Recovery from acidification in aquatic mesocosms after reducing ammonium and sulphate deposition

Seven small-scale indoor ecosystems, simulating hydrologically isolated soft waters, were exposed to different artificial rain solutions over a 2-year period, and later on to a 10-year period of clean, artificial rain water. Two main types of rain water were used; water with sulphuric acid (pH 5.6-3.5) and water with ammonium sulphate (pH 5.6), The treatments with ammonium sulphate, particularly, caused acidification down to pH 3.5 in both water and sediment as a consequence of the nitrification of ammonium. This acidification led to increased levels of Ca, Mg, Mn, Al, Cd and Zn in the water layer. Plants typical of soft waters declined and were overgrown by Sphagnum species and Juncus bulbosus L. A slow but gradual recovery to the pre-treatment water chemistry was observed during a 10-year period of treatment with clean artificial rain. The species composition of the small-scale ecosystems which were treated originally with sulphuric acid did not differ from the control, although the abundances of Juncus bulbosus, Hydrocotyle vulgaris L. and Sphagnum species were still higher after 10 years. The recovery of the water quality was less distinct in the small-scale systems originally treated with ammonium sulphate. In these systems almost all plant species of soft-water communities had disappeared in the first few years following ammonium sulphate application. After 10 years of clean-rain treatment Sphagnum species were still very abundant and the water volume was strongly reduced owing to the luxurious growth of Molinia caerulea (L.) Moench and Juncus effusus L. It is concluded that the recovery of the water chemistry and vegetation of soft-water ecosystems is very slow after ammonium sulphate addition, compared with that of the recovery following sulphuric acid addition.