Phosphorus control and dredging decrease methane emissions from shallow lakes.

Freshwater ecosystems are an important source of the greenhouse gas methane (CH ₄ ), and their emissions are expected to increase due to eutrophication. Two commonly applied management techniques to reduce eutrophication are the addition of phosphate-binding lanthanum modified bentonite (LMB, trademark Phoslock©) and dredging, but their effect on CH ₄ emissions is still poorly understood. Here, this study researched how LMB and dredging affected CH ₄ emissions using a full-factorial mesocosm design monitored for 18 months. The effect was tested by measuring diffusive and ebullitive CH ₄ fluxes, plant community composition, methanogen and methanotroph activity and community composition, and a range of physicochemical water and sediment variables. LMB addition decreased total CH ₄ emissions, while dredging showed a trend towards decreasing CH ₄ emissions. Total CH ₄ emissions in all mesocosms were much higher in the summer of the second year, likely because of higher algal decomposition and organic matter availability. First, LMB addition lowered CH ₄ emissions by decreasing P-availability, which reduced coverage of the floating fern Azolla filiculoides, and thereby prevented anoxia and decreased surface water NH ₄ ⁺ concentrations, lowering CH ₄ production rates. Second, dredging decreased CH ₄ emissions in the first summer, possibly it removed the methanogenic community, and in the second year by preventing autumn and winter die-off of the rooted macrophyte Potamogeton cripsus. Finally, methanogen community composition was related to surface water NH ₄ ⁺ and O ₂ , and porewater total phosphorus, while methanotroph community composition was related to organic matter content. To conclude, LMB addition and dredging not only improve water quality, but also decrease CH ₄ emissions, mitigating climate change.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
(Copyright © 2022 The Author(s). Published by Elsevier B.V. All rights reserved.)