Contribution of microphytobenthos to the carbon sink in brackish and freshwater tidal flats of the Yangtze Estuary

Estuarine and coastal wetlands have great potential as carbon sinks; however, the carbon fluxes that occur in unvegetated tidal flats remain largely unknown. The CO₂ flux of tidal flats is mainly controlled by CO₂ uptake by microphytobenthos (MPB) and CO₂ emission by sediment respiration. In this study, a brackish tidal flat (B-TF) and a freshwater tidal flat (F-TF) in the Yangtze Estuary were selected to investigate the effects of salinity on MPB growth (indicated by the content of chlorophyll a and pheopigment) and the CO₂ flux of tidal flats (during daytime). The results showed that the sediment salinity at the B-TF was 5−19 times higher than that at the F-TF, depending on the season. Diatoms showed a dominant saltwater community at the B-TF and a freshwater community at the F-TF. Over spring, autumn, and winter, CO₂ absorbed by the MPB was on average 113 and 230% of the CO₂ emitted through sediment respiration of the B-TF and F-TF, respectively, showing a CO₂ sink capacity. High salinity in winter inhibited sediment respiration and improved the physiological status of the MPB, making the B-TF a strong CO₂ sink, whereas the F-TF was a strong CO₂ sink in summer. The freshwater and dissolved (SiO₃^2-) silicon input to the estuary via the river had significant effects on MPB growth and sediment respiration, which were also influenced by light, temperature, and pH. Our study highlights the importance of estuarine salinity fluctuations in the CO₂ sinks of estuarine tidal flats, which should be given additional attention when assessing the role of tidal flats in CO₂ mitigation.