Temporal variability and within‐plant heterogeneity in blade biomechanics regulate flow‐seagrass interactions of Zostera marina

Seagrasses are marine flowering plants that have important roles in the ecological and physical processes of many coastal areas. Seagrass modelling to date has mostly assumed that seagrasses have uniform biomechanical traits in space and time. In this study we compare the biomechanical traits of Zostera marina blades collected in late summer and spring from a lagoon in southern Denmark. Then, we describe how biomechanics vary depending on (i) seasonality, (ii) storage in laboratory conditions with high nutrient levels, (iii) blade rank and (iv) position along blades. The data collected with these direct measurements are fed into a numerical structural model that simulates seagrass response to an idealized flow and accounts for plant non uniformity. The model is used to assess the effects of temporal variability and within-plant heterogeneity in blade biomechanics on flow-seagrass interactions. Results show that seagrass biomechanics are affected considerably by seasonality and laboratory storage. This biomechanical variability has a key role in defining flow-seagrass interactions, enhancing light availability in summer and reducing potential drag force in spring. Significant within-plant heterogeneity associated with both blade rank and along-blade position is reported. Compared to temporal variability, within-plant heterogeneity has a secondary role in determining flow seagrass interactions; however, blade rank is associated with a consistent reduction in the drag force. The results presented improve the understanding of flow-seagrass interactions by clarifying the importance of variations in seagrass blade biomechanical traits and their origin.