Direct and indirect effects of tidal elevation on eelgrass decomposition

In urbanised coastal environments, where shoreline migration is impeded by hard structures, sea-level rise is expected to impact decomposition processes by changing the patterns of tidal inundation. We assessed how tidal elevation directly (by determining the decomposition environment) and indirectly (by determining leaf traits) influences the decay of the south-east Australian seagrass Zostera muelleri. First, we assessed how the standing biomass and quality of Z. muelleri leaves varied across a tidal elevation gradient. Second, we used a litter-bag experiment to assess how the effects of tidal elevation on leaf traits and the decompositional environment interact to influence the decomposition rate. Surveys of 3 estuaries revealed that the size and stiffness of seagrass blades increased with depth, and the carbon and fibre content of blades was greater at subtidal and low intertidal zones than at high intertidal elevations. The differences in leaf traits among tidal elevations were, however, less important than the differences in aerial exposure for controlling the decomposition rate. The litter bags incubated at subtidal and low intertidal elevations had a faster rate of decomposition than those incubated in the high intertidal, irrespective of litter quality. Across elevations, the decay rates of the intertidal seagrass and the stiffer subtidal Zostera were similar. These results suggest that sea-level rise will influence decay processes by changing the decompositional environment rather than by changing tissue quality. Studies investigating differences in litter production among tidal elevations and the capacity of hydrodynamic processes to transport excess litter are now required to facilitate ecosystem-level predictions of the effects of sea-level rise.