Fish functional responses to eutrophication and mass mortality events in a Mediterranean coastal lagoon.

Coastal lagoons are increasingly vulnerable to anthropogenic pressures, with eutrophication emerging as a key driver of ecological degradation. In systems with restricted marine connectivity, understanding how biological communities respond to critical eutrophic events is essential for preserving ecological functions and guiding effective management strategies. This study applies a fish trait-based approach to assess the functional consequences of mass mortality events caused by eutrophication in the Mar Menor coastal lagoon (Western Mediterranean Sea). Based on 588 surveys, we evaluated long-term functional shifts in fish assemblages across three critical eutrophic phases -Pre-Mortality (2018-2019), Mortality (2020-2021) and Post-Mortality (2022-2023)- compared with a reference period (2002-2004). Four functional diversity indices were assessed -functional richness (FRic), specialization (FSpe), originality (FOri), and divergence (FDiv)- and compared their sensitivity with taxonomic metrics and a multi-metric fish index. FSpe and FOri were the most sensitive indices to critical eutrophic events, indicating declines in the uniqueness of traits and ecological functions. Functional homogenization and the loss of specialist taxa were widespread across the lagoon, with minimal influence from site-specific anthropogenic pressure or spatial confinement. These findings suggest limited recovery of ecological functions under and after persistent critical eutrophic conditions. Our study demonstrates the usefulness of incorporating functional diversity metrics to detect and characterize the ecological responses of fish assemblages to eutrophication and mass mortality events in transitional coastal systems.