The effect of acidification, liming and reacidification on macrophyte development, water quality and sediment characteristics of soft-water lakes

Abstract

Rapid expansion of Juncus bulbosus L. and the concomitant suppression of isoetid plant species has often been observed in acidifying soft water lakes in Western Europe. Experimental studies have shown that this mass development of J.bulbosus was caused by changes in the carbon and nitrogen budgets in these ecosystems. Acidification leads to temporarily strongly increased carbon dioxide (CO₂) levels in the slightly calcareous sediment and to accumulation of ammonium as a result of a reduced nitrification rate in acidifying waters. Many acidifying Scandinavian soft water lakes, however, have a well developed macrophyte vegetation. It is suggested that this is related with the non-calcareous sediments 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 mineralisation rates, CO₂ production, sediment pore water levels of phosphate and ammonium and to a decrease of the nitrate concentrations in the sediment. These changes have been earlier observed in acidifying West 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 CO₂ levels in the water layer of the limed lakes already a few months after liming.