Methane emission patterns following invasive species remediation in a tropical lentic ecosystem

Abstract

Tropical reservoirs are significant sources of methane (CH₄), yet the impacts of invasive macrophyte management on CH₄ dynamics remain poorly resolved. Las Curias Reservoir (San Juan, Puerto Rico) experienced a severe infestation of <em>Salvinia molesta</em> beginning in 2016, followed by mechanical and biological control measures that left ~3,449 tons of biomass to decompose in situ under anoxic hypolimnetic conditions. This study evaluates how this biomass deposition influenced CH₄ production and emissions across the reservoir. During peak summer stratification, 30 sites were surveyed using floating chambers, ebullition traps, and depth-profiled water-quality measurements; in vitro sediment incubations quantified methanogenic potential along the riverine–lacustrine reservoir gradient. Sediment assays showed increasing CH₄ production potential downstream, from 0.018 to 0.211 μmol CH₄ g⁻&sup1; DW d⁻&sup1;, consistent with enrichment of labile <em>Salvinia</em>-derived organic matter. In contrast, in situ CH₄ fluxes were highest in transitional sites and were frequently ebullition-dominated: at 77% of sites, more than half of total CH₄ flux occurred as bubbles. The reservoir-wide median total flux was 23.33 mg CH₄ m⁻&sup2; h⁻&sup1; ( ≈560 mg CH₄ m⁻&sup2; d⁻&sup1;), placing Las Curias within the upper range of CH₄-emitting U.S. reservoirs. Strong thermal stratification and persistent hypolimnetic anoxia limited oxidative CH₄ loss and facilitated efficient bubble transport to the atmosphere. These findings reveal a decoupling between methanogenic potential and realized fluxes driven by substrate distribution, water-column structure, and depth constraints on ebullition, emphasizing that macrophyte control strategies that leave biomass to decompose in place can have substantial and long-lasting impacts on tropical reservoir CH₄ emissions.