One of the most frequently quoted ecosystem services of seagrass meadows is their value for coastal protection. Many studies emphasize the role of above-ground shoots in attenuating waves, enhancing sedimentation and preventing erosion. This raises the question if short-leaved, low density (grazed) seagrass meadows with most of their biomass in belowground tissues can also stabilize sediments. We examined this by combining manipulative field experiments and wave measurements along a typical tropical reef flat where green turtles intensively graze upon the seagrass canopy. We experimentally manipulated wave energy and grazing intensity along a transect perpendicular to the beach, and compared sediment bed level change between vegetated and experimentally created bare plots at three distances from the beach. Our experiments showed that i) even the short-leaved, low-biomass and heavily-grazed seagrass vegetation reduced wave-induced sediment erosion up to threefold, and ii) that erosion was a function of location along the vegetated reef flat. Where other studies stress the importance of the seagrass canopy for shoreline protection, our study on open, low-biomass and heavily grazed seagrass beds strongly suggests that belowground biomass also has a major effect on the immobilization of sediment. These results imply that, compared to shallow unvegetated nearshore reef flats, the presence of a short, low-biomass seagrass meadow maintains a higher bed level, attenuating waves before reaching the beach and hence lowering beach erosion rates. We propose that the sole use of aboveground biomass as a proxy for valuing coastal protection services should be reconsidered.

Our study aims to enhance process understanding of the long‐term (decadal and longer) cyclic marsh dynamics by identifying the mechanisms that translate large‐scale physical forcing in the system into vegetation change, in particular (i) the initiation of lateral erosion on an expanding marsh, and (ii) the control of seedling establishment in front of an eroding marsh‐cliff. Short‐term sediment dynamics (i.e., seasonal and shorter changes in sediment elevation) at the mudflat causes variation in mudflat elevation over time (δz_TF). The resulting difference in elevation between the tidal flat and adjacent marsh (ΔZ) initiates lateral marsh erosion. Marsh erosion rate was found to depend on sediment type and to increase with increasing ΔZ and hydrodynamic exposure. Laboratory and field experiments revealed that seedling establishment was negatively impacted by an increasing δz_TF. As the amplitude of δz_TF increases towards the channel, expanding marshes become more prone to lateral erosion the further they extend on a tidal flat, and the chance for seedlings to establish increases with the distance that marsh has eroded back towards the land. This process‐based understanding, showing the role of sediment dynamics as explanatory factor for marsh cyclicity, is important for protecting and restoring valuable marsh ecosystems. Overall, our experiments emphasize the need for understanding the connections between neighbouring ecosystems such as mudflat and salt marsh.