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

Abstract

Summary 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.

Interactive effects of nitrate concentrations and carbon dioxide on the stoichiometry, biomass allocation and growth rate of submerged aquatic plants

“…Changes in the CO 2 and HCO 3 À availabilities can cause shifts within the macrophyte communities Vadstrup & Madsen, 1995), and particularly for nutrient poor soft water lakes strong effects of increasing CO 2 concentrations were found (Lucassen, Roelofs, Schneider, & Smolders, 2016;Spierenburg, Lucassen, Lotter, & Roelofs, 2009). Changes in the CO 2 and HCO 3 À availabilities can cause shifts within the macrophyte communities Vadstrup & Madsen, 1995), and particularly for nutrient poor soft water lakes strong effects of increasing CO 2 concentrations were found (Lucassen, Roelofs, Schneider, & Smolders, 2016;Spierenburg, Lucassen, Lotter, & Roelofs, 2009).…”

“…The present data indicate that increasing CO 2 availabilities will reduce the nitrogen demand and accumulation by submerged plants, which can have drastic effects for the ecosystem function and competition between macrophytes and algae. Changes in the CO 2 and HCO 3 À availabilities can cause shifts within the macrophyte communities Vadstrup & Madsen, 1995), and particularly for nutrient poor soft water lakes strong effects of increasing CO 2 concentrations were found (Lucassen, Roelofs, Schneider, & Smolders, 2016;Spierenburg, Lucassen, Lotter, & Roelofs, 2009). There, increasing CO 2 concentrations were found as the major parameter explaining the recent inva-…”

Juncus Bulbosus Tissue Nutrient Concentrations and Stoichiometry in Oligotrophic Ecosystems: Variability with Seasons, Growth Forms, Organs and Habitats

“…Reading through the original publications [24,39,40], no factorial experiments were ever carried out to test the independent and interactive effects of CO 2 , N and P on the Juncus bulbosus growth rate and yield. The liming of lakes in Norway promoted J. bulbosus mass development through the mineralisation of the organic matter with the production of CO 2 , NH 4 and inorganic P in sediment pore water [24] until the organic matter became less reactive over time [41]. In the presence of sufficient N and P, CO 2 can be a limiting factor [39,40,42].…”

“…The repeated story that the mass development of J. bulbosus would only occur under high CO 2 and sediment pore water NH 4 concentrations [24,27,41,48] can only make sense if the plant can access enough P for its growth because aquatic plant tissue C:N:P stoichiometry is rather homeostatic [14,26], whatever the ecosystem, based on principles of ecological stoichiometry. Thus, in Norwegian lakes and rivers, it is likely that CO 2 , N and/or P could limit or colimit the growth rate of J. bulbosus.…”

Non-native Minnows Threaten Quillwort Populations in High Mountain Shallow Lakes

“…The quillwort biomass allocation ratio measured in minnow lakes are up to 2.5 times higher than under natural conditions and well above the risk of uprooting described for other isoetids (Røelofs et al, 1994 ). This is usually associated to processes not only of light impairment but of decreasing sediment compactness that occurs along with eutrophication (Spierenburg et al, 2013 ; Lucassen et al, 2016 ).…”