Effects of atmospheric sulphur compounds on natural and man-made terrestrial and aquatic ecosystems

Amounts of atmospheric sulphur compounds including the gaseous sulphur dioxide, hydrogen sulphur and methyl mercaptan, and particulate sulphate depend upon the activities of man, volcanic emissions, releases from waterlogged soils and anaerobic estuarine and marine environments 3. The gases are transferred to vegetation, soil and other surfaces by dry deposition whereas particulate pollutants are removed mainly in, or on, raindrops, snowflakes 3. i.e. wet deposition.

The balance of dry to wet deposition varies regionally, SO₂ being the predominant sulphur pollutant near emission sources, particulate sulphate and acid rain gaining in importance at greater distances. Amounts of deposited sulphur may minimise the occurrence of sulphur deficiencies when crops are cultivated intensively.

Although effects of SO₂ have been assessed in series of controlled fumigations, few observations have relevance to field conditions where concentrations fluctuate diurnally and seasonally, and where episodic extreme concentrations may be more important than protracted exposures to mean concentrations. Additionally SO₂ usually occurs in mixtures with oxides of nitrogen (NO_x), also ozone; changes in U.K. concentrations of NO_x, but not ozone, tend to parallel those of SO₂. Little is known about the effects of mixtures; there is, however, evidence showing that damage done by mixtures of SO₂ and NO_x, also SO₂ and ozone, is sometimes greater than the summation of the damage done by each constituent. Plant growth can be decreased by concentrations of pollutants which do not cause blemishes.

In parts of Scandinavia, the U.K., the U.S.A. and probably elsewhere in the industrialised world, rain is commonly acid (pH 4.5, sometimes 4.0). Where it contained biologically significant concentrations of bisulphite (HSO₃⁻) ions, vegetation (Sphagnum spp.) seems to have been damaged: in the absence of these concentrations, rain, unless it is more acid than pH 3.0, neither blemishes foliage nor decreases yields of field-grown crops including trees. The role of acid rain in areas with relatively large concentrations of mixed atmospheric pollutants has not been identified. Acid inputs are, it seems, beginning to affect some mechanisms/processes in field soils. These need to be quantified in relation to plant production. Lakes and streams (a) lacking dissolved calcium and magnesium (as happens when they are dependent upon slowly weathering granitic and porphyritic bedrocks) and (b) subject to acid rain, have become more acid in recent years with a progressive switch from carbon dioxide/bicarbonate to aluminium/strong acid buffering systems. With increasing acidity, assemblages of plankton and macrophytes change but without greatly affecting plant biomass. Similarly there is a change among species of invertebrates with mayflies, stoneflies, Gammarus sp. (Malacostraca) and Daphnia sp. (Cladocera) becoming less abundant, and caddis flies, Bosmina sp. (Cladocera) and the copepods Cyclops and Diaptomus more plentiful. The loss of their preferred food (species of Daphnia and Gammarus) does not explain the diminishing abundance of fish, notably salmonids; instead it is closely associated with toxic concentrations of aluminium which deleteriously affect fish reproduction. Further corroborative evidence is required to detail the sequence of events from the wet deposition of rain and snow to the drainage of acid discharges into streams and lakes.

Globally, present day outputs of sulphur pollutants are unlikely to appreciably affect the chemistry of the oceans. They may, however, have discernible local estuarine effects on crabs and lobsters where ‘acidified’ streams/rivers mix with saltwater.