Methane flux patterns following zooplankton addition in lake mesocosms with contrasting primary producers

Abstract

In lake ecosystems, different dominant primary producers may exert distinct effects on methane (CH₄) emissions. As key secondary producers and the principal grazers of phytoplankton and bacteria, zooplankton could modulate CH₄ cycling. However, comparative studies are currently lacking that evaluate the impacts of zooplankton on ecosystem CH₄ flux, and whether these impacts vary among systems containing different dominant primary-producer species. We posit that the influence of zooplankton on CH₄ emissions is modulated by the functional and structural attributes of the dominant primary producers. In this study, we conducted a mesocosm experiment to examine the individual and combined effects of three dominant primary-producer species (macrophyte-dominated: Hydrilla verticillata, Vallisneria natans; phytoplankton-dominated: Chlorella pyrenoidosa) and zooplankton addition on ecosystem CH₄ emissions. In treatments without zooplankton addition, H. verticillata mesocosms exhibited significantly higher CH₄ fluxes across the water–air interface (104.7 ± 50.5 μmol m⁻² day⁻¹) than those of V. natans mesocosms (78.2 ± 43.4 μmol m⁻² day⁻¹). C. pyrenoidosa mesocosms exhibited intermediate CH₄ fluxes, with no significant differences compared with either macrophyte treatment. Zooplankton addition was associated with higher CH₄ flux in the H. verticillata treatment (160.8 ± 126.5 μmol m⁻² day⁻¹), whereas no significant effect was detected in the V. natans or C. pyrenoidosa treatments. Correlation analyses revealed weak but significant positive relationships between CH₄ flux and both zooplankton biomass and H. verticillata biomass. Our findings suggest that lake CH₄ flux responses to zooplankton addition are influenced by the identity and associated traits of the dominant primary producer.