The influence of rising CO2 availability on softwater vegetation

ABSTRACT: Many softwater lakes are found in boreal and temporal regions, and at higher altitudes in the (sub) tropics. These lakes contain oligotrophic water with low alkalinity and carbon availability. Therefore, the production of submerged macrophytes is thought to be often carbon limited. The macrophyte vegetation is mostly dominated by slow-growing, specialized species, such as Iso봥s spp. and Littorella uniflora, all capable of utilizing sediment pore water CO2. During the last decades isoetid-dominated vegetations are invaded and massively overgrown by faster growing elodeid species, such as Myriophyllum alterniflorum and Callitriche hamulata. Commonly, deterioration of isoetid vegetation is attributed to eutrophication and acidification of softwater lakes. A multi-proxy paleolimnological study showed, however, that neither eutrophication nor acidification was responsible for the observed replacement of isoetids by C. hamulata in a SW Norwegian lake, during the last few decades. Instead rising CO2 availability in the water column might have induced this replacement. To explore this role of CO2, growth experiments with M. alterniflorum and C. hamulata were conducted at different CO2 levels. At CO2 concentrations ?20 mol L-1, which are frequently measured in pristine softwater lakes, neither M. alterniflorum nor C. hamulata could sustain itself. However, the growth rates became positive and increased with rising CO2 concentrations above 40 mol L-1. Raised CO2 levels can therefore be a prerequisite for elodeid invasion and expansion in softwater lakes. The high oxygen loss by isoetid roots generally results in sediment oxidation and reduced P and N availability to rooted macrophytes. The presence of isoetids might, therefore, decrease the invasion success of elodeids. Indeed, in a competition experiment with M. alterniflorum and L. uniflora, growth of M. alterniflorum was reduced by approximately 50% when planted together with ?553 L. uniflora plants m-2. Nevertheless at CO2 concentrations >90 mol L-1 M. alterniflorum growth rates were higher compared to L. uniflora, implying that, at raised CO2 availability, M. alterniflorum can invade and expand in isoetid dominated softwater lakes. Littorella uniflora vegetation can be threatened by massive uprooting, as observed in a Dutch softwater lake. The uprooting was caused by increased plant shoot production, and decreasing root:shoot ratios. This way, the relatively high buoyancy of isoetids was not longer sufficiently counterbalanced by root anchorage in the sediment. The shoot biomass production was positively related to high sediment organic matter content, and the resulting high sediment pore water CO2, P, and N concentrations. In several Norwegian isoetid dominated softwater lakes Sparganium angustifolium appeared at sites with a relative low sediment redox potential and higher sediment pore water concentrations of CO2, nutrients and Fe2+, indicative for in-lake alkalinity generation due to higher iron reduction rates. Eco-physiological experiments showed that S. angustifolium is capable of utilizing pore water CO2, enabling the production of floating leaves and uptake of atmospheric CO2, securing the carbon supply for further growth and reproduction. In softwater lakes the macrophyte community is strongly structured by CO2 in both surface- and pore water. Therefore, rising CO2 availability might form a serious threat to isoetid vegetations in softwater lakes.