Diatom and geochemical indicators of acidification in a tropical forest stream, singapore 2

There is an emphasis on the impacts of acid deposition on freshwater ecosystems as it is directly relevant to this study of the acidification of a tropical forest stream.. Table 2-1: Exa

2.1 Overview

Chapter two focuses on acid deposition It begins by explaining what acid deposition is and provides a brief history of the phenomenon It then goes on to examine why acid deposition is such a pertinent issue by looking at its causes and impacts There is an emphasis on the impacts of acid deposition on freshwater ecosystems as it is directly relevant to this study of the acidification of

a tropical forest stream As the problem of acid deposition was first observed in North America and Europe, much of this initial research is based in these regions during the 1970s to 1990s

Following the 1980s and 90s, with effective pollution control legislation, acid deposition was alleviated in Europe and North America However, researchers realised that Asia, a region that was beginning to industrialise rapidly, was in danger of the same acidification problems North America and Europe once faced This lead to a boom in research on acidification in Asia during the 1990s

Unfortunately, by the end of the 1990s, the amount of research on acid deposition in Asia declined sharply However, this is not because acid deposition

is no longer occurring in the region Rather, as the effects of acid deposition have not yet manifested themselves and caused widespread environmental degradation, it is possible that countries are choosing to focus on economic development at the price of environmental health This is compounded by the fact that the large-scale research projects in Europe and North America were also completed by the early 1990s, and studies on acidification in general declined As atmospheric contaminants can be transported over long distances before being deposited, atmospheric pollution and acidification in Asia is a cause for concern

to the Singapore environment

2.2 Acid deposition

The phenomenon of acid precipitation was first observed in the early 1850s by Robert Angus Smith while he was studying the effect of coal combustion on air and rain in England (Gorham, 1989; Mannion, 1999) He coined the term “acid rain” to describe precipitation that had unusually low pH due to atmospheric pollution While the term “acid rain” is widely used in both scientific literature and the popular press, it is somewhat misleading as this pollution also occurs as snow, hail, gas clouds, fog, mist and dry dust (Bell and Walker, 1992) Furthermore, uncontaminated rain already has a pH below 7 and

is, therefore, acidic

“Pure” water generally has a pH of between 5.6 and 5.7 because water molecules combine with carbon dioxide present in the atmosphere to form carbonic acid (Boyle and Boyle, 1983) In addition, sulphur compounds from volcanic eruptions and nitrogen oxides from events such as lightning strikes are converted to sulphuric acid and nitric acid in the atmosphere, which then contributes to the acidity of precipitation (Jones, 2000; Galloway, 1995) Thus, this form of atmospheric pollution is more accurately referred to as “acid deposition” (Bell and Walker, 1992)

Unfortunately, while Smith recognized the environmental damage caused

by acid rain, his research was largely forgotten until a century later when the continental scale of acid rain effects was discovered (Schindler, 1988) Depending on prevailing wind directions, acid deposition can be carried hundreds

of kilometres from the source area (Mannion, 1992) Acidification is so extensive that its presence at the hemispheric scale can be found in polar ice core records These show an increase in N2O from the 1800s with current high levels

unprecedented for the last 200,000 years (Raynaud et al, 1993) Furthermore, in

the Northern Hemisphere, many lakes have become acidified by at least one pH unit (table 2-1, Mannion, 1999)

Table 2-1: Examples of changes in pH of lake waters that have occurred since 1840 in the UK, Europe and North America (data from Mannion, 1999)

Thus, during the late 1970s and early 1980s, acidification gained widespread public attention due to environmental concerns – studies were emerging that linked the acidification of lakes due to industrial pollution to declining fish population (Antoniades, 2007) Acidification became an a source of hostility between acid-producing and acid-receiving nations (Mannion, 1992) Table 2-2 shows the annual sulphur dioxide (SO2) emissions and deposition during the 1980s in some countries within Europe, Russia and North America The major producers of SO2 were Russia and the United States of America (USA), both producing over 20x106 tonnes annually Of the sulphur deposition within Russia, at least 50% was produced domestically and 32% from foreign sources On the other hand, Sweden and Norway only produced 483x103 and 141x103 tonnes of SO2 respectively, and foreign sources account for at least 58% and 63% of sulphur deposition respectively, twice that of Russia In a similar vein,

the acid-producing nations are not always the ones that suffer the greatest damage (Mannion, 1992)

Table 2-2: Annual SO 2 emissions and deposition in selected countries (data from Mannion, 1992)

This made acidic pollution and acid deposition an issue of huge economic and international political importance, studied by thousands of scientists utilising many hundreds of millions of dollars of research funds (Schindler, 1988) Boyle and Boyle (1983: 12) called it “the single most important threat to the United States and Canada”

In 1979, a convention on Long-range Transboundary Air Pollution was signed in Geneva by 35 countries While it was merely a token treaty, it provided important recognition among industralised nations that tackling the problem of acidification required collective action (Mannion, 1992) In 1983, the “30% Club” was established and implemented a protocol which required signatory countries

to reduce sulphurous emissions by at least 30% of 1980s levels by 1993 The United Kingdom (UK) and the USA did not sign this protocol In 1989, a second protocol, which included the UK and the USA, was signed that included an agreement to also reduce the emissions of nitrogen gases The result was a significant reduction in the output of SO2 and nitrogen oxides (NOx) from most industralised nations (Mannion, 1992)

Sulphur dioxide emissions

Country

2.3 Causes

Sulphur dioxide and the two nitrogen oxides – nitric oxide (NO) and nitrogen dioxide (NO2) – are the basic elements involved in acid deposition (Bell and Walker, 1992) These pollutants originate from the burning of fossil fuels and petroleum products, the smelting of metallic ores, the petrochemical and associated industries and from vehicular exhausts (Bell and Walker, 1992) In other words, acid deposition is the product of a fuel-powered urban-industrial system (Figure 2-1, Manion, 1992)

Figure 2-1: The formation and deposition of acid pollution (modified from Mannion, 1992)

These oxides of sulphur and nitrogen combine with water in the atmosphere, creating sulphurous and sulphuric acids and nitrous and nitric acids (Mannion, 1999) With regard to SO2, it dissolves in cloud water, producing sulphurous acid:

SO 3 + H 2 O → H 2 SO 4⇌ H + + SO 4 -⇌ 2H + + SO 4

2-With regard to NOx, they are involved in numerous chemical processes in the atmosphere, some of which damage the ozone layer in the stratosphere (Mannion, 1999) Nitric oxide can be converted to nitrogen dioxide through a reaction with ozone:

O 3 + NO ⇌ NO 2 + O 2

On the other hand, nitrogen dioxide can also be converted back to nitric oxide through a reaction with light The oxygen atom released then reacts to form ozone:

NO 2 → NO + O

O + O 2→ O 3

Nitrogen dioxide may dissolve in cloud water to produce nitric and nitrous acids:

2NO 2 + H 2 O → HNO 3 + HNO 2

The above equations are based on Mannion (1999)

It should be noted that both SO2 and NOx also have natural sources For example, SO2 can originate from gases released during volcano eruptions (Jones, 2000) and NOx from lightning and microbial processes (Galloway, 1995) However, while there is significant uncertainty in the magnitude of these natural fluxes, there is a general agreement that anthropogenic fluxes are larger than natural ones (Galloway, 1995)

According to Galloway (1995), in 1990, anthropogenic SO2 emissions exceeded natural emissions of sulphur dioxide by approximately three times (≈75

Tg S/yr compared to ≈25 Tg S/yr) and emissions of nitrogen oxides followed a similar pattern with contribution from fossil-fuel combustion totalling ≈20 Tg N/yr,

light

over twice that of natural emissions which were ≈8 Tg N/yr While this study is dated, anthropogenic emissions have increased rather than dropped, making their magnitude over natural fluxes greater, not smaller (figure 2-2) Figure 2-2 shows the global emissions of sulphur and nitrogen since the mid-1800s along with the acidification potential of these emissions

Figure 2-2: SO 2 and NO x emissions along with their acidification potential (data from Galloway,

1995; Galloway, 2001 and Smith et al, 2011)

Acid deposition has the greatest impact on freshwater ecosystems that experience high precipitation rates and have an acid bedrock (such as granite) as acid bedrocks provide little buffering capacity Conversely, alkaline bedrocks

0"

500" 1000" 1500" 2000" 2500" 3000" 3500" 4000" 4500"

have a neutralising effect on acid deposition (Mannion, 1992) Freshwater systems are also already naturally acidic though processes such as the dissociation of carbonic and humic acids (Ho and Todd, 2010)

Aside from acid deposition, there are two other possible causes of acidification in a freshwater ecosystem that need to be taken into account in any acidification study – long-term acidification and land use change (Renberg and Battarbee, 1990) It was found in the 1920s that lakes in catchments which had base-poor or slow-weathering bedrock had a gradual tendency to become more acidic during the post-glacial time period (Renberg and Battarbee, 1990) This was because, as soils became stabilised by forest cover, the mineral elements within, namely calcium, magnesium, sodium and potassium, were leached out, causing the brown forest soils to become acid brown earths and podsolic soils

As some areas underwent paludification, which is “the expansion of a bog caused

by the gradual rising of the water table as accumulation of peat impedes water drainage” (Goudie, 2000: 356), the accumulation of acidic organic matter significantly increased the acidity of the water moving through it, causing the subsequent acidification of freshwater ecosystems (Pennington, 1984)

Landscape stabilisation and paludification as a cause of long-term acidification is a process that has thus far only been observed in post-glacial temperate landscapes, particularly with the formation of upland peat bogs However, such a process could occur in the tropics This is because mainland east Asia, the Caribbean, southern Africa, Central and South America and

Southeast Asia contain extensive peatland (Page et al, 2006) Most tropical

peatlands occur in low altitude, coastal and sub-coastal (between 0m to 50m above sea-level) areas They are ombrogenous and convex in shape, “analogous

to raised bogs of the Northern Hemisphere” (Dommain et al, 2011: 999) There

are also peatlands in mountainous regions For instance, in Central Kalimantan,

peatlands “extend up to 200km or more inland from the coast and occupy thousands of square kilometres, covering the gently sloping landscape in a

manner analogous to temperate zone blanket peat” (Page et al, 2006: 151) The

majority of these peatlands formed during the mid-Holocene as a response to post-glacial sea-level changes, coupled with an intensification of the Asian

monsoon (Dommain et al, 2011)

The difference is that while paludification is a common process of peatland formation in the tropics, in temperate regions, it is terrestrialisation that

is the primary mechanism of peatland initiation, which is then followed by paludification (Kamal and Varma, 2008) This would mean that in the tropics, which did not experience the widespread glaciation that temperate environments undergo, the weathering system may be in quasi-equilibrium, and would not experience the dramatic geochemical changes temperate environments undergo

It is therefore uncertain whether tropical peatland environmental dynamics would cause similar long-term acidification as seen in temperate environments

In any case, long-term acidification is usually a very slow process, implying that a quasi-equilibrium exits in most catchment soils between cation generation from weathering and cation loss from leaching (Renberg and Battarbee, 1990) For example, based on data from acidification studies in Europe, at sites that exhibited signs of long-term post-glacial acidification, prior to the 1800s, acidification was 0 or less than 0.1 pH units per 1000 years (Renberg and Battarbee, 1990) Recent (post-1850) acidification, on the other hand, is a significantly more rapid process (Renberg and Battarbee, 1990) Furthermore, according to Smol (2008), natural changes cannot account for the current highly acidic state of various lakes in Europe, though it can make lakes more vulnerable

to anthropogenic acidification That being said, as Jungle Falls stream is not in,

surrounded or affected by peatlands, this factor will not be considered further in this study

Other opponents of the severity of acid deposition often ascribe present day acidification of freshwater ecosystems to land use change For instance, Rosenqvist (1978) concluded that the cause of acidification in Norway was a response to an increase in timber production and export along with changing agricultural practices from semi-nomadic cattle farming to sedentary farming According to Rosenqvist (1978), the growing trees, as vegetation changed from grassland to forests, extracted bases from the ground and increased the amounts

of humic acids, thus causing the acidification of river waters Debates regarding the relative importance of land use and acid deposition in causing recent lake

acidification has since been highly contentious (Sullivan et al, 1996)

Another study area where land-use change was discovered to have caused surface water acidification, is at the Trossachs region of Scotland By comparing afforested (Loch Chon) and non-afforested (Loch Tinker) sites, Kreiser

et al (1990) found that the main acidification of Loch Chon occurred after conifer

afforestation while the pH at Loch Tinker, an adjacent moorland “control” site

remained largely unchanged Kreiser et al (1990) concluded that the afforestation

of sensitive catchments can cause freshwater ecosystems to acidify, but only in regions that receive high levels of sulphur deposition

A similar research project was conducted in 1983 at the Llyn Brianne catchment in Wales Researchers found that not only did afforestation of Sitka

Spruce enhance stream water acidification in the area (Whitehead et al, 1988),

but that forest age also had an impact on acidification levels (Waters and Jenkins, 1992) Differences in stream chemistry between afforested young forests and moorlands were due to disturbances caused during site preparation,

such as ploughing, planting and fertilisation, which alters soil drainage, increasing sulphate and nitrate mineralisation, consequently causing stream acidity to rise (Waters and Jenkins, 1992) As a forest aged, and a canopy developed, not only did uptake and retention of base metals and inorganic nitrogen occur, along with the accumulation of biomass, scavenging of dry and occult deposition of sulphur and nitrogen also exacerbated acidification (Waters and Jenkins, 1992)

In order to investigate the significance of land use change to freshwater acidification, several studies in Norway, the UK and Sweden, as part of the Royal Society Surface Waters Acidification Programme (SWAP), were designed to either eliminate or independently vary the influence of one or the other factor This included choosing sites with no decrease in grazing intensity, sites that were above the treeline and sites that had minimal catchments (Renberg and Battarbee, 1990) It was found that “all paleolimnological tests designed to disprove the land-use hypothesis for surface waters acidification have succeeded while those designed to disprove the acid deposition hypothesis have consistently failed, both in Europe and North America” (Renberg and Battarbee, 1990: 296)

In other words, while long-term acidification has increased the sensitivity

of numerous lakes to acid deposition and it is also possible that afforestation in areas of high acid deposition has affected the timing and intensity of acidification, these factors have not been a direct cause of the recent acidification of freshwater ecosystems (Renberg and Battarbee, 1990) This recent acceleration

in acidification of numerous freshwater ecosystems is mainly due to anthropogenic causes (Mannion, 1992) However, it is vital to recognise that “the importance of acidic deposition as an agent of acidification does not preclude the fact that land use and landscape changes may also be important, and in some

cases more important than acidic deposition” (Sullivan et al, 1996: 233)