Periodic solutions in modelling lagoon ecological interactions.

In this paper we present and analyze a nutrient-oxygen-phytoplankton-zooplankton mathematical model simulating lagoon ecological interactions. We obtain sufficient conditions, based on principal eigenvalue criteria -- for the existence of periodic solutions. A decoupled model which arises in the high nutrient regime is then considered in further detail for gathering some explicit conditions on parameters and averages of exogenous inputs needed for coexistence. An application to Italian coastal lagoons is finally obtained by parameter estimation and comparison with real data. A biological interpretation of the mathematical results is also presented.