MASSIVE EXPANSION OF JUNCUS-BULBOSUS L AFTER LIMING OF ACIDIFIED SW NORWEGIAN LAKES

Isoetes-Lobelia clear water lakes in an ultraoligotrophic area of sout h-western Norway were studied. Most lakes in the area are acidified, w ith a vegetation dominated by small isoetids such as Lobelia dortmanna L., Littorella uniflora (L.) Aschers. and Isoetes lacustris L. After liming, however, dense stands of Juncus bulbosus L. often have develop ed, filling up the water layer completely at relatively shallow sites and resulting in suppression of the original vegetation. In this study a comparison is made between plant growth and physical and chemical p roperties of water and sediment pore water in six limed and five non l imed 'reference' lakes. It appears that liming leads to a strong incre ase in CO2 production and mobilisation of nitrogen, phosphorus and iro n in the sediment. Probably as a result of the high mobilisation of ir on and the increased iron cycle no increased levels of phosphate in th e water layer occur. As a result the waters are still very clear after liming. The rooted macrophyte Juncus bulbosus can benefit from this s ituation, as it is able to take up nutrients from the sediment pore wa ter by its roots. It is discussed that the success of Juncus bulbosus is not only caused by the increased carbon dioxide, phosphate and nitr ogen levels in the sediment pore water (internal eutrophication), but also by the fact that the sediment becomes anoxic, leading to a decrea se in nitrate concentrations and a very strong increase in ammonium le vels. It is known from experimental studies that Juncus bulbosus can o nly reach a very high biomass at high carbon dioxide levels in combina tion with high ammonium concentrations.