Tidal effects on ecosystem CO₂ exchange at multiple timescales in a salt marsh in the Yellow River Delta

The tide plays a crucial role in maintaining the carbon sink strength in salt marsh ecosystems. Furthermore, the effects of tides on ecosystem carbon exchange could vary with different timescales. Using the eddy covariance technique combined with wavelet analysis, we analyzed the time-frequency characteristics of net ecosystem CO₂ exchange (NEE) to address the tidal effects on NEE at multiple timescales. The wavelet analysis showed that NEE displayed a tidal-driven pattern with distinct characteristics at the multiday scale (i.e., 8–16 days) and the seasonal scale (i.e., 64–128 days). Moreover, a more significant controlling effect of light rather than air temperature on NEE was found at the diel scale. Tides also affected the variation of the diurnal pattern of NEE. Tidal flooding inhibited nighttime CO₂ emissions (NEE_nighttime) as well as decreased temperature sensitivity (Q₁₀) of NEE_nighttime from 1.37 to 1.16. In contrast, the response of daytime NEE (NEE_daytime) to tidal activities was more complicated, as the NEE_daytime reacted differently with different months during the growing season. Overall, our findings can contribute to a better understanding of tidal effects on ecosystem carbon exchange in salt marshes.