise in the last 3- 5 decades” 3. 7 Summary With a lack of historical records on pH levels in lakes, paleolimnological investigation has become a vital tool in investigating lake acidification histories Such a history is important as in order to make sound lake management policies, one should ideally have an understanding of the natural state of the lake under consideration, the variability... (Urban, 1994) Carbon accumulation rates in a lake also need to be constant with changing sulphur concentrations levels to eliminate eutrophication as a potential cause of the changing sulphur cycle in a lake (Urban, 1994) While there are reservations associated with sulphur analysis in paleolimnological acidification studies, there is value in studying sulphur accumulation in lake sediments and total ... it has found general acceptance and use among diatomists and can act as a guide to interpret diatom findings (Battarbee and Charles, 1987) The use of fossil diatom assemblages to infer lake acidity... in an area (Battarbee, 1984) 37 The use of diatoms in paleolimnological investigations of lake acidification is most often based on Hustedts classical study on the diatom flora of Java, Bali and. .. load and the natural background pH level of the lake In a review of diatom analysis and lake acidification, Battarbee (1984) found that a decrease in the diatom plankton component is often a