Long‐term effects of liming in Norwegian softwater lakes: the rise and fall of bulbous rush (Juncus bulbosus) and decline of isoetid vegetation

Acidification has been recognised as a serious environmental problem in Scandinavia since the 1970s, and liming has been the main strategy to counteract negative effects on biota by improving water quality. We studied the short‐ and long‐term effects of liming on sediment and water quality, as well as macrophyte development, by comparing five limed with five unlimed lakes during the period 1993–2013. In the limed lakes, massive development of bulbous rush (Juncus bulbosus: Juncaceae) occurred during the first 6–9 years after the start of liming (1984–1993). Juncus bulbosus then started to decline around 1999 and eventually reached its original abundance in 2010. In addition, the cover of the original isoetid vegetation declined by around 60% between 1995 and 2010. In contrast, changes in submerged aquatic vegetation in the unlimed lakes were minor. Liming initially stimulated anaerobic breakdown of sediment organic matter with Fe as the electron acceptor. Juncus bulbosus probably benefitted from the higher CO₂ fluxes from the sediment to the water layer in combination with a high NH₄ ⁺ concentration in the limed sediment. However, the organic matter became less reactive over time, thus eventually providing less CO₂. Together with the significant increase in water pH over time, this would have led to a decline in C availability in sediment and water, thus discouraging excessive growth of J. bulbosus. This may explain why J. bulbosus thrived only during the first years after liming. The decline of isoetids in limed lakes was probably caused by a combination of uprooting and sediment anoxia. In limed lakes, isoetids developed a shoot : root ratio that was above the threshold known to promote uprooting of these high buoyancy species on organic sediment. In addition, isoetids eventually became covered by algae and dead J. bulbosus, affecting light conditions and suffocating the remaining isoetid vegetation on limed reductive sediments. In the future, renewed excessive growth of J. bulbosus seems unlikely due to the lack of sufficient carbon sources. It remains, however, unclear whether the original isoetids vegetation can recover. We hypothesise that the isoetid Littorella uniflora may eventually recolonise the lake from the shore and improve the oxygen content of limed sediments; this, in turn will facilitate the germination of other plant species, through high radial oxygen loss in combination with asexual reproduction.