Study on seasonal variation characteristics and driving factors of greenhouse gas fluxes in Qiandao Lake

Carbon budget balance is an inherent requirement for the sustainability of reservoir fisheries. To explore the greenhouse gas (GHG) budget characteristics of large-scale ecological fishery reservoirs, we conducted a two-year in-situ monitoring study in Qiandao Lake with static floating chamber method and headspace equilibrium method, so as to measure the exchange fluxes of GHGs at the water-air interface, as well as the dissolved concentrations of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) in water samples from the water-sediment interface and upper/lower water columns.The results show significant seasonal variations in GHG emissions at the water-air interface. CO₂ was absorbed in summer but emitted in all other seasons, with an annual average flux of (62.840±5.050) mg·m⁻²·h⁻¹. Its variation was significantly negatively correlated with chlorophyll a (Chl a) and chemical oxygen demand (COD) (p<0.01), and positively correlated with water temperature (WT) and total organic carbon (TOC) (p<0.05). Both CH₄ and N₂O were emitted throughout the year. The peak CH₄ emission occurred in summer, with an annual average flux of (0.260±0.020) mg·m⁻²·h⁻¹. CH₄ emission was extremely significantly negatively correlated with total nitrogen (TN) (p<0.01), and positively correlated with WT and COD (p<0.05). The highest N₂O flux was observed in winter, with an annual average flux of (0.050±0.010) mg·m⁻²·h⁻¹. N₂O emission was extremely significantly negatively correlated with WT and COD (p<0.01), and negatively correlated with Chl a and TOC (p<0.05).The water-sediment interface acts as the primary production zone for greenhouse gases (GHGs). The annual average concentrations (mg∙L⁻¹) of CO₂, CH₄ and N₂O were approximately 21.510±13.000, 0.032±0.014 and 0.011±0.009, respectively. The dissolved concentrations of GHGs in the upper water column remained at a low level and stable throughout the year, with the annual average concentrations (mg∙L⁻¹) of CO₂, CH₄ and N₂O being 9.671±4.790, 0.014±0.004 and 0.009±0.008, respectively. The lower water column functioned as a transition zone for GHG production and accumulation, where the gas concentrations were generally higher than those in the upper water column and exhibited significant seasonal fluctuations. The annual average concentrations (mg∙L⁻¹) of CO₂, CH₄ and N₂O in this layer were 18.240±7.480, 0.016±0.007 and 0.008±0.003, respectively. The results indicate that temperature is the key driving factor regulating GHG emissions from fishery reservoirs. Silver carp (Hypophthalmichthys molitrix) and bighead carp (Aristichthys nobilis) can directly and indirectly modulate the patterns of GHG emissions by altering aquatic environmental factors through their biological activities.