Detect coastal disturbances and climate change effects in coralligenous community through sentinel stations

Moreover, the use of representative taxa as proxies for entire communities has proven to be a reliable alternative for evaluating the state of rocky bottom communities comprising algae because the loss of information associated with the identification of only some specific taxa does not greatly alter the results compared with results obtained with complete datasets based on entire communities [3]. In hard bottom invertebrate communities, it has also been found that sampling efforts can be reduced without significant losses of information via the selection of indicator and representative species [129]. A. calycularis, P. clavata, and Eunicella sp. (which were clearly more abundant in the moni- tored vertical enclaves) play an important structural role in coralligenous assemblages. These species colonise both horizontal bottoms and vertical walls and offer ideal habitats for numer- ous other organisms [43, 130, 131]. Several previous studies of coralligenous zones in different areas of the Mediterranean, such as the coasts of Italy and France, have determined that the facies of Paramuricea and Eunicella are distinctive [87, 132]. None of these species exhibited significant cover alterations over the entire monitoring period. Although P. clavata and Eunicella sp. have similar biological and ecological characteris- tics, including similar rates of growth and production [133–135], this similarity does not extend to their turnover rates. For P. clavata, this rate oscillates between 7 and 9 years [133,135],

whereas for Eunicella sp., the rate is significantly lower with a range of 3 to 4 years [134]. For this reason, during the long-term temporal monitoring of fixed, limited points, we would expect Eunicella sp. to exhibit greater oscillations in cover percentages than those that we observed due to mortality among the colonies within the monitoring quadrats. Nevertheless, this phenome- non was not recorded in our work, possibly because the magnitude of the studied area allowed for the replacement of the dead organisms by new recruits to buffer the potential differences. Also, it is important to point out that the two main indicator species, A. calycularis and P. clavata (particularly the latter), are sensitive to changes in normal temperature conditions [122, 136–138], although they both withstand seasonal fluctuations well [120]. The differences found in the covers of certain species can also be explained by natural pro- cesses. Mesophyllum alternans is among the organisms responsible for the greatest percentage of cover (particularly in the horizontal orientation), but it is considered to be opportunistic [51], which might explain any oscillations in cover in the absence of any significant alterations to the reference species. The other two species that exhibited significant differences, i.e., Salma- cina incrustans and Crambe sp., are organisms that account for very low percentages of the overall cover. Variations in abundances in deep rocky bottom communities are usually much more noticeable among minor species than among the highly abundant or characteristic spe-

cies [139]. In any case, these two species can be classified as tolerant and relatively resistant to environmental impacts [43]. Recent studies of coralligenous areas have only used vertical enclaves [132], but, despite com- prising the same species, the biological communities that occur at the two orientations are clearly different due to the varying percentages of cover formed by those species. Encrusting algae domi- nate horizontal enclaves, whereas populations of P. clavata and A. calycularis are more abundant on vertical walls; these differences have previously been mentioned by other authors [120, 140]. Therefore, wherever possible, stations should be set up in both orientations, although vertical cor- alligenous enclaves provide more information because they have greater biodiversity, are better structured spatially and trophically, and have greater numbers of sensitive colonial species. Applications of fixed quadrats for the monitoring of rocky bottom habitats The method tested here would be particularly useful for the WFD/MSFD in relation to the environmental control of littoral zones based on biological indicators. The WFD/MSFD has PLOS ONE Detect coastal disturbances in coralligenous PLOS ONE | https://doi.org/10.1371/journal.pone.0231641 May 5, 2020 13 / 23