Mediterranean macroalgal forests are important habitats, but the map of their condition is surprisingly incomplete. This study turns scattered observations into a basin-wide baseline—and shows exactly where the biggest gaps remain. The worldwide decline of macroalgal forests is raising major concerns because of potentially negative consequences for biodiversity and ecosystem functions, pushing toward specific conservation and restoration measures.

Protecting and restoring these habitats requires detailed information on their distribution, ecological status, and drivers of decline. This study provides updated information on Mediterranean Cystoseira s.l. forests through a comprehensive bibliographic survey of literature published from two thousand nine to two thousand twenty-one, complemented by unpublished data.

The results show that most Mediterranean coasts remain understudied or under-studied, while available information is concentrated in spatially limited coastal areas and restricted to very few species. Reported ecological status is highly heterogeneous, making comparisons problematic and supporting the use of easy, standardized monitoring methods for comparative purposes.

Marine algal forests produce important biomass and create three-dimensional structures of high fractal complexity that host and feed many associated species. These forests represent some of the most productive and biodiverse coastal marine ecosystems in temperate and polar regions, while also providing essential ecosystem goods and services for human society.

In the Mediterranean Sea, mainly Cystoseira, Ericaria, and Gongolaria species shape rocky seascapes from the upper infralittoral to the upper circalittoral zones; the study reports these together as Cystoseira s.l. Many brown canopy-forming macroalgal species are increasingly affected by cumulative local and global anthropogenic stressors, raising major concern for their conservation.

The loss of these forests often triggers a transition toward less diverse and productive habitats, such as sea urchin barrens or turf-dominated grounds, where stabilizing feedback processes reinforce their persistence. Cystoseira s.l.

species are highly vulnerable to several human disturbances, and widespread Mediterranean declines have been linked mainly to habitat destruction, overgrazing, and deterioration in water quality. Macroalgal restoration attempts are less common than those for other marine habitats, although restoration knowledge has significantly progressed through funded projects during the last decade.

The primary eligibility criterion for spatially prioritizing restoration sites should be the historical presence of the habitat or species targeted for restoration. Site suitability also requires assessing current and future biotic conditions, including potentially harmful invasive species and herbivory pressure, together with abiotic conditions such as bottom type, hydrodynamics, and seawater temperature.

Crucial information includes fine-scale data on past and present species distribution, ecological status, and the causal stressors responsible for population declines. Comprehensive knowledge remains limited to a few species and regions, with mostly short-time data series, partly because long-term monitoring and cartographic information are scarce.

This work combines a systematic scientific literature review, unpublished data, and expert judgment to provide an updated picture of accessible knowledge about Cystoseira s.l. forests at the Mediterranean scale. The aims are to overview distribution and ecological status, identify the drivers of forest loss, and determine geographic patterns of those drivers.

The results are intended as an essential baseline for identifying gaps in species and stressor information, and for showing where more investment in funds, monitoring, and sampling effort is needed. The baseline can also assist in determining where impact mitigation should focus and where restoration and conservation plans should be prioritized.

The systematic literature review consisted of relevant article identification, eligibility evaluation, and detailed revision of selected articles. Article identification used Web of Science, SCOPUS, and ScienceDirect, limited to peer-reviewed articles published from two thousand nine to two thousand twenty-one.

The starting cutoff date was chosen to assess the current state of Cystoseira s.l. forests and avoid older publications that might contain outdated data. The same search query was used for all three databases, with searches covering titles, abstracts, and keywords, and with SCOPUS and ScienceDirect also searching the text.

Because Cystoseira s.l. taxonomy has been changing, four consecutive searches replaced the word Cystoseira with Carpodesmia, Treptacantha, Ericaria, and Gongolaria. Because of this reinstatement, most species names used in publications before two thousand twenty are no longer accepted, so the study uses accepted species names according to AlgaeBase.

After the selection process, only one of the different articles was kept for data extraction. As a result, one hundred thirty-eight articles were used for data extraction. For each article, the study extracted available information on Cystoseira s.l.

species and geographical details, including coordinates, site name, region, and country. The extraction also recorded ecological status as Good, Moderate, or Bad, along with stressor presence, stressor type, population dynamics, and sampling year.

To study spatial availability, the compiled data were placed on a grid of ten kilometers by ten kilometers projected over the Mediterranean Sea, keeping only cells that intersected the coastline. Each cell received a distinct colour based on the number of georeferenced points it contained, producing the basis for the data-availability map.

The number of presence, absence, and extinction occurrences was also represented in a bar plot for each Cystoseira s.l. species. Points containing information about ecological status were classified as Poor, Moderate, or Good.

The study assigned numerical values to those categories: Poor received one, Moderate received two, and Good received three. Figure one maps the availability of sampled Cystoseira s.l. populations across the Mediterranean. Each coastal grid cell covers ten square kilometres, and its colour indicates how many georeferenced sampling points it contains, across sub-basins such as the Alboran, Adriatic, Aegean, and Levantine seas.

This matters because the spatial pattern reveals where evidence has been collected—and where geographic gaps may limit assessments of Cystoseira s.l. forests. The study illustrates primary stressors in each sub-basin and outlines their expected impact trends over the next fifty years, referred to as the mid-term.

Primary stressors are those frequently reported or considered to have substantial implications. The expected trends combine information collected in the study with expert consensus in each sub-basin. Of one thousand five hundred thirty-six georeferenced points, one thousand three hundred sixty-one reported the presence of at least one Cystoseira s.l.

species, while absence was explicitly mentioned in one hundred seventy-five points. Among the absence records, fifty specifically indicated local extinction, while one hundred twenty-five did not state whether absence resulted from natural factors or population decline.

Thirty-two percent of points reporting past or present presence were not based on species-level identification and were reported simply as Cystoseira sp. or spp. The data produced two thousand three hundred eighty-two past or present occurrences, covering thirty-two of the thirty-six Cystoseira s.l.

species present in the Mediterranean Sea. Cystoseira compressa, Ericaria amentacea, and Ericaria crinita were the most frequently reported species, while Ericaria funkii, Cystoseira masoudii, and Cystoseira jabukae were each reported in only one location.

Figure two is a bar chart of reported occurrences for Cystoseira s.l. species, separating presence, unspecified absence, and absence attributed to extinction. It includes records such as Cystoseira compressa with three hundred thirty-four presence reports and nineteen absence-due-to-extinction reports, while the broad Cystoseira sp.

or spp. category includes four hundred seventy-nine presence reports and one hundred thirty-three unspecified absences. The chart matters because it shows how the compiled evidence is distributed across taxa and absence types.

Of one thousand four hundred eleven points reporting present or past forest presence, one thousand one hundred seventy-nine, or eighty-three percent, had information on ecological status. Among those points, five hundred ten, or thirty-six percent, were classified as Good, three hundred eleven, or twenty-two percent, as Moderate, and three hundred fifty-eight, or twenty-five percent, as Poor.

Forests in different ecological-status categories were reported in all sub-basins, but Poor status predominated in the Adriatic, Aegean, and Levantine Seas. In the North-Western Mediterranean, the sub-basin with the highest number of occurrences, forests considered Good were the most reported.

Figure three maps the ecological status of Cystoseira s.l. populations across the Mediterranean. Each ten-square-kilometre cell is coloured according to the average status of sampling points inside it, with green indicating good, yellow moderate, and red poor conditions; areas without data are also shown.

The distribution spans all defined sub-basins, while the visible mixture of categories highlights substantial local variation—supporting the authors’ interpretation that ecological changes may be linked mainly to local stressors rather than a single basin-wide process.

Figure four counts records of past or present Cystoseira s.l. forest across Mediterranean sub-basins, separating Good, Moderate, Poor, and Unknown ecological status. The chart shows that all status categories are represented across the sub-basins, while records without ecological-status information remain explicitly identified as Unknown.

This matters because, among the one thousand four hundred eleven reported presence points, status information was available for one thousand one hundred seventy-nine, allowing the authors to map the geographic distribution of ecological condition while keeping data gaps visible.

Information about stressors was the least reported category: only five hundred ninety, or thirty-eight percent, of the one thousand five hundred thirty-six occurrences provided specific information on stressor presence or type. Only five percent, or seventy-six cases, recognized Cystoseira s.l.

populations as not threatened by any stressor. Water quality and habitat loss were the most reported impacts, followed by overgrazing by sea urchins and overgrazing by invasive fishes, including Siganus species. Climate-change impacts were reported less often than those stressors, despite increasing in recent years.

Figure five maps where the main reported stressors affecting Cystoseira s.l. populations are concentrated across the Mediterranean. Water quality and habitat destruction appear broadly distributed, with denser reporting in parts of the western basin, while climate change and grazing stressors show more localized concentrations, especially in the eastern Mediterranean.

The authors emphasize that these kernel-density patterns reflect reporting frequency and intensity, so areas without color may indicate missing documentation rather than an absence of the stressor. None of the sub-basins is expected to experience an improvement in any considered stressor, except for overgrazing by Sarpa salpa in the Aegean sub-basin.

The expected reduction in Sarpa salpa overgrazing is likely due to out-competition with invasive fish species. There is also a prevailing perception that the effects of global stressors will intensify across the Mediterranean sub-basins. Figure six maps reported stressors across eight Mediterranean sub-basins, using shading for detected presence and symbols for the expected direction of change: decreases, no change, or varying levels of increase.

Habitat loss and climate change appear broadly across the region, while several overgrazing pressures are more unevenly documented. The authors emphasize that global stressors are generally expected to intensify, especially in the eastern basins, whereas the Aegean is the only sub-basin showing a possible mitigation of Sarpa salpa overgrazing.

The study highlights the challenge of establishing reliable baselines for present distribution, ecological status, and stressors threatening Cystoseira s.l. forests in the Mediterranean Sea. Although this information is essential for regional conservation or restoration, it is generally scarce, and knowledge is highly limited, spatially skewed, and biased toward a few species.

Certain regions, including Italy, Catalonia, and the Mediterranean French coast, have received more research attention, while information and long-term monitoring for much of the remaining Mediterranean coastline are lacking. Most available information relies on surveys providing scattered spatial data without information on population area or extension, and reported data points frequently have low spatial resolution.

Gathering this knowledge together with an accurate understanding of current stressors can support ecosystem-based management of marine forests, with mitigation of local and global stressors such as water quality and herbivory pressure as a critical step. Appropriate management and threat mitigation are prerequisites for conservation and restoration plans, and naturally recovered Cystoseira s.l.

populations provide examples following reduced eutrophication, pollution, or herbivory. Climate change already affects some populations and represents a challenging factor that cannot be easily mitigated. Leading research challenges include promoting resilience, identifying more tolerant species and populations, locating potential climate-change refugia for thermally sensitive species, and determining areas expected to be more severely affected by human activities.

The central message is that conservation and restoration cannot rely on isolated records: the Mediterranean needs standardized monitoring, better coverage, and stressor-specific management before priorities can be set confidently.