Isoetid mediated radial oxygen loss prevents iron reduction and the related mobilisation of ammonium and methane: an experimental approach

Isoetids are slow growing ecosystem-engineers that maintain an oligotrophic environment through high radial oxygen losses (ROL) from their roots. This ROL could potentially reduce methane (CH4) emission by maintaining sediment redox conditions unfavourable for methanogenesis. Isoetids have declined in many European countries as an effect of eutrophication and inorganic carbon enrichment. To assess the effect of sudden disappearance of isoetids on biogeochemical carbon and nutrient cycling in organic rich sediments, we set up a greenhouse experiment. Intact vegetated sediment patches with Lobelia dortmanna and Littorella uniflora were collected from ten boreal Norwegian softwater lakes. From half of the patches, isoetids were manually removed. We determined the effect of removal on sediment redox potential, pore water chemistry and CH4 emission. In addition, germination success of L. dortmanna seeds was tested on the sediments where isoetids were removed. The absence of ROL after isoetid removal led to a drop in sediment redox potential, resulting in anaerobic breakdown of organic matter using alternative electron acceptors, especially iron(hydr)oxides. Consequently, increased porewater concentrations of ammonium, iron, inorganic carbon and CH4 were observed in sediments where isoetids had been removed compared to the isoetid vegetated sediments. Moreover, over 20 times higher emission of CH4 was observed from the sediments where isoetids had been removed. On the highly reduced sediments, the germination of L. dortmanna was hampered. When isoetids are unable to recolonise after disturbance events, it is expected that the absence of ROL eventually results in a long-term increase in lake productivity and CH4 emission.

Automated summary

This study investigated the impact of sudden disappearance of isoetids on biogeochemical carbon and nutrient cycling in organic rich sediments through a greenhouse experiment. The findings revealed that the removal of isoetids led to a decrease in sediment redox potential, resulting in anaerobic breakdown of organic matter and increased concentrations of ammonium, iron, inorganic carbon and methane in porewater.