Beach sand addition hinders coastal seagrass regeneration

A study published August 1 in the journal Water considers the curious case of the creation of an artificial beach in the French town of La Vieille. The annual addition of sand to the coast appears to disrupt the recovery of the marine ecosystem. According to the researchers, this is an important wake-up call for decision-makers in regions where marine underwater meadows exist.

La Vieille seagrass reef

Posidonia oceanica is an endemic seagrass species that forms extensive meadows in the Mediterranean Sea. It occurs up to 30-40 meters deep and is one of the region’s most important marine ecosystems, providing numerous ecosystem services and acting as natural coastal protection. Remnants of Posidonia form a distinctive structure, known as matte – made up of living and dead rhizomes and sediment, which in shallow bays can come up to the water surface to form coastal reefs, and when thrown ashore helps reduce wind and wave swell energy and acts as a seed bank for dune formation.

In front of La Vieille, an artificial beach in Saint-Mandrier-sur-Mer (southern France), a fragment of such a reef has been preserved. Research based on aerial photographs from 1953-2023 and field reconnaissance by divers has revealed its gradual degradation, a surprising development compared to positive trends observed elsewhere in the Mediterranean.

How the reef has changed – half a century of observations

The earliest photo of the region dates back to 1953 and indicates the presence of a darker area at the site of today’s reef. In 1972, that is, a year after the beach was created and sand was first applied to it, there were five distinct patches with a total area of 251 m². The western boundary was covered by a layer of sand, suggesting that part of the reef was buried at that time.

Since 2000, the two northern patches have disappeared, the central one has merged into one larger fragment, and the southern one has shrunk and disintegrated into small islands of vegetation. Within the main section, two areas of dead matte have appeared, and the edges bear signs of structural intrusion. The 2023 photo shows that the main part of the reef now covers 127 sq m, and the southern fragments cover 36 sq m, a total of 35 percent less than in 1972. Significantly, the rate of natural expansion of the meadows (3-20 cm per year) should have yielded a growth of 1.5-10.2 m at that time. However, this phenomenon has not been observed. In the southern part, the regression process continues.

Why is the reef not growing back? Is it caused by topping up sand?

Water quality along the French coast has improved in recent decades, thanks to the implementation of EU directives (Habitat, Water Framework and Marine Strategy Framework) and investments in wastewater treatment. In many places, including Marseille and Hyères, Posidonia meadows are recovering faster than expected. Even more surprising is the lack of regeneration in La Vieille.

Analysis by a team of French scientists led by Dominique Calmet points to several potential causes:

• Pollution – although the ecological status of the area’s waters (according to CARLIT: CARtography LITtoral) is good, sediments in the Little Red of Toulon (Petite Rade) still contain elevated concentrations of heavy metals (Pb, Hg, Zn), PAHs and TBT. These may periodically find their way into the water column as a result of sediment uplift by storms and ship traffic;
• Water turbidity – caused by resuspension of sediment due to water surges, ship traffic and bathers. However, the impact on the shallow reef is limited;
• Trampling by beachgoers is particularly damaging for such a shallow site; buoys were set up in 2024 to limit access to the northern part of the reef;
• overgrazing – the sarpa fish(Sarpa salpa) feeds year-round in the study area, and the grass leaves bear numerous bite marks. Small, isolated patches are more prone to grazing, and the lack of other grasslands in the area increases foraging pressure;
• sand sprinkling on the beach – every year in early summer, approx. 20 m³ of sand from the quarry. The material quickly flows into the sea, depositing on the rhizomes and tops of plant growth. As little as a 4-cm layer, persisting for several weeks, can reduce the survival rate of Posidonia grass.

The authors identify the annual feeding of the beach with sand as the most likely cause of the lack of reef restoration and the further loss of the southern portions of the beach. The process creates a vicious cycle: sand protects the beach only temporarily, but destroys the seagrasses that naturally dampen waves and reduce erosion. As a result, erosion continues, prompting authorities to continue adding sand.

What’s next for La Vieille?

La Vieille Reef, despite its small size, could become a valuable laboratory in the field – its monitoring will allow assessing the effects of attempts to reduce anthropogenic pressure on underwater grass meadows . Already, a ban on their trampling has been introduced, and in the near future there are plans to stop the annual recharge of the beach with sand. This will make it possible to see if natural regeneration is possible under such conditions.

The study shows that even in regions with overall environmental improvements, local factors can completely block recovery processes. The results from La Vieille may become an important benchmark for other vulnerable locations where similar practices and hydrodynamic conditions are causing seagrass meadows to become buried.

It is worth mentioning that a similar phenomenon is observed in Polish lakes, where artificially created beaches, fed by aggregate extracted from quarries, cover up plant communities of the shallow littoral, including valuable ramshorn meadows. Such a phenomenon can be observed, for example, in lakes Bialy Gostyninski, Kownatki or Skorzecinski. It is nice to spend the summer on a beautiful sandy beach, but won’t the consequences of this pleasure one day turn against us? Excessive degradation of underwater vegetation will lead to a deterioration of water quality and an increase in the frequency of phytoplankton blooms, and take away our pleasure in swimming.

Source:

Calmet, D.; Calmet, P.; Boudouresque, C.-F. Does Beach Sand Nourishment Have a Negative Effect on Natural Recovery of a Posidonia oceanica Seagrass Fringing Reef? The Case of La Vieille Beach (Saint-Mandrier-sur-Mer) in the North-Western Mediterranean. Water 2025, 17, 2287. https://doi.org/10.3390/w17152287