ABSTRACT: In a long-term and a short-term study of 2 estuarine intertidal flats (sandy and muddy), sediment carbon contents were measured every 2 wk over 2.5 yr and daily for a 2 wk period, respectively. During the long-term study, current velocity was measured twice (for periods of 8 wk and 6 wk). During the short-term study, sediment traps were also deployed. In both tidal flats, sediment carbon content fluctuated but had no long-term trend, indicating dynamic equilibrium. Amplitude, period and mechanisms of dynamic equilibrium differed between the 2 tidal flats. In the sand flat, the carbon content of the surficial sediment (0 to 1 cm) fluctuated, ranging from 0.03 to 0.17%. It is likely that physical transport of particulate carbon (e.g. by deposition) increases sediment carbon content under calm conditions, while carbon is washed out of the sediment through sand resuspension during spring tides. These processes are probably responsible for the dynamic equilibrium in the sand flat. In the mud flat, the carbon content of the surficial sediment varied from 0.5 to 2.4%, showing sharp peaks after summer floods, and gradual increases in winter followed by a decrease in spring to a lower limit of 0.5 to 1.0%. In the mud flat, the consolidated sediment layer probably governs the dynamic equilibrium of sediment carbon content: it prevents a long-term increase in carbon content by withstanding the transport of deposited particulate carbon into the sediment layer and destabilizing it on the sediment surface; it also maintains the lower limit of sediment carbon content by compaction of organic matter. Sediment organic content is crucial for benthos occupation; thus, our results suggest that the dynamics of sediment quality in tidal flats should be taken into account when assessing or managing benthic communities of tidal flats.