Estuaries are highly productive ecosystems that play an important role in carbon fixing. The amount of carbon fixed by temperate brackish marshes depends, among others, on the biomass produced, its decomposition and the organic carbon stored in the soil. Here, we assumed that the functional trait composition of the vegetation both responded to environmental drivers and affected production, decomposition rate of native biomass and soil organic carbon, in addition to direct links between environmental drivers and these ecosystem properties. We tested a set of detailed hypotheses with a partial least squares structural equation model and quantified wave height, inundation period, salinity, soil nutrients, species abundances and as traits the leaf area, specific leaf area, stem bending properties and investment per plant organ, as well as above-ground standing biomass, decomposition rates of native and control biomass (hay) and soil organic carbon (SOC).

There was no direct relationship between environmental drivers and ecosystem properties, except with the decomposition of control hay. All other linkages involved the functional composition of the vegetation. Increasing inundation period simultaneously decreased plants traits like stem stiffness, leaf area, specific leaf area and mass fraction of stems and leaves, which were aggregated in a new trait ‘aboveground module’. Total plant biomass decreased in communities on higher elevations with reduced wave height. Here, increased investment in the aboveground trait module resulted in higher aboveground community biomass and SOC but decreased the decomposition of native plant material, which in turn was higher in the low lying vegetation zones. Increasing total plant biomass also increased SOC, but not aboveground community biomass. Both the aboveground trait module and total plant biomass were response and effect traits, by responding to the environmental drivers and affecting carbon related ecosystem properties.

At lower elevation with higher inundation and higher wave height, SOC was lower than the product of standing biomass and decomposition rate, which could be a proxy of the expected SOC. This may indicate an export of dead plant material to higher elevations or further downstream.

Due to sea level rise, the area of estuarine vegetation may decrease because a fixed dike line along the river prevents landward migration, which can reduce the amount of biomass produced and with it the potential to store SOC. Restoration of the tidal marshes or realignment of the dikes may be necessary to protect the ecosystem properties and services of estuarine vegetation.