Effects of a severe storm on seagrass meadows.

Extreme environmental events can strongly affect coastal marine ecosystems but are typically unpredictable. Reliable data on benthic community conditions before such events are rarely available, making it difficult to measure their effects. At the end of October 2018, a severe storm hit the Ligurian coast (NW Mediterranean) producing damages to coastal infrastructures. Thanks to recent data collected just before the event on two Posidonia oceanica seagrass meadows hit by the storm, it has been possible to assess the impact of the event on one of the most valuable habitats of the Mediterranean Sea. By means of seagrass cover and depth data gathered along four depth transects positioned within the two meadows in areas differently exposed to the storm waves, and by using models (WW3® + SWAN + XBeach 1D) to evaluate wave height and bed shear stress, we showed that meadows experienced erosion and burial phenomena according to exposure. Paradoxically, meadows in good conditions suffered more damage as compared to those already suffering from previous local anthropogenic impacts. Besides the direct effect of waves in terms of plant uprooting, a major loss of P. oceanica was due to sediment burial in the deepest parts of the meadows. Overall, the loss of living P. oceanica cover amounted to about 50%. Considering that previous research showed that the loss of the original surface of P. oceanica meadows in 160 years due to anthropogenic pressures was similarly around 50%, the present study documented that an extreme environmental event can generate in a single day a loss of natural capital equal to that produced gradually by more than a century of human impact.

Macroalgal forest vs sea urchin barren: Patterns of macro-zoobenthic diversity in a large-scale Mediterranean study.

The study aimed at contributing to the knowledge of alternative stable states by evaluating the differences of mobile and sessile macro-zoobenthic assemblages between sea urchin barrens and macroalgal forests in coastal Mediterranean systems considering a large spatial scale. Six sites (100 s km apart) were selected: Croatia, Montenegro, Sicily (Italy), Sardinia (Italy), Tuscany (Italy), and Balearic Islands (Spain). A total of 531 taxa, 404 mobile and 127 sessile macro-invertebrates were recorded. Overall, 496 and 201 taxa were found in macroalgal forests and in barrens, respectively. The results of this large-scale descriptive study have met the expectation of lower macrofauna complexity and diversity in barrens rather than in macroalgal forests, and have allowed estimating the differences in levels of diversity and the consistency of variability across Mediterranean sites. Some peculiar patterns in barrens, related to both abundance of specific taxa and to high values of beta diversity, have been evidenced.

Early warning response of Posidonia oceanica epiphyte community to environmental alterations (Ligurian Sea, NW Mediterranean).

The epiphyte community structure of the Posidonia oceanica leaves in three impacted meadows occurring in urbanised coastal areas was contrasted with that observed in three meadows located within Marine Protected Areas in the Ligurian Sea. Samplings were carried out in two distinct periods, at the beginning and at the end of the summer season, in order to individuate early changes in the epiphyte community structure. Differently from the descriptors commonly adopted for evaluating the health status of P. oceanica ecosystem, the epiphyte community structure was able to detect alterations in the water quality already after 4 months. The impacted meadows showed an immature epiphyte community characterised by large development of brown algae. Leaf P. oceanica epiphytes are, thus, proposed as appropriate biological quality elements (BQEs) able to show early responses to environmental alterations and they should be included in the monitoring programs for the conservation and the management of coastal areas.