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SUMMARY. 1. The sulphate reduction capacity of six shallow soft water sediments, differing in pH and organic matter content, was studied under controlled pH adjustments ranging from pH 3 to pH 8. 2. In the acid sediments, relatively rich in organic matter, the sulphate reduction capacity reached values of 0.09-0.12 μmol g−1 d−1. In the circumneutral mineral sediments the values ranged between 0.04 and 0.08 μmol g−1 d−1. 3. The latter group of sediments was very sensitive to the effects of experimental acidification as sulphate reduction was almost fully inhibited when pH decreased from 7 to 5. In the acid sediments inhibition occurred at lower pH values, in the range pH 5 to pH 3. 4. Sulphate reduction governed the production of free sulphides, whereas putrefaction processes were only of minor importance. It is suggested that in acid sediments, relatively rich in organic matter, the sulphate reducing bacterial population is less sensitive to acidification than in circumneutral mineral sediments. 5. The presence of organic matter appeared to be important in counteracting the inhibiting effects of acidification on sulphate reduction. This is important for the in situ sulphate reduction in sediments of soft waters which become enriched with organic matter during the long-term process of acidification.

The atmospheric transport of NH3, emitted from breeding farms and strongly manured cropland, causes a severe loading of neighboring non-agricultural environments. Particularly the wet deposition of ammonium sulphate is enhanced, while the pH of rainwater is only slightly acidic. Due to the variation of gaseous air pollutants over the Netherlands large differences are noticed in the relative contribution of each acidifying component in rainwater. In areas with high atmospheric NH3 concentrations, ammonium sulphate contributes the major part to the potential acidifying influence of wet deposition. Preliminary experiments show a causal relation between ammonium sulphate and water acidification, ammonium enrichment and the expansion of ammonium tolerant shore macrophytes. For the Dutch situation it is clear that, particularly in areas with enhanced atmospheric NH3 concentrations, the deposition of NH3 and SO2 derived compounds has provided the major contribution to acidification and N-enrichment of hydrologically isolated, poorly buffered pools. In all countries, particularly in areas with high NH3 emission, it will be useful to pay more attention to the environmental impact of airborne ammonium sulphate deposition.

In a greenhouse, seven identical mini-ecosystems, simulating soft water ponds, were exposed to different types of artificial rain water. The effects of rain water containing H2SO4 and nitrate, and rain water containing ammonium sulphate on water quality and vegetation were studied and compared. Causal relations were established between rain water quality, water chemistry and changes in floristic composition. Ammonium sulphate deposition, particularly, strongly affected water quality and vegetation development. Although ammonium sulphate deposition was only slightly acid, due to nitrification it acted as an important acid source, causing acidification to pH=3.8. Under acidified conditions, ammonium sulphate deposition lead to a luxuriant growth of Juncus bulbosus and Agrostis canina. In the mini-ecosystems, H2SO4 deposition with a pH of 3.5 only decreased the pH of the water to 5.1 within 1 yr. The acidification of water appeared to be coupled with changes in alkalinity, sulphate, Al, Cd, Ca, Mg, K and inorganic-N. It is concluded that in NH3-affected regions in The Netherlands, the high atmospheric deposition of ammonium sulphate probably contributes to a large extent in the acidification, eutrophication and floristic changes of oligotrophic soft waters.

During 1983–1984 about 90% of the Dutch soft waters appeared to be acid, and 35% of them showed pH-values below 4.0. The alkalinity of most waters (70%) was less than 0.1 meq.L−1. Evidence for recent acidification is derived from temporal trends in water quality, fish population status, shifts in aquatic biota and application of current empirical models for lake acidification. It has been concluded that at least 59% of the Dutch soft waters have been recently acidified. This is a conservative estimate. Depending on the criterion used for assessment of acidification this percentage might be as high as 96%.