Phytoplankton dynamics in a shellfish farming lagoon in a deltaic system threatened by ongoing climate change.

Global climate change has generated an increasing number of environmental problems, especially in Mediterranean coastal areas, such as the Po Delta (PD), where shellfish production has undergone an overall decline because of strong environmental changes. The present study is centred on assessing the fundamental ecological aspects in one of the most crucial European shellfish production lagoons, Sacca degli Scardovari (SC), addressing phytoplankton community parameters directly affecting shellfish production, namely, chemotaxonomic composition, size fractions, and total biomass, in relation to the physicochemical properties of the water column and mussel filtering activity. Our findings suggest that the phytoplankton community structure, its role within the lagoon food web and its production cycles depend on two distinct allogenic inputs, which shape the community differently and exert substantial control on shellfish production. At the same time, the suspended mussel biomass strongly controls the phytoplankton size composition, as their growth is largely supported by nanophytoplankton. As the Po River collects the drainage waters of the Italian side of the entire Alpine Arch, the phytoplankton dynamics reported here represent a useful baseline for further addressing issues of climatic changes affecting lagoon ecology. We believe that our study presents an innovative tool for the planning and management of interventions aimed at enhancing national mussel production without neglecting aspects of environmental protection or the integrity of the coastal system, with significant scientific implications.