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

very few methodological tools for monitoring sessile benthic invertebrates associated with hard substrates, although the tools that do exist are excellent [141]. The environmental infor- mation provided by these sessile benthic species is significant because it covers a wide range of sessile organisms that are long-lived and sensitive to changes in the environment. These spe- cies can be used as reliable ecological sentries that keep watch over the quality of littoral envi- ronments because they are not able to flee or be displaced (as adults) when conditions deteriorate or significantly change. The effectiveness of this monitoring method was demonstrated over the 10-year period used to obtain a complete quantitative temporal series. As indicated by Gatti [87,142], such series are very useful for evaluating both reference conditions and environmental changes in ecosystems. Indeed, the main macrobiota species in the sublittoral community in this study were stable and remained in excellent condition from 2005 to 2014. The use of images is advantageous because, unlike the removal of physical samples by scrap- ing, it is neither destructive nor aggressive to the surroundings. The scraping approach is not effective as a monitoring method in zones in which the fauna is distributed in patches because it entails the destruction of relatively large areas [106]. In protected locations and areas of spe- cial interest, this scraping option is particularly undesirable. The present image-based monitoring method is “low cost” in addition to being “low effort”

and requires minimal maintenance (repairs or replacements of the fixed quadrats were neces- sary only twice over the 10-year monitoring period). Panayotidis [22] indicated that the use of simple monitoring programs (i.e., those involving taxonomic efforts that are limited to repre- sentative species and simple statistical treatments) and low budget methods is demonstrably effective for other rocky bottom communities. According to recent studies, the minimum replication area required for biodiversity studies of communities dominated by Paramuricea clavata [143], as well as other hard-substrate ben- thic invertebrates [92, 144,145], is less than the area used in the replicates in our present work. Additionally, the captured images remain available for subsequent, more detailed descriptions of the community. Higher quality images facilitate the visual identification of more of the macrobenthic species that are present in the community, and the system of photo-quadrats is more efficient than in situ visual census methods [92] while simultaneously avoiding the destruction of the monitored areas. As a final summary, Table 3 presents a comparison of the data from the monitoring area of the present study with the data from several other studies of coralligenous bottoms in terms of time and depth. Accounting for both area and time, the present study represents a long-term monitoring of coralligenous assemblages. Table 3. Comparison of the data of monitoring area, time and depth of several studies on coralligenous bottoms. Study Location Area (m2) Depth (m) Time (years)

[139] Peckol P (1984) North Carolina 0.132 20 2 [51] Garrabou J (2000) Medes Islands 1.085 15–19 2 [44] Garrabou J (2002) Marseilles, France 0.4 27 21 [66] Garrabou J (2002) NW Mediterranean 0.372 17–20 2 [146] De Biasi A M (2004) Aegean Sea 33.6 10–35 1 [112] Bussotti S (2006) Southern Italy 13.25 6–8 1 [147] Parravicini V (2009) Ligurian Sea 25 4–5 1 [36] Piazzi L (2017) Ligurian Sea 180 30–40 1 [92] Sant N (2017) Cabrera Archipelago 4.96 0–50 1 Present study 8 27–30 10 https://doi.org/10.1371/journal.pone.0231641.t003 PLOS ONE Detect coastal disturbances in coralligenous PLOS ONE | https://doi.org/10.1371/journal.pone.0231641 May 5, 2020 14 / 23