Treffer: The Importance of Seasonality in Seagrass Properties for Coastal Hydro‐Morphodynamics—A Case Study in a Wadden Sea Basin.

Seagrass meadows fulfill many essential ecological functions, of which an important one is to stabilize sediment. Therefore, they are perceived as a nature‐based addition or alternative to conventional rigid coastal protection. The impact of seagrass meadows depends on their morphology, such as canopy height, shoot density, and spatial extent. However, deciduous, intertidal seagrass species are often simplified in modeling studies by adopting their annual mean height and density. This can lead to an erroneous estimate of their impact on hydro‐morphodynamics and misconceptions about their contribution to coastal protection. Here, we assess the importance of seasonal changes in seagrass properties for morphological development, using a tidal basin in the Wadden Sea as an example. We applied numerical modeling to simulate the annual growth cycle of seagrass meadows and their interaction with hydro‐morphodynamics. Based on validated seasonal changes in seagrass properties from field surveys and comparisons between scenarios of seagrass growth, our results show that adopting static seagrass parameters in modeling can lead to over‐ or underestimation of morphological changes induced by seagrass meadows. In some cases, it may even predict results contrary to simulations that consider seasonal changes in seagrass properties, particularly regarding the net sediment volume change in the intertidal zone. This highlights the essential necessity of considering the natural growth and decline cycles of seagrass meadows when assessing their role in coastal protection, especially in temperate zones where seasonal changes in seagrass properties are distinct. Plain Language Summary: Seagrasses grow on the coast, either fully or temporarily submerged in the water. They have leaves, rhizomes, and roots that stabilize sediment by reducing erosion and increasing sedimentation by trapping sediment particles through a reduction of near‐bottom current velocities. The impact of seagrasses on the sediment movement depends on their properties, such as height, shoot density, and the area they cover. Many modeling studies simplify seagrass properties by using yearly average values, even in areas where the seagrass has a strong seasonality. This study looks at seasonal changes in seagrass properties and their impact on the sediment movement in a Wadden Sea basin. Using numerical modeling, we simulated the yearly growth cycle of seagrass meadows and their interactions with water and sediment movement. Our results, supported by field measurements, show that using year‐round averages for seagrass in models can lead to significant over‐ or underestimations of the changes caused by seagrass in sediment movement. In some cases, simplified models that adopt average values can even give opposite predictions compared to models that include seasonal changes. Therefore, it is essential to include the natural growth and decline cycle of seagrass in models to accurately assess its role in coastal protection. Key Points: Seagrass properties in temperate zones feature distinct seasonal changesSeasonal changes in seagrass meadows are often overlooked in numerical modelingOmission of seagrass seasonality can lead to false assessment of their impact on coastal morphology [ABSTRACT FROM AUTHOR]

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