Effects of boat anchoring in Posidonia oceanica seagrass beds in the Port-Cros National Park (north-western Mediterranean Sea)

1. A study was set up in the Port‐Cros National Park in order to determine the effects of boat anchoring on Posidonia oceanica seagrass beds. 2. Experiments on the effects of anchors on the seagrass meadows revealed that, on average, 34 shoots were destroyed during an anchoring cycle (lock‐in and retrieval), especially when the seagrass mat compactness is weak and the extent of rhizome baring is high. 3. Five parameters of the Posidonia oceanica beds (meadow cover, shoot density, extent of rhizome baring, proportion of plagiotropic rhizomes, degree of meadow fragmentation) were considered and it was shown that the extent of rhizome baring was not correlated with anchoring pressure. Meadow cover and mean shoot density were positively correlated with high anchoring pressure. 4. The proportion of plagiotropic (i.e. horizontally growing) rhizomes and the degree of meadow fragmentation were positively correlated with moderate anchoring pressure. Copyright © 1999 John Wiley & Sons, Ltd.