BACI design reveals the decline of the seagrass Posidonia oceanica induced by anchoring.

The key species Posidonia oceanica is the dominant endemic seagrass in the Mediterranean Sea and its meadows are considered as one of the most important and productive ecosystems in coastal waters. Covering the seabed from the surface down to about 40 m, meadows of P. oceanica are often affected by mechanical direct damages caused by boat anchoring and mooring activities. Negative effects of these activities have been shown to be recorded by P. oceanica at two different levels: the individual level (phenology of the plant) and the population level (structure of the meadow). We investigated the effect of an anchoring chain system on the P. oceanica meadow of Prelo cove (NW Mediterranean Sea) at two different depths (shallow, deep) and at three different situations of P. oceanica bottom cover (high, medium, low). Several standardized descriptors of the meadow health, working either at the individual or at the population level, were analysed in order to quantify the impact of the deployment of the chain system. A symmetrical BACI design was adopted to detect the impact, where multiple disturbed sites were contrasted with multiple controls in two distinct times, i.e. right few days after the chain settling (early) and 4 months later the disturbance (late). The anchoring chain system has been shown to strongly affect the meadow in terms of shoot density decline and rhizome baring, especially in the deep portions and where the cover of the meadow was low. All descriptors working at the population level proved effective in detecting the impact of the anchoring system. In contrast, descriptors working at the individual level did not show a consistent response to the impact. Our results pointed out the imperative necessity to proper regulate the boat anchoring and mooring activities on the P. oceanica meadows and the adoption of seagrass friendly mooring technology is thus recommended.