ENHANCED GROWTH OF THE MACROPHYTE JUNCUS-BULBOSUS IN S NORWEGIAN LIMED LAKES - A REGIONAL SURVEY

The effects of liming on the aquatic macrophyte vegetation have been i nvestigated in S and SW Norway. In the western part of the study area, Juncus bulbosus was considerably more frequent in the limed than in t he unlimed lakes, whilst in the eastern part there were no such differ ences, and the J. bulbosus populations were generally not so vital. In some southwestern areas a luxuriant and massiv nuisance growth of Jun cus bulbosus in the depth zone 0-4 m was recorded. The most vital plan ts produced up to 1 m long annual shoots, and developed extensive, den se and vital surface mats in shallow areas (depth zone 0-3 m) after 4- 5 years. The original isoetid vegetation had disappeared in the areas of dense J. bulbosus populations, and this development seems to be mor e or less irreversible. The massive J. bulbosus expansion is seen main ly in directly limed lakes with a sometimes visible layer of calcium c arbonate on the sediment surface, but enhanched growth has been observ ed also in lakes downstream liming. The massive expansion is believed to be due to an increase of CO2 and ammonium in the sediment pore wate r, combined with a mild climate with a very high precipitation. In man y areas the liming has led to an increase in species diversity, and a (re-)establishment of some acid-intolerant species such as Myriophyllu m alterniflorum and Potamogeton spp.