High thermodynamical sensitivity of CO emissions from a large oligotrophic-hardwater lake (Nam Co) on the Tibetan Plateau.

The Tibetan Plateau (TP) has the world's largest distribution of high-alpine and saline (generally hardwater) lakes, which are expected to affect regional carbon cycling profoundly. However, the variability, and especially underlying factors controlling CO dynamics, across widespread hardwater lakes is poorly understood on the TP. Here, we present year-round records of surface water pCO from a representative hardwater lake (Nam Co) on the TP, and analyze relationships between ambient variables and pCO during open water (i.e., ice-free) and ice-covered months. Surface pCO (233.3 μatm on average) was a little oversaturated to atmosphere (219 μatm on average) during the open water season. As a CO source, Nam Co emitted 8.73 ± 1.06 Gg C annually, but this flux only accounted for 0.53 ± 0.06 ‰ of its total dissolved inorganic carbon pool (1.64 × 10 g C). Regression results indicate that, during open water months, both seasonal and diurnal varying patterns of surface pCO were influenced predominantly by water temperature, in a quasi-marine mode, by controlling gas solubility and dissolved carbonate equilibria. Therefore, CO evasion was elevated during summer months, despite the lake being autotrophic (i.e., CO consumption via photosynthesis). By contrast, during ice-covered months the surface pCO was strongly related to under-ice thermodynamics, and declined nonlinear with increased inversed stratification. In the hypolimnion, as a result of extremely weak metabolism (as indicated by low dissolved oxygen depletion rates) and a combined high carbonate buffering effect, accumulation of CO was negligible, leading to an absence of peak effluxes of CO during turnover periods, compared to eutrophic freshwater lakes. We argue that, under future global warming scenarios, consideration of the impact of gradually warming lake water on thermodynamics and dissolved carbonate equilibria are vital in order to understand the future CO dynamics of these widespread high-altitude oligotrophic-hardwater lakes situated across the TP.