Assessment of Salmonella spp., E. coli, Vibrio parahaemolyticus and algal biotoxins in Pacific Cupped Oysters (Magallana gigas) along with monitoring of phytoplankton and hydrological parameters in water samples from an experimental facility in the C

• Hydrological parameters and phytoplankton were investigated. • Biotoxins and pathogenic bacteria in oysters were determined. • The potentially toxic microalga Dinophysis acuminata was found. • No accumulation of biotoxins in oysters was pointed out. • Escherichia coli was always present overlapping the EU limits of 230 MPN/100 g. This study aimed to assess the presence ofof Salmonella spp., Escherichia coli, Vibrio parahaemolyticus and algal biotoxins in Pacific cupped oysters ( Magallana gigas while also monitoring the phytoplankton community (both toxic and non-toxic), as well as key environmental parameters including temperature, dissolved oxygen, chlorophyll a , and pH in the Calich Lagoon (Sardinia, Italy). An experimental pilot farm was set up in the frame of the EU Interreg program “RETRALAGS” and samples were collected from December 2018 to November 2019. Overall, . E. coli was always over the EU limit of 230 MPN/100 g provided for Class A areas Salmonella spp. was never detected and V. parahaemolyticus was detected only in summer samples. Only once the presence of the mixotrophic dinoflagellate Dinophysis acuminata was found, without accumulation of any biotoxins. The results confirmed the suitability of Pacific cupped oysters as bioindicators of the health status of coastal lagoon ecosystems.Microbiological parameter concentrations were strongly influenced by the season of collection and by anthropogenic activities within the lagoon's catchment area, highlighting the dynamic interaction between environmental conditions and human impact.