What kind of underwater desert is this? To understand it, one can look to the landscapes described in classic cinema and Western films. Picture a typical American desert, blanketed in cactus and with little other vegetation. Now translate that to the sea: cacti become chestnuts, the sand dunes turn into bare rocks. These whitish ponds are becoming more common in waters of the Mediterranean, as confirmed by researchers from CEAB-CSIC (Centre d’Estudis Avançats de Blanes).
Underwater deserts are just one of the planet’s many shifting lights. Alarm bells ring as climate change accelerates, say the “sea watchers.” They verify this at every inspection. In total, this effort has grown to 5,489 volunteers (six years ago they could not reach 1,500), with 22,000 data entries recorded and later approved by scientists.
As one researcher states, science cannot stay inside research centers. We need powerful tools and, crucially, citizen science. The Sea Watchers project brings scientists together with divers, fishermen, and marine enthusiasts to provide data that can strengthen the protection of Spain’s coastline and beyond.
Bare and lifeless rocks can be a somber image, yet they frame the broader story of change. The project’s volunteers and observers help paint a clearer picture of what is happening beneath the waves.
Combining science with community participation is essential, according to Ignacio Torres, deputy director of the Biodiversity Foundation at the Ministry of Ecological Transition. He notes that engaging people is easier when conversations cross sectors—from fishing to tourism—and that the evidence gathered is indeed worrying at times.
1.Rabbits and deserts
Thousands of underwater forests are dying. Invasive species, driven by unusually high temperatures, move into areas once colder. A clear example is the rabbitfish, including species such as Siganus luridus and Siganus rivulatus. Why rabbitfish? They not only replace native herbivores but also feed on algae at a rapid pace, turning rocks pale. Overfishing compounds the issue by reducing predators that would normally keep herbivore populations in check. Salps, which resemble the cows of the sea, also graze on algae, adding to the disruption. When researchers warned about underwater deserts, they went out, collected data, and analyzed it. These are ecosystems that have lost their functionality and biodiversity.
2. Unusual bloom
Posidonia meadows usually reproduce non-sexually. Yet high autumn temperatures have sparked major bloom periods. Technicians from CEAB-CSIC explain that this phenomenon involves extra effort from the organisms: we do not yet know the consequences of this energy expenditure, but it is possible that many rhizomes and reproductive events are disrupted, hindering recolonization through non-sexual means. These seagrass meadows form protected ecosystems, and it remains uncertain how sudden temperature drops will affect them in the future.
3. Tropicalization
Ocean warming is tropicalizing the Mediterranean, and observers are noting these rare sightings as increasingly normal. An example is the green fish, once limited to warmer zones, which now tolerates more northern waters. Communities are being reorganized as warm-water species move north and cold-water species retreat deeper. Greenfish are native to the Mediterranean, just as the loggerhead turtle, which now nests more readily in eastern Spain. Invasive species are arriving through channels like the Suez Canal, with parrotfish appearing in Balearic waters.
Researchers emphasize that coastal communities are adapting to this shift. Warm-water fish move north, cold-water fish retreat deeper, and biodiversity patterns shift in response to changing temperatures. The presence of greenfish and other survivors demonstrates the sea’s dynamic reconfiguration.
4. Mortality rate
Volunteers have documented mass mortalities, including reductions in sponges and other benthic organisms as heat waves persist. The red gorgonian faces stress, and sponges show signs of decline under higher temperatures. The census indicates a spike in mortality rates and highlights the fragility of reef-like structures beneath the waves.
Documentation remains vital. As Marc Garrabou and CEAB’s director Marc Rius note, the Sea Watchers program is among the most important because it blends biodiversity protection with public awareness. The initiative underscores that real-time observations by everyday people can drive meaningful conservation work.
5.How to collaborate
Participation is simple. Visit the Sea Observers website and register. Each observation should include a photo and georeferenced data. Scientists verify entries and add them to a data warehouse used to support multiple projects. Some projects focus on invasive algae, others on jellyfish, and some on underwater deserts. In total, 17 lines of research are advanced thanks to volunteer observations. You do not need to be an expert to contribute; program leaders stress that not everyone must identify every species to become a marine observer.
Marked citations identify the core findings and ongoing assessments from the CEAB-CSIC and the Biodiversity Foundation, reflecting a coordinated, citizen-inclusive approach to monitoring coastal ecosystems across the region. These notes serve to anchor the narrative in ongoing scientific work and local conservation efforts.