Eutrophication mitigation through aluminium sediment injections did not reduce methane bubble emissions in a shallow eutrophic lake

Methane emissions are of growing concern due to global warming and the important role methane has as a greenhouse gas. Methane is 27 times more potent as a greenhouse gas compared to carbon dioxide and it is produced by anaerobic archaea living in oxygen-free environments, such as within lake sediments. Lakes are a significant natural source of atmospheric methane. Methanogenesis have also been found to be positively correlated with eutrophication in lakes. Eutrophication in lakes is caused mainly by elevated phosphorous concentrations, often caused by sewage water and fertilizers used within the watershed. Historically eutrophied lakes, as a result, often have a problem with internal loading of phosphorous. Internal loading is when labile portions of legacy phosphorous cycles back into the water column from the sediment surface instead of being buried. This can occur when lakes become stratified and the bottom water turns anoxic, leading to mobilization of labile phosphorus. To reduce internal P loading, aluminium salts can be dispersed through the water column or injected directly into lake sediments to bind to labile phosphorous, rendering it inert and non-bioavailable. The inert aluminium-phosphorus complex prevents the phosphorus from leeching back into the water and facilitates its’ burial. At present, there is no knowledge on the effect of lake sediment aluminium treatment on methane emissions. I hypothesise that by limiting labile P through aluminium treatment, methanogens will be deprived of bioavailable phosphorus, limiting methanogenic activity. The hypothesis was tested by measuring methane bubble emissions in a shallow eutrophic lake called Ullnasjön outside Stockholm from July to November 2022. Two 1-hectare grids had been injected with polyaluminium chloride in April 2021. This study found no significant difference in methane bubble emissions between the aluminium-treated site compared to the control site. The results suggests that sediment methanogens are likely not limited by access to labile P and that the link between methanogenesis and eutrophication is due to later ecosystem-scale effects of eutrophication, such as high production of organic matter, rather than the elevated phosphorus concentration itself. Furthermore, these results highlight the necessity of long-term studies to evaluate whether aluminium treatments could be a potential tool for greenhouse gas emission mitigation.