Community-based environmental monitoring is a participatory approach engaging volunteers through citizen science (CS) programs to enhance the ability of decision-makers and non-government organizations to monitor and manage natural resources, track endangered species, and protect biodiversity (Conrad and Hilchey, 2011; Chandler et al., 2017). Therefore, community-based monitoring engages citizen scientists and other stakeholders in the ecosystem-based management of natural heritage, not only aiming to increase the chance of obtaining biodiversity data for conservation purposes but also to raise public awareness and support for environmental protection (Keough and Blahna, 2006; Freiwald et al., 2018; Alexander et al., 2019). Marine citizen science (MCS) may provide a valuable contribution to community-based monitoring in marine environments, given the vastness of the world's oceans and coastlines and the diversity of their habitats, communities, and species (Thiel et al., 2014; Garcia-Soto et al., 2017). Involving millions of people worldwide, MCS programs are becoming increasingly important to conservation science not only by providing monitoring of biodiversity, ecosystem services, and functions but also by influencing and improving the management of marine protected areas and fishery resources (Freiwald et al., 2018). Despite a worldwide increase in program number and extent of MCS (Thiel et al., 2014), the collected information is rarely used for institutional monitoring programs or to inform decision-making processes in marine conservation (Conrad and Hilchey, 2011). This disconnect is partly due to persisting skepticism about the reliability of data collected from volunteers (Burgess et al., 2017) and a co-creation approach in the supply and demand of environmental monitoring data that is still not well-integrated in CS processes (Bonney et al., 2015). However, many studies demonstrate that well-trained citizens can provide valuable data on marine environmental issues and that suitable protocols for volunteer projects can provide results consistent with the methods used by the professional researchers (e.g., Holt et al., 2013; Forrester et al., 2015; Done et al., 2017). Still, there are limits to accessing the MCS data (Thiel et al., 2014), which are not always well-organized and readily available according to the FAIR (findable, accessible, interoperable, and reusable) data principles (sensu Wilkinson et al., 2016).

Here, we describe a well-established MCS protocol i.e., the Reef Check Mediterranean Underwater Coastal Environment Monitoring (RCMed U-CEM) protocol, which is refined and applied since 2001 by trained snorkelers, freedivers, and scuba diver volunteers (hereafter called as EcoDivers) to collect data on the occurrence, distribution, abundance, and bathymetric range of selected key marine species along the Mediterranean Sea coasts. The obtained dataset is openly accessible across different platforms and allows for various uses, such as complementing scientific papers on the species distribution and abundance, aiding distribution modeling, and comparing historical series (Lucrezi et al., 2018 and references therein). The protocol is implemented, and the data are maintained by the non-profit organization Reef Check Italia onlus, collaborating with the other European Reef Check organizations, members of the worldwide Reef Check Foundation, and within the Reef Check Mediterranean Sea network.

Reef Check Mediterranean Underwater Coastal Environment Monitoring (RCMed U-CEM) protocol is intended to collect data on the abundance, and geographical and bathymetric distribution of selected taxa. It requires trained participants (certified EcoDivers) to collect standardized data and send it to the online database. The data are then processed and made available on the various open access sharing platforms.

As of December 2020, the dataset consisted of 50,255 records (including absence records) unevenly distributed among 43 taxa in the Mediterranean Sea collected in 4,898 single survey events from 2001 and carried out by 692 EcoDivers (Ponti et al., 2021; Turicchia et al., 2021b). The data comes from Croatia, France, Greece, Italy, Spain, and Tunisia, covering part of the following ecoregions (sensu Spalding et al., 2007): Western Mediterranean (52.3% of the surveys), Adriatic Sea (42.2%), Ionian Sea (4.9%), Alboran Sea (0.2%), Aegean Sea (0.2%), and Tunisian Plateau/Gulf of Sidra (0.2%). The possibility to focus on a few target species during the underwater surveys ensures a higher accuracy of the data collection: EcoDivers select the species based on confidence (thereby reducing identification errors), personal interest (increasing satisfaction), or because some species are more charismatic than others (KrŽelj et al., 2020). Although never assessed, a reduced number of species to consider may reduce psychological stress during the surveys; however, this generates skewed distribution efforts among the searched taxa and surveyed coastal habitats. Indeed, the most-searched taxa are common species like seabreams (Diplodus spp.) and the edible sea urchin (Paracentrotus lividus), the noble pen shell (Pinna nobilis), the red coral (Corallium rubrum), and sea fans (Paramuricea clavata and Eunicella cavolini). Less conspicuous but highly concerning species, such as invasive algae in the genus Caulerpa, are also frequently surveyed (Figure 6A). As expected, the most investigated habitats are those most attractive to divers, namely, rocky coasts, cliffs, and wrecks (Figure 6B).

Over the years the dataset has been successfully used to complement studies on the spatial and the temporal distribution of key marine species such as the habitat-forming corals in the central-eastern Mediterranean (Özalp and Alparslan, 2016; Di Camillo et al., 2018), the pink sea-fan Eunicella verrucosa (Chimienti, 2020), as well as of rare and/or endangered species like the gold coral Savalia savaglia (Giusti et al., 2015), the zooxanthellate soft coral Maasella edwardsii (Özalp and Ateş, 2015), and the sponge Geodia cydonium (Turicchia et al., 2013). These data can also help in tracking mass mortality events and assess the possible effects of climate change (Pairaud et al., 2014; Ponti et al., 2018; Turicchia et al., 2018; Garrabou et al., 2019) and the invasion of the non-indigenous species Caulerpa taxifolia and Caulerpa cylindracea (Montefalcone et al., 2015; Cerrano et al., 2017). Moreover, the dataset can be used in assessing the protected and sensitive species richness within the marine protected areas (Turicchia et al., 2015, 2016; Cerrano et al., 2017), and in offering an effective monitoring tool for the Mediterranean subtidal rocky coastal habitats through the MedSens biotic index (Turicchia et al., 2021a).

The RCMed U-CEM protocol integrates the Reef Check family of protocols, which already includes those for the California coasts (Gillett et al., 2012) and tropical coral reefs around the world (Hodgson, 2001) extending its range to the Mediterranean Sea. These standard protocols aim to report the presence and the abundance of key species and to assess and monitor over time the ecological status of the investigated sites based on the relative abundance of selected indicator species. They are optimized for the habitats and operating conditions for which they are intended. In tropical reefs and California, standard transects are made in shallow waters under the supervision of a scientist and/or a team leader, while in the Mediterranean, volunteers independently apply a roving visual census that allows them to explore from shallow to deep habitats (see further comparisons in Table 5). RCMed U-CEM is not the only protocol that can be adopted by snorkelers and divers to report marine species in the Mediterranean Sea; however, the aims and methods applied differ widely. Among those listed by Earp and Liconti (2020), the most popular and internationally applied alternatives in the Mediterranean Sea are: REEF, Sea Watchers, and iNaturalist. Their main features are summarized in Table 5 for comparison. REEF, started in 1993 with the fish visual census in the tropical western Atlantic, was one of the forerunners in involving volunteer divers (Pattengill-Semmens and Semmens, 2003). Over the years, the protocol has been extended to various regions of the world, and since 2014, it also includes the Eastern Atlantic and the Mediterranean Sea. As RCMed U-CEM, REEF is based on roving visual census carried out by trained volunteers. While the main focus of REEF's program is marine fish, they also survey selected invertebrates and algae in the temperate water regions. In addition to the different lists of considered species, the major difference between the two protocols lies in the less explicit and not always precise location of the sites (based on hierarchical area codes and not geographic coordinates), the lack of information on the bathymetric distribution of species, and the reduced findability, accessibility, and interoperability of the REEF data. Sea Watchers, launched in Spain in 2009 as “Observadores del Mar,” includes several thematic sub-projects (e.g., massive mortalities of corals, death of pen shells, alien fishes, invasive algae, decapod crustaceans, sharks and rays, and zooxanthellate scleractinian; see Mariani et al., 2018); they are based on visual observations supplemented by photographs that are sent to experts for identification and analysis. Involved experts assess and archive the observations, possibly integrating with additional information interactively requested to the participants. Data collected from some sub-projects are already distributed by EMODnet or other data portals. iNaturalist, which began in 2008, is the widest social network of nature enthusiasts that share and cross-validate photographic observations and it is one of the more FAIR initiatives worldwide (Bowser et al., 2014). Unfortunately, despite initiatives on marine organisms are multiplying within it (Reef Check Italia has launched two calls for contributions on Mediterranean nudibranchs and corals), most of the collected data concerns terrestrial species. In addition, many of the marine species' records lack important additional data such as observation depth or surveyed habitat. Notably, except for RCMed U-CEM, none of the other initiatives mentioned here for the Mediterranean Sea provide information on the absence of key species, a “dark diversity” (sensu Partel et al., 2011) usually neglected but potentially very useful in assessing biological integrity, ecological status, and the effects of global changes.

The application of the RCMed U-CEM protocol may generate a range of direct societal impacts, including a higher public awareness of environmental threats and the involvement of stakeholders (e.g., tourists, divers, and diving centers) in the monitoring and conservation of coastal marine environments (Turicchia et al., 2021a). Therefore, it may enhance the collaboration between coastal management authorities, stakeholders, and researchers, increasing the acceptability of management decisions and enabling more participatory conservation tactics (Markantonatou et al., 2013; de Francesco et al., 2017; Lucrezi et al., 2018).

To obtain and distribute reliable and scientifically sound data, RCMed U-CEM protocol was designed to minimize taxonomic and geolocation errors. However, taxonomic and spatial biases, recognized as major issues in CS projects and biodiversity databases, remain intrinsically unavoidable for this and most CS initiatives (Beck et al., 2014; Troudet et al., 2017). New technologies may help improve the protocol and reduce the possibility of common errors. For example, the widespread use of the smartphone app has already reduced localization errors, thanks to the integrated GPS. The increasing adoption of waterproof cases for smartphones and tablets suggests the possibility of entering data directly during the in-water activity and collecting photos to verify the species identification later using a specifically preconfigured app (Max and Gualdesi, 2013). Advances in image analysis and deep learning algorithms coupled with photo databases can support the development of apps for identifying marine species, as has been the case for identifying plants and terrestrial animals (e.g., the app Seek by iNaturalist; see Waldchen and Mader, 2018 for a review about automatic terrestrial plant identification), and this technology could be implemented on the Reef Check Med app and possibly used directly underwater.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

MP conceived the early version of the protocol, which was refined together with CC, GR, CGDC, and ET. MP and ET wrote the first draft of the manuscript. All authors contributed to and approved the final version of the manuscript.

MM was employed by company Studio Associato Gaia Snc. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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