Summer profundal hypoxia determines the coupling of methanotrophic production and the pelagic food web in a subtropical reservoir

We investigated how profundal redox conditions determine the inter-annual variation in methane oxidising bacteria (MOB) contribution to zooplankton production in a deep subtropical reservoir. Two hydrological regimes which affect MOB activity are considered: (i) reduced MOB activity resulting from high profundal oxygen saturation promoted by water column disturbance during the period of summer stratification; and (ii) increased MOB activity due to high profundal oxygen saturation during winter mixing. Four years of field stable-isotope analyses revealed that oxygen saturation of profundal waters during summer stratification was negatively correlated with winter MOB contribution. This relationship is consistent with our theoretical ecosystem modelling. Although the ecosystem model simulation predicted positive effects of profundal oxygen supply during winter mixing on MOB contribution, the stable isotope mixing model indicated that the effects were secondary. Winter oxygen supply did not enhance MOB activity when methane accumulation during the preceding summer was low. Our findings suggest that summer profundal hypoxia plays the primary role in determining methanotrophic food-web activity and methane-derived carbon cycling in deep lakes.