Submersed macrophyte density regulates aquatic greenhouse gas emissions

Shallow freshwater ecosystems emit large amounts of greenhouse gases (GHGs), such as carbon dioxide (CO₂) and methane (CH₄), yet emissions are highly variable. The role that aquatic macrophytes play in regulating aquatic GHG emissions is uncertain despite their ability to dominate shallow waterbodies. Here, we studied the effects of submersed macrophyte (Ceratophyllum demersum) density on CO₂ and CH₄ concentrations and fluxes. We conducted a 61-days experiment using mesocosms containing one of the following C. demersum density treatments: 0, 10, 20, or 30 individual shoots (n = 3). We found that high density C. demersum had the highest CO₂ and CH₄ surface water concentrations and emissions while there was no significant difference in CH₄ in the low and medium densities and no plant control. The high density treatment lost biomass over the course of the experiment, indicating die-off and additions of organic matter to the sediment. High organic matter loading and low dissolved oxygen likely stimulated GHG production in the high density treatment. Our results emphasize that submersed macrophyte density and periods of growth and dieback are important in regulating GHG emissions, which may help explain why shallow waterbodies are high yet variable sources of GHGs to the atmosphere.