A photographic record of decline
Surrounding the small island of Tabarca, the first marine reserve established in Spain, are extensive underwater meadows of Posidonia oceanica. This marine plant, an angiosperm endemic to the Mediterranean, is not seaweed but a true flowering plant with roots, stems, and leaves. These meadows are important to the local marine ecology, is a nursery for fish, stabilizing the seabed, and oxygenating the coastal waters. Their presence indicates high water quality.
Despite the area's protected status, granted in 1986, the Posidonia meadows are in a state of regression. Scientists use time-series aerial photography, with images dating back to 1956, to map the extent of the seagrass. This historical record shows a persistent decline, with a acceleration in recent decades. Globally, the rate of seagrass loss has increased from a median of 0.9% per year before 1940 to 7% per year since 1990. Around the Mediterranean, some estimates suggest a 34% loss of Posidonia habitat over the last 50 years. The decline around Tabarca is part of this larger, basin-wide trend.
Human pressures and ecological costs
The primary drivers of the seagrass loss around Tabarca are linked to human activity on the nearby mainland. Increased coastal urbanization and development have led to higher levels of marine pollution. One of the most damaging factors is the discharge of poorly treated sewage. This effluent increases the nutrient concentration in the water, a process known as eutrophication. The excess nutrients fuel the growth of phytoplankton and algae, which cloud the water and block the sunlight that Posidonia oceanica needs to photosynthesize, effectively starving the plant.
Physical damage also contributes significantly to the meadows' decline. Illegal bottom trawling, where heavy nets are dragged across the seafloor, can uproot and destroy vast swaths of the slow-growing plant. Boat anchoring in the highly touristic area is another major source of mechanical damage. Each anchor drop can create a bare patch that takes many years to recover, as Posidonia grows at an average rate of only one centimeter per year. The loss of these meadows has direct consequences, including increased coastal erosion, as the plant's root system no longer binds the seabed sediment, and a reduction in the biodiversity that the habitat supports.