Enhanced growth of the macrophyte Juncus bulbosus in S Norwegian limed lakes. A regional survey

Abstract

The effects of liming on the aquatic macrophyte vegetation have been investigated in S and SW Norway. In the western part of the study area, Juncus bulbosus was considerably more frequent in the limed than in the unlimed lakes, whilst in the eastern part there were no such differences, and the J. bulbosus populations were generally not so vital. In some southwestern areas a luxuriant and massiv nuisance growth of Juncus bulbosus in the depth zone 0–4 m was recorded. The most vital plants produced up to 1 m long annual shoots, and developed extensive, dense 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 more or less irreversible. The massive J. bulbosus expansion is seen mainly in directly limed lakes with a sometimes visible layer of calcium carbonate on the sediment surface, but enhanched growth has been observed also in lakes downstream liming. The massive expansion is believed to be due to an increase of CO₂ and ammonium in the sediment pore water, combined with a mild climate with a very high precipitation. In many areas the liming has led to an increase in species diversity, and a (re-)establishment of some acid-intolerant species such as Myriophyllum alterniflorum and Potamogeton spp.