Abstract: Dissolved inorganic carbon (DIC) formed by carbonate weathering in karst water has a significant fertilization effect on aquatic photosynthetic organisms (mainly phytoplankton), which can promote the fixation of inorganic carbon through the biological carbon pump (BCP) and increase carbon sinks. However, the source-sink attributes and their driving mechanisms in karst lakes and reservoirs under different temporal scales and disturbance events remain unclear. Taking Tianchi Lake in Huaying Mountain, Sichuan Province as the study area, we combined high-frequency online monitoring, the Bookkeeping model and the generalized linear mixed model to investigate the dynamics and driving mechanisms of net ecosystem productivity (NEP, net carbon sink) under the DIC fertilization effect at multiple scales (annual, seasonal, diurnal, and rainfall events). The results showed that the annual NEP of Tianchi Lake was 9.15 g C·m^-2·a^-1 during the monitoring period, with significant seasonal variations. The net carbon sink in summer was 53.86 g C·m^-2, which could offset the total carbon emissions in spring, autumn, and winter (NEP was -6.74, -36.95 and -4.02 g C·m^-2, respectively). On the diurnal scale, there was a pattern of “carbon fixation during daytime and carbon emission during nighttime”, with the largest amplitude in summer (64.06 g C·m^-2) and the smallest in winter (2.30 g C·m^-2). Rainfall events drove NEP to form a three-stage pulse response of “inhibition-rebound-decay”, with the increases of NEP after heavy rainstorm (10.41 g C·m^-2·d^-1) being 9.7 times that of moderate rain event (1.07 g C·m^-2·d^-1). On the annual scale, solar radiation dominated the changes in carbon sink/source. On the seasonal scale, carbon limitation in summer was dominated by the coupling effect of “high pH and low CO₂ partial pressure”, with a contribution rate of more than 80%; other seasons were affected by solar radiation and temperature factors. On the diurnal scale, the periodic rhythm of solar radiation was the core driver. Rainfall events alleviated carbon limitation through exogenous DIC input and pH decrease, thus driving the dynamic changes of NEP.

Key words: karst carbon sink, biological carbon pump (BCP), dissolved inorganic carbon (DIC), multi-scale, Tianchi Lake of Huaying Mountain