All data supporting the findings of the crayfish and GHG experiment, data analysis and Scripts used for Monte Carlo modelling.

We investigated how the invasion of crayfish influence methane (CH₄) emissions in freshwater macrocosms. From May to July 2024, an experiment was conducted using twelve outdoor mesocosms (1.20 m × 1.20 m × 0.50 m; surface area = 1.44 m²) at the Sinderhoeve research facility, located in the Netherlands (www.sinderhoeve.org). Each mesocosm simulated a macrophyte-dominated freshwater ecosystem. Adult male Procambarus clarkii were introduced to simulate an invasion and we tracked their effects on CH4 emissions and other water and vegetation parameters for 6 weeks. Mesocosms were randomly assigned to one of three density treatment groups (n = 4 per group): No crayfish, Low density invasion, High density invasion. Crayfish presence led to a 2.7-fold increase in methane release compared with systems without crayfish. This increase was associated with the removal of submerged vegetation and sediment resuspension, processes that reduce methane oxidation in the water column. To explore potential implications at larger spatial scales, a Monte Carlo simulation was conducted using pre-invasion, country-level data for the Netherlands. This analysis indicated that accounting for emissions from invaded waterbodies results in national methane emissions more than 35% higher than pre-invasion estimates.