Introduction

UV irradiance at the earth's surface is intimately related to stratospheric ozone. This gas tends to be concentrated in the lower stratosphere (hence the notion of an ozone layer) and is primarily responsible for the absorption of solar UV radiation (UVR). UVR has been recognised for many years (e.g. Worrest, Dyke & Thomson, 1978; Worrest et al., 1981; Calkins, 1982) as a potential stress for organisms in a variety of environments and as a factor in biogeochemical cycling (Zepp, Callaghan & Erickson, 1995). The trend in recent years of an intensifying, but periodic, anthropogenic-induced decline in stratospheric ozone concentrations with concurrent enhanced UV-B radiation is quite alarming. Altered solar radiation regimes can potentially upset established balances in marine ecosystems and thus presents a new problem. Most attention has been given to the ‘ozone hole’ over Antarctica that has been recorded annually since the 1980s. However, recent observations have confirmed measurable ozone losses over other regions, including the development of an Arctic ozone hole. The major factor responsible for the destruction of the ozone layer is anthropogenic emissions of chlorofluorocarbons (CFCs). These gases, having no natural sources, are non-toxic and inert in the troposphere, but are photolysed in the stratosphere, thereby releasing reactive chlorine atoms that catalytically destroy ozone. Other anthropogenic contributions to ozone depletion may include global changes in land use and the increased emission of nitrogen dioxide as a result of fertiliser applications (Bouwman, 1998). Paradoxically, the anthropogenic emissions of greenhouse gases that tend to cause a temperature increase at the earth's surface also produce a decrease in stratospheric temperatures.

Introduction

Given the anticipated decreasing concentrations of stratospheric ozone, the concern regarding the effects of UV radiation (UVR) in the environment is very topical. The preceding chapters in this book provide a state-of-the-art review of UVR effects in the marine environment (see Figure 11.1). This last chapter synthesises briefly the potential UV-induced responses at community and ecosystem levels and gives an insight on how the global changes can interact with the direct impacts of UVR.

The increase of anthropogenic chlorofluorocarbons (CFCs) is responsible for the destruction of ozone in the upper atmosphere. This decrease in the ozone layer causes in an increase of UV-B radiation in the wavelength range 280 to 320 nm (Chapter 1). The past decade has been witness to a concentrated effort from the international scientific community tomeasure the increase of UVR and to understand its effects on different ecosystems. These efforts were supported by signatories to international agreements, such as the Montreal Protocol, to control and eliminate the production and emission of ozone-depleting substances. However, each spring, an ‘ozone hole’ larger than the size of Canada appears over the Antarctic region (Smith et al., 1992). In the Arctic and into the north temperate zone, the ozone layer diminished by more than 45% during the 1997/8 winter, the most important decrease in 35 years (Wardle et al., 1997). Notable ozone depletion has now been observed in both polar regions, as ozone loss rates in the Arctic region in recent years have reached values comparable to those recorded over the Antarctic (Rex et al., 1997, and references therein).

Introduction

Tributyltin is not a compound in its own right, but only a constituent part of molecules in this class of organotin substances. It comprises three n-butyl chains attached to a single tin atom via covalent Sn–C bonds. As the tin exists in the form Sn(IV), the moiety displays an univalent positive charge. Commercial products are typically available in the form of bis(tributyltin) oxide (known as TBTO), acetate (TBT–OAc), halides (TBTF, TBTCI) and as the co-polymer with methylmethacrylate (TBTM). Industrial applications of TBT compounds followed the recognition of its biocidal properties, first noted in the early 1950s (Bennett, 1983). Although present applications include molluscicides, stone preservation, and disinfectants, the most important usage remains in wood preservatives and anti-fouling paints (Blunden & Evans, 1990). The production and use of TBT compounds in these two contexts are reviewed here in Chapter 2. Several other organotin compounds are routinely utilised in agricultural biocides, catalysts and as heat and light stabilisers for PVC. Reviews of environmental behaviour of such substances can be found elsewhere (Blunden, Hobbs & Smith, 1984; Maguire, 1991).

Because TBT can act as a broad spectrum biocide, it is useful in the preservation of wood, which is subject to decomposition via bacteria, fungi and insects. Unprotected soft woods degrade relatively quickly giving useful lifetimes of only one to two years. TBT treatment can extend this time to decades. TBT formulations have also proved useful for the preservation of marine timbers (Hill & Killmeyer, 1988).

Atoms and atomic structure

The atom

For many years it was speculated that the ultimate state of subdivision of matter was discrete indivisible particles termed atoms. In the early nineteenth century John Dalton gave quantitative support to these ideas by assigning a self-consistent set of masses to atoms of elements. Elements are substances which cannot be decomposed by ordinary types of chemical change, or made by chemical union. However, subsequent work by Faraday, Crookes and Goldstein pointed out the electrical nature of atoms and that they are divisible. The simplest picture of an atom now shows a central nucleus surrounded by a cloud of electrons.

Electrons

When a neutral atom is supplied with sufficient energy it will ionise yielding positively and negatively charged fragments. The former, the positive ion, is characteristic of the particular parent atom, whilst the negative fragment, the electron, is the same irrespective of the parent atom. The charge and mass of electrons can be determined by studying their behaviour in electrical and magnetic fields. Early experiments revealed the ratio of charge to mass, e/m, which for all electrons is 1.7588 × 108 coulombs per gram (Cg−1).

To evaluate the mass of an electron, experiments were carried out to determine the applied potential required to prevent oil droplets carrying negative charges from settling due to gravity.

Natural abundance of the elements

Our knowledge about the elemental composition of the earth is restricted to the composition of the crust, atmosphere and oceans. Although the mantle and core represent over 99% of the earth's mass, their elemental compositions are not accurately known. Hence natural abundances of elements quoted in Table 3.1 refer only to the crust. Since the crust (35 km average thickness under continents and 10km under oceans) is not homogeneous and different parts of the earth contain different minerals, these values are averages of a large number of estimations. The relative homogeneity of ocean waters (Tables 6.3 and 6.4) and the atmosphere (Table 3.2) renders their compositions far less variable for major components.

Oxygen is the major element in the crust, with silicon coming second in abundance. Formation processes may have enriched the crust with certain elements in comparison with the mantle and core. The water and air masses are very different in composition from the crust, and, although elemental abundances may vary slightly with water depth or atmospheric height, the overall relative proportions of the major elements remain almost constant in the oceans and in the lower and middle atmosphere.

Although many problems in environmental chemistry are related to the abundance of elements in the environment, the chemical behaviour and properties of elements are largely independent of their abundances.

General principles of analytical chemistry

Introduction

Analytical chemistry encompasses the science of characterising a sample. Two broad categories can be considered. Qualitative analysis deals with the question of what elements or compounds are present. In the simplest case, this may involve only sample identification. For instance, is an unknown water sample fresh or saline? Several screening tests may rely on determining whether or not a particular substance is present or absent. This has applications in quality control during food processing where the analysis might serve to distinguish foreign bodies. Urine samples from athletes and racehorses are routinely examined for the presence of prohibited drugs. Qualitative analyses may also be used to trace the source of a sample. Pollen analysis of honey indicates the country of origin. Comparing the ‘fingerprint’ of substances in an oil slick with that in the bilge tanks of ships may enable the responsible vessel to be identified. Quantitative analysis involves determining how much of an element or molecule is present. Several types of quantitative analyses can be contemplated. These are exemplified below using the analysis of petrol (gasoline). A complete analysis would quantify the amount of each constituent in the sample. This would involve the determination of hexane, octane, toluene, tetraethyllead, etc. Such analyses are generally impossible and for environmental samples usually unnecessary. For an ultimate analysis the amount of each element is quantified.

Introduction to biogeochemical cycles

Much of the discussion of environmental processes in Chapter 6 will deal with single environmental reservoirs, such as the atmosphere, or the oceans. Many important processes involving chemicals, some of which have given rise to significant environmental problems, have involved transfer of chemicals from one reservoir or compartment to another. The transfer of chemical elements and compounds between compartments, taking into account the important influences of the biota, is termed biogeochemical cycling. Much work has been carried out to elucidate the biogeochemical cycles of the elements, but much remains to be done to describe the cycles in adequate detail.

The major transfers taking place in a biogeochemical cycle appear in Figure 5.1. An element is washed from rock and soil and enters fresh water streams and lakes in runoff, and to a lesser extent, in water flowing from ground waters into surface waters. The major part of that entering the fresh waters is carried by streamflow into the oceans. Direct coastal discharges of pollutants can also be important, especially in fully and partially enclosed seas like the North Sea. Volatile elements (e.g. mercury) and compounds are degassed from land surfaces and the sea. Less volatile materials enter the atmosphere from the land surface in wind-blown dusts (and man-made emissions) and from the sea surface in sea spray, the smaller particles of which form marine aerosol.