Tidal effects on ecosystem CO₂ exchange in a Phragmites salt marsh of an intertidal shoal

Understanding the mechanisms and controlling factors of ecosystem CO₂ exchange in tidal wetlands is of great benefit for research concerning the global carbon cycle and climate change. In spite of this, the multiple controls of ecosystem-atmosphere CO₂ exchange in coastal wetlands subject to subdaily tidal flooding have yet to be adequately addressed. In this study, we investigated the tidal influence on ecosystem CO₂ exchange in a Phragmites salt marsh of an intertidal shoal in the Changjiang estuary, based on eddy covariance (EC) measurements. The results revealed that the study area acted as a strong sink for atmospheric CO₂ (net ecosystem exchange, NEE = -901 g C m⁻²) in 2018. Photosynthetically active radiation (PAR), air temperature (T_a), and vapor pressure deficit (VPD) were major drivers of NEE on both diel and multi-day scales. The tides, along with the bio-meteorological variables, strongly affect ecosystem photosynthesis (gross primary production, GPP) and ecosystem respiration (R_eco) in the tidal wetland, especially on the multi-day scale. Regardless of which flux partitioning method was utilized, tidal inundation generally imposed inhibitory effects on GPP, which were directly attributed to tidal water level (TWL) and salinity. The daytime data-based estimates of R_eco was also suppressed on average under the tidal inundation condition when T_a was higher relative to the non-inundation condition, reflecting the influence of TWL on R_eco and the reduced sensitivity of R_eco to T_a. We observed that NEE responded positively or negatively to tidal flooding, depending on the magnitude of tidal suppression on GPP and R_eco. When T_a was roughly between 28 and 32°C and PAR was > 1200 μmol m⁻² s⁻¹, GPP was suppressed by tides more than R_eco during the early and rapid vegetative stage, while during the peak vegetative stage, the opposite was true. This study not only shows the unique impact of tidal salt marsh wetlands on carbon uptake, but it also represents an example of a coastal wetland in which tidal inundation promotes the net uptake of CO₂.