La mer slovène: petite, précieuse et sous pression

The Adriatic Sea, often referred to as the Slovenian Sea due to its proximity to Slovenia’s coastline, is a unique and complex marine environment. Despite being small in size, it holds immense ecological value and serves as a crucial indicator of climate change. However, this sea is also under significant pressure from both environmental degradation and rising temperatures. Recent discussions have highlighted the dual nature of this body of water: on one hand, it remains a popular destination for swimming, diving, and other recreational activities, while on the other, it faces serious threats to its ecosystem and biodiversity.

The Slovenian part of the Adriatic Sea has been classified as “excellent” in terms of microbiological quality over the period 2021–2024, according to official reports and the European Environment Agency (EEA). This classification supports Slovenia's tourism industry and public health, as it indicates clean waters suitable for human use. Yet, beneath this surface-level assessment lies a more troubling reality. The chemical condition of the marine organisms in the area is poor, primarily due to high levels of brominated diphenyl ethers (PBDEs) and mercury. These substances, which were once widely used in plastics, electronics, automobiles, and furniture, have left a lasting impact on the marine environment.

The Trstenik Bay, which includes the Slovenian portion of the Adriatic Sea, possesses unique physical and chemical characteristics. It is relatively shallow, with an average depth of around 17 meters, making it highly susceptible to climate change. Seasonal temperature fluctuations in this region are among the highest in the entire Mediterranean, reaching up to 22 degrees Celsius during summer months compared to winter lows of 4–8 degrees Celsius. In contrast, the western Mediterranean experiences a much smaller range of about 11 degrees Celsius. During the summer, when the Soča River brings large amounts of freshwater into the bay, salinity levels drop below the world ocean average, further increasing the vulnerability of this area to climate change.

One of the most pressing issues affecting the Slovenian Sea is the presence of mercury contamination. The Soča River continues to carry approximately 1.5 tons of mercury annually into the Trstenik Bay, originating from the former Idrija mercury mine, which operated from 1493 to 1995. Mercury concentrations in some sediments reach up to 14 mg/kg, far exceeding the thresholds set by the National Oceanic and Atmospheric Administration (NOAA) and recognized internationally. This level of pollution is significantly higher than in other parts of the Mediterranean and is considered unique even on a global scale.

A central question raised in recent debates is whether this level of contamination poses a threat to food safety. According to the Slovenian Environmental Agency (ARSO) and the National Plan for Managing the Marine Environment 2022–2027, mercury levels in marine organisms do not exceed the EU regulation limits for human consumption. However, these levels surpass the stricter environmental standards aimed at protecting aquatic ecosystems. The distinction between “safe for consumption” and “healthy for the ecosystem” remains a critical point of discussion.

The Adriatic Sea is experiencing rapid warming, particularly in the northern regions. Data collected from monitoring stations along the Slovenian coast and in Dalmatia indicate that surface temperatures have exceeded 30 degrees Celsius, marking a significant shift in the marine ecosystem. This extreme heat wave, which occurred at the end of June and beginning of July 2026, confirmed the transformation of the Adriatic Sea into a new normal characterized by unprecedented temperatures.

Historical data shows that the northern part of the Adriatic, including the Trstenik, Koper, and Piran Bays, has become a hotspot for heat accumulation. On July 1, 2026, the temperature recorded in Trieste reached 31°C, with similar critical readings noted in Gradče and Lignano. Monitoring stations in Koper registered exactly 30°C, approaching the historical record of 31.1°C from July 2010. Hydrologist Maja Koprivšek from ARSO emphasized that such records might soon become routine rather than exceptional, given the rapid warming observed in just four days, where temperatures rose by three degrees Celsius.

Long-term averages reveal a dramatic shift in the Adriatic Sea's thermal conditions. Average annual temperatures in Koper reached 18°C in 2023, the highest since measurements began in 1957, far exceeding long-term averages. The northern Adriatic, encompassing the Trstenik, Koper, and Piran Bays, acts as a heat trap due to its shallowness and semi-enclosed location. Surface temperatures regularly exceed 29°C, with recent measurements in Koper reaching 29.9°C, nearing historical extremes.

Scientists from the Piran Marine Biological Station warn of a powerful "attached" heatwave in the Trstenik Bay. Under the oceanographic buoy Vida, researchers measured a record temperature of 24.65°C at a depth of 22 meters, the highest recorded since continuous measurements began in 2002. Such temperatures are catastrophic for bottom-dwelling organisms, which lack the ability to escape the heat.

These developments underscore the urgent need for comprehensive strategies to address the environmental challenges facing the Slovenian Sea. As temperatures continue to rise and pollution persists, the balance between human activity and ecological preservation becomes increasingly delicate. Future actions will likely focus on mitigating climate impacts, reducing industrial pollutants, and implementing sustainable practices to protect this vital marine ecosystem.