Alkalinity generation and sediment CO2 uptake influence establishment of Sparganium angustifolium in softwater lakes

1. Softwater lakes are generally dominated by slow growing, small, isoetid plant species that are adapted to the carbon‐ and nutrient‐limited conditions in these lakes. We investigated the strategy of a fast growing species, Sparganium angustifolium, for occupying softwater lakes. A field survey was carried out in Norwegian carbon‐limited Isoëteto‐Lobelietum softwater lakes to compare abiotic conditions at locations with and without S. angustifolium. In addition, long term abiotic changes (1995–2008) related to the sudden establishment of the species on experimentally limed plots were studied. Based on the results, the carbon acquisition mechanism of S. angustifolium was tested in eco‐physiological laboratory experiments. 2. The redox potential was significantly lower at locations with S. angustifolium (220 ± 2.3) compared to locations without S. angustifolium (338.1 ± 13.9). The lower redox potential was accompanied by significantly higher concentrations of HCO₃^?, CO₂ and Fe²⁺ in the sediment pore water, indicating in‐lake alkalinity generation due to higher iron reduction rates in the generally iron‐rich sediments. In addition, the lower redox potential was accompanied by a higher nutrient availability (NH₄⁺ and PO₄^3?) in the sediment pore water. Since there were no differences in water quality between the lakes, the ability of S. angustifolium to grow in softwater lakes very likely depends upon the higher dissolved inorganic carbon (DIC) and nutrient concentrations present in the sediment pore water. 3. Results from the liming experiment revealed that appearance of S. angustifolium on limed plots was related to the dissolution of Ca and Mg carbonates and development of a lower redox potential in the sediment. These processes were accompanied by a sustained increase in the availability of DIC in the sediment pore water. 4. The eco‐physiological experiments indicated that S. angustifolium can increase in biomass and produce floating leaves at a relatively high DIC availability in the root medium. In addition, it appeared that S. angustifolium can take up CO₂ by the roots. As far as we know, the ability to use sediment CO₂ has only been described as an adaptation typical for isoetid plant species. Use of the relatively large sediment CO₂ pools present in these sediment types (>1000 μmol L^?1) to enable development of long floating leaves for additional uptake of atmospheric CO₂ is a very different strategy to colonise softwater lakes as compared to isoetid plant species.