Acoustic surveys as a multidisciplinary tool to study the pelagic ecosystem

The pelagic ecosystem plays a crucial role in the global carbon budget, contributing about half of total net primary production, while coastal areas regulate nutrient cycles, carbon sequestration, and sustain over 90% of global fish catches. However, continental shelves are heavily impacted by human activities, making ecosystem monitoring essential. Zooplankton are central to pelagic food webs, linking primary producers to higher trophic levels, recycling nutrients, and contributing to carbon export through sinking fecal pellets and carcasses. Because ecosystem services depend on community functional traits, understanding zooplankton dynamics is key to preserving trophic coupling under climate change. This research used the MEDIAS acoustic survey conducted annually by the IRBIM-CNR as a multipurpose platform to investigate the pelagic environment of the Adriatic Sea, integrating acoustic, environmental, and biological data. The first study analyzed stable isotopes (δ¹⁵N, δ¹³C) in three small pelagic fish (anchovy, sardine and sprat) to assess trophic niches and resource partitioning. High productivity, particularly in the northern basin, supported niche differentiation. Anchovy showed the greatest trophic plasticity, suggesting higher adaptability to future climate-driven changes, though warming and reduced productivity may intensify competition and stock fluctuations. The second study compared mesozooplankton communities in the North Adriatic between 2019 and 2022, years with contrasting climatic conditions. Warmer temperatures and altered salinity in 2022 coincided with a shift from a Noctiluca-dominated to a crustacean-dominated community, demonstrating sensitivity to river inputs and climate anomalies. The third study linked acoustic backscatter in the South Adriatic to zooplankton biomass and environmental variables, highlighting aggregation near the pycnocline and strong correlations with chlorophyll, oxygen, salinity, and temperature. Finally, combining acoustics and zooplankton imaging in the North Adriatic revealed gas-bearing zooplankton as major sound scatterers. However, correlations between acoustic signals and plankton metrics depended strongly on community composition, highlighting the complexity of interpreting pelagic acoustic data.