Diurnal Variation in Water–Air Greenhouse Gas Fluxes Across Different Aquatic Vegetation Habitats in a Shallow Subtropical Lake

Lakes are important sources of greenhouse gases (GHGs), but diurnal flux dynamics across different aquatic vegetation habitats are not well quantified, leading to uncertainties in ecosystem-scale budgets. Here, we used high-frequency monitoring (static chamber coupled with Picarro G2301) to examine diurnal CO2 and CH4 fluxes at the water–air interface in three habitats—submerged macrophytes (SM), emergent macrophytes (EM), and non-vegetated control (BC)—in the shallow lake (Changshu Emergency Water Source Lake). During the study period, the lake was a consistent net CO2 sink (mean flux: −17.53 ± 1.64 μmol·m−2·d−1) but a net CH4 source (mean flux: 5.86 ± 1.70 μmol·m−2·d−1). Pronounced diel variability was observed: CO2 uptake was strongly enhanced during the day, whereas CH4 emissions peaked at night. Vegetation type exerted a strong control on flux magnitudes, with the SM habitat showing the highest CO2 uptake and the EM habitat the lowest CH4 emissions. Generalized linear models (GLMs) revealed that the regulatory effects of key environmental drivers (e.g., temperature, dissolved oxygen, turbidity) on gas fluxes varied significantly by habitat type and diurnal cycle, exhibiting distinct patterns of differentiation. Our findings highlight that accurate assessment of GHG fluxes from shallow lakes—and thus reliable carbon budgeting—must explicitly account for both diurnal cycles and the distinct regulatory roles of aquatic vegetation types.