Assessing biogeochemical anomalies in 2024 at an eLTER Site in the Northern Adriatic Sea: the Senigallia-Susak transect and TeleSenigallia pylon

The year 2024 presented a series of environmental conditions that attracted the interest of the scientific community, particularly concerning alterations in the biogeochemical parameters of the Adriatic Sea. In particular, the northern Adriatic Sea experienced record-high surface temperatures, reaching 30°C near the coast of Ancona (Italy). This rise in temperature could be linked to multiple factors, including marine heatwaves and the increased occurrence of African anticyclones. The impacts of this warming are numerous, causing significant stress on the marine ecosystem, with certain fish species struggling to survive under such extreme conditions. Furthermore, a warmer sea leads to increased evaporation, raising atmospheric humidity and potentially triggering intense rainfall when cold air masses collide with the warm, humid air. In addition, in 2024, the phenomenon of mucilage was observed again along the western coast of the Adriatic Sea, something that had not been seen on such a large scale since the early 2000s. The European network of eLTER (Long-Term Ecosystem Research: https://elter-ri.eu/) stations provides long-term data that allow for the analysis of environmental variations and the identification of potential anomalies compared to historical averages. The long-term data series (1988–2024) of physical, chemical and biological parameters were analyzed to assess trends and variability in oceanographic conditions at the coastal station of the eLTER Senigallia transect and TeleSenigallia pylon, located in the northern Adriatic Sea within the Mediterranean Sea. This transect is an important monitoring site due to its distance from the Po Delta and its location within an area where the dense water mass, known as the North Adriatic Dense Water (NAdDW), forms during winter and crosses the region as a bottom current. This analysis aimed to highlight and enhance our understanding of anomalies in biogeochemical processes, including seasonal and interannual changes (i.e., temperature, salinity, nutrients, chlorophyll-a and phytoplankton abundance). In this study, data from the coastal station (SG1) and the TeleSenigallia pylon were analyzed to assess whether 2024 exhibited significantly different biogeochemical characteristics compared to previous years (1988–2023) through the analysis of key parameters such as physical parameters (temperature, salinity), chemical parameters (dissolved oxygen, nitrites, nitrates, ammonium, orthophosphates, and orthosilicates), and biological (chlorophyll-a derived from fluorescence sensor and phytoplankton abundance). For data processing, the database was divided into seasons. The temporal assessment of temperature (T) and salinity (S) shows a strong difference between the bottom layer and the surface layer in summer, when stratification is at its maximum. In winter, the two layers are more homogeneous, as expected. Along the western Adriatic coast, the impact of river plumes with freshwater is evident, varying depending on the season. The trend analysis of temperature and salinity data for the period 1988–2023 has shown a general increase in both temperature and salinity in the majority of the analyzed datasets. In particular, it has been observed that SG1 exhibits a significant increase in temperature both at the surface and at the bottom in all seasons except for summer. The increases range from 0.22% yr⁻¹ (winter, surface) to 0.66% yr⁻¹ (winter, bottom). Salinity shows a significant decrease only in spring (0.06–0.09% yr⁻¹) and increases in other seasons within a range of 0.06% yr⁻¹ (autumn, bottom) to 0.11% yr⁻¹ (autumn, surface; winter, surface). Phytoplankton trend confirmed the increase of small sized taxa reflecting the tendency to oligotrophication. Correlation analyses between nutrients (NO₃ and Si(OH)₄) and salinity carried out in different seasons have highlighted the impact of river inputs along the coast. Station SG1 shows a significant correlation with salinity in winter (surface and bottom) and autumn (surface) for both analyzed nutrients. In spring, only nitrates show a significant correlation. The preliminary temporal analysis of temperature and salinity has clearly highlighted how, over 35 years, the investigated area has undergone a marked increase in both temperature and salinity. The effects of climate change during 1988–2023 seem to impact the increase in temperature in all seasons except summer. It should be noted that sampling carried out during the summer period was often missing in August (summer holiday), when sea warming is at its maximum. Salinity has shown a significantly increasing trend in the area, except for spring, where a significant decrease has been observed near the coast. These observations seem to indicate that coastal inputs have decreased over the years due to lower precipitation. On the other hand, spring has often been characterized by extreme rainfall events and river floods. The 2024 recorded temperatures were approximately 1°C above the historical average, with anomalous summer peaks (about 30°C recorded at the end of July). The anomalies observed in 2024 could be attributed to exceptional climatic factors, such as rising atmospheric temperatures, changes in water mass circulation patterns, and variations in the rainfall regime. The preliminary results suggest that 2024 was an anomalous year compared to historical conditions, with significant ecological implications for the Northern Adriatic ecosystem. It will be crucial to continue long-term monitoring to understand whether these variations represent a new trend or a temporary fluctuation.