THE EFFECT OF ACIDIFICATION, LIMING AND REACIDIFICATION ON MACROPHYTEDEVELOPMENT, WATER-QUALITY AND SEDIMENT CHARACTERISTICS OF SOFT-WATERLAKES

Rapid expansion of Juncus bulbosus L. and the concomitant suppression of isoetid plant species has often been observed in acidifying soft wa ter lakes in Western Europe. Experimental studies have shown that this mass development of J. bulbosus was caused by changes in the carbon a nd nitrogen budgets in these ecosystems. Acidification leads to tempor arily strongly increased carbon dioxide (CO2) levels in the slightly c alcareous sediment and to accumulation of ammonium as a result of a re duced nitrification rate in acidifying waters. Many acidifying Scandin avian soft water lakes, however, have a well developed macrophyte vege tation. It is suggested that this is related with the non-calcareous s ediments of these lakes. After liming, however, mass development of J. bulbosus and/or Sphagnum spec. has been observed in Swedish and S.W. Norwegian lakes. From field experiments it has become clear that part of the lime is deposited on the sediments leading to an increase of mi neralisation rates, CO2 production, sediment pore water levels of phos phate and ammonium and to a decrease of the nitrate concentrations in the sediment. These changes have been earlier observed in acidifying W est European waters. Rooted species like J. bulbosus can only benefit from the higher nutrient levels in the sediment when the CO2 level of the water layer is relatively high as this species is adapted to leaf carbon uptake. It is demonstrated that gradual reacidification by the acid water from the catchments and the increased flux of carbonic acid from the limed sediments to the overlying water leads to increased CO 2 levels in the water layer of the limed lakes already a few months af ter liming.