31 Luglio 2025
Giulia Nannini
Record-Breaking July: How Extreme Heat Threatens Marine Biodiversity in the Mediterranean.
June 2025 saw exceptionally high temperatures throughout Italy and the Mediterranean basin, with increasingly evident effects on marine ecosystems. Earlier and prolonged heatwaves are profoundly altering the biodiversity of our sea, one of the richest in species in the world but also one of the most vulnerable to climate change.
According to data from the European Copernicus (Copernicus) service, the surface waters of the Mediterranean reached average values of over 27 °C already in the second half of June, +3.7 °C above average, with local peaks of over 30 °C, particularly along the Italian coast and in more closed waters such as the Adriatic.
This anomalous warming is not an isolated episode: for years, scientists have been observing a rising trend in the temperature of the Mediterranean Sea, which is faster than that of the oceans, due to its small size and strong geographical closure.
The consequences of this phenomenon on marine biodiversity are serious and wide-ranging. One of the most visible is the mass mortality of benthic organisms, i.e. those organisms attached to the seabed, such as gorgonians, sponges, madrepores and non-tropical corals. These organisms are unable to adapt quickly to temperature changes, especially when the heat lasts for weeks. The result is the loss of key habitats for hundreds of species, with a direct impact on the complexity and functionality of marine ecosystems. (Garrabou et al., 2009).
Another worrying effect is the disappearance and migration of native species to deeper or northern waters, where temperatures remain more tolerable. This causes a depopulation of southern coasts and a depletion of local trophic networks, making ecosystems more fragile and less resilient (WWF, 2021). Added to this is the invasion of thermophilic alien species, coming from the Red Sea and favoured by global warming, such as the rabbitfish (Siganus), which is colonising vast areas of the Mediterranean and destroying Posidonia oceanica meadows (Tiralongo et al., 2022).
These prairies, endemic to the Mediterranean, are among the most important ecosystems in our sea: they produce oxygen, offer shelter and nourishment to many species, stabilise the seabed and protect the coastline from erosion (Darmaraki et al. 2022).
Their decline, due both to warming and to pollution and anthropogenic pressure, has cascading effects on all marine biodiversity and the health of the sea. Abnormal algal blooms, stimulated by high temperatures and excess nutrients, also upset the ecological balance. Some of these blooms are toxic and lead to areas of anoxia (lack of oxygen), causing fish and invertebrate die-offs and serious damage to fisheries and coastal tourism (Hilbert et al. 2021). The climate crisis is thus increasingly evident in Mediterranean waters. Protecting marine biodiversity today means defending the life, food security, economy, and very identity of Mediterranean populations. In Italy, several Marine Protected Areas (MPAs) (National Geographic, 2019) have been established to preserve ecosystems, but enforcement and management often remain insufficient, with limited funding and weak surveillance.
Moreover, illegal fishing, pollution, and the effects of climate change continue to threaten delicate marine habitats. To truly protect our seas, we need constant and transparent monitoring systems, a stronger implementation of marine protection laws, and policies aimed at reducing emissions and land-based pollution. Italy must also invest in sustainable fisheries, support scientific research, and foster cross-border cooperation with other Mediterranean countries. Only through integrated and forward-looking governance can we ensure the resilience of marine ecosystems and the well-being of future generations.