The large penumbra: Long-distance effects of artificial beach nourishment on Posidonia oceanica meadows

- We evaluate environmental impact of artificial beach nourishment on seagrasses.
- No lethal effects on seagrasses were detected within two years of the nourishment.
- Nourishment almost doubled seagrass vertical rhizome growth.
- On the long term, low frequency storms can cause critical burial of seagrasses.

We used modelling and field approaches to assess the influence of beach nourishment on a relatively distant Posidonia oceanica seagrass meadow in the NW Mediterranean. Both sediment transport models and in situ observations showed that, two years after the nourishment and under wave climates prevalent during the study period, sediment movement was restricted to shallow waters (<7 m), above meadow shallow limit. The only significant impact on seagrass meadows during this period was an increase in fine sediments, associated with vertical rhizome growth rates 1.5-1.7 times higher than normal. However, running the model with data of wave climate over several decades indicates that strong storms able to transfer these sediments much deeper, potentially burying meadows, occur with a return period of about 25 years. Taken together, our results suggest that beach nourishment could result in significant sub-lethal and lethal consequences for seagrasses that may go unnoticed with short-term evaluations.