Etude in situ haute fréquence des interactions entre herbiers et processus hydro-sédimentaires

Seagrasses are known to reduce currents and turbulence near the bottom, but they also play a key role in the sediment dynamic in coastal environments. The Arcachon lagoon shelters the largest European seagrass meadow, mainly Zostera noltii, but declining since the late 80s. The EMPHASE EC2CO project was conducted to investigate the interaction between seagrasses and hydrodynamics and sediment processes, and examine possible consequences this decline may have on the sediment balance within the lagoon. As part of this study, three in situ field campaigns were conducted on two close sites (vegetated and bare mud) for different growth stages during a fortnightly cycle in order to examine interactions between the vegetation, hydrodynamics and sediment processes. Results showed i) a significant attenuation (up to 95%) of the bottom currents correlated with the seagrass growth, ii), the development of a “skimming flow” characterized by increased turbulence in its limit lower, and iii) a reduction of sediment resuspension compared to the unvegetated sediment. The current orientation was used to assess the origin of the particles in suspension, but failed to clearly identify sediment movements above the tidal flats.