RECOVERY FROM ACIDIFICATION IN AQUATIC MESOCOSMS AFTER REDUCING AMMONIUM AND SULFATE DEPOSITION

Seven small-scale indoor ecosystems, simulating hydrologically isolate d soft waters, were exposed to different artificial rain solutions ove r a 2-year period, and later on to a 10-year period of clean, artifici al rain water. Two main types of rain water were used; water with sulp huric 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 nit rification of ammonium. This acidification led to increased levels of Ca, Mg, Mn, Al, Cd and Zn in the water layer. Plants typical of soft w aters declined and were overgrown by Sphagnum species and Juncus bulbo sus L. A slow but gradual recovery to the pre-treatment water chemistr y was observed during a 10-year period of treatment with clean artific ial rain. The species composition of the small-scale ecosystems which were treated originally with sulphuric acid did not differ from the co ntrol, although the abundances of Juncus bulbosus, Hydrocotyle vulgari s L. and Sphagnum species were still higher after 10 years. The recove ry of the water quality was less distinct in the small-scale systems o riginally treated with ammonium sulphate. In these systems almost all plant species of soft-water communities had disappeared in the first f ew years following ammonium sulphate application. After 10 years of cl ean-rain treatment Sphagnum species were still very abundant and the w ater volume was strongly reduced owing to the luxurious growth of Moli nia caerulea (L.) Moench and Juncus effusus L. It is concluded that th e recovery of the water chemistry and vegetation of soft-water ecosyst ems is very slow after ammonium sulphate addition, compared with that of the recovery following sulphuric acid addition.