Dynamic behaviour of lagoon ecosystems

The dynamic behaviour of primary producers in coastal lagoons due to changes of external control parameters—phosphorous external load and maximum tidal flow rate—is investigated by using a eutrophication model. The eutrophication model allows the simulation of the temporal evolution of the concentration of the following species: a) in the water column: dissolved oxygen, vegetal organic carbon, particulate and dissolved organic carbon, dissolved phosphorous and hydrogen sulphide; b) in sediments: dissolved oxygen, particulate and dissolved organic carbon, dissolved and adsorbed phosphorous, hydrogen sulphide. The vegetal organic carbon is distinguished in three components related to the main groups of primary producers, namely eelgrass, macroalgae and microalgae, having different modalities of nutrient uptake and kinetics parameters depending on the S/V ratio. Diagrams representing the ecosystem dynamic response to the changes of the control parameters were constructed by simulations. The response diagrams suggest the existence of regime shifts which border different ecosystem stability ranges, characterized by the dominance of a specific group of primary producers and by different ecosystem vulnerability at summer water anoxia. A catastrophic bifurcation or abrupt regime shift—evidenced by Poincar`e sections obtained mapping, at a prefixed time period, the values of the concentration of the species—occurs for a critical value of the control parameter and manifests itself as an abrupt change of the dominant species from eelgrass to macroalgae. An analogous abrupt change from eelgrass to macroalgae occurs for a critical value of the tidal flowrate assumed as control parameter in simulations conducted for an assigned value of external phosphorous load.