Ocean wave reflection by seagrass patterns may protect coasts against flooding

• New study shows that spatial patterns in seagrass meadows such as Posidonia oceanica can be explained by the interaction of seagrass growth and ocean wave reflection.
• The researchers propose that this mechanism may improve the capacity of these coastal ecosystems to reflect off wave energy, thus protecting coasts from flooding and erosion.

A new study by researchers from IFISC (CSIC-UIB) and IMEDEA (CSIC-UIB), published in the journal Scientific Reports, shows that spatial patterns in seagrass meadows such as Posidonia oceanica can form through interactions between seagrass growth and incoming ocean waves. The authors propose that this mechanism can increase the potential of these coastal ecosystems to reflect ocean wave energy, which may amplify their value as nature-based coastal protection against the increasing flood risks associated with climate change. 

Seagrass meadows are found globally and provide vital ecosystem services, such as carbon storage, water quality improvement, habitats for many marine species, and coastal safety by damping ocean waves and reducing shoreline erosion. In the Mediterranean Sea, Posidonia oceanica is one of the species that fulfills these ecosystem functions. However, anthropogenic pressure is threatening the survival of seagrass ecosystems worldwide. Deeper understanding of their functioning is therefore required to safeguard their persistence. 

The team of scientists has found that striped and hexagonal spatial patterns of alternating high and low seagrass densities can be explained by interactions between ocean wave propagation, seagrass growth, and seabed topography. The researchers observed in Posidonia oceanica meadows around the islands of Mallorca and Sardinia that the scale of these spatial patterns is proportional to the lengthscale of incoming ocean waves. Based on this finding, the team developed a mathematical model that simulates how ocean waves, propagating towards the coast, partially reflect when travelling over a seabed with seagrass-induced height differences. Interactions between the incoming and reflected water waves create a spatial pattern in water flow speeds, leading to local seagrass growth where flow speeds are low and local seagrass mortality where flow speeds are high. Since seagrass presence reduces sediment erosion and leads to vertical sediment buildup, this water flow pattern creates a pattern of alternating high and low seabed elevation, coinciding with high and low seagrass density. This seabed pattern, in turn, strengthens ocean wave reflection, triggering a self-reinforcing feedback loop that further amplifies the seagrass and seabed pattern over time. Dependent on water depth and wave conditions, seagrass meadow patterns have elevation differences between 1 and 10 meters, have high and low regions spaced out between 10 and 100 meters, and shape seagrass meadows that cover several square kilometers of seafloor. 

The authors conclude that the formation of spatial patterns allows seagrass meadows to more strongly reflect back the energy of incoming waves. This adds to their value as natural forms of coastal protection against the global change-induced increase in ocean wave power. This study further emphasizes the need to protect these crucial coastal ecosystems, and may inspire coastal managers to better employ seagrass meadows as nature-based flood defenses. 

van de Vijsel, R. C., Hernández-García, E., Orfila, A., & Gomila, D. (2023). Optimal wave reflection as a mechanism for seagrass self-organization. Scientific Reports, 13(1), 20278. https://doi.org/10.1038/s41598-023-46788-4. 

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