Modelling the interactions between nutrients and the submersed vegetation in the Orbetello Lagoon

Abstract

The main problem in the Orbetello Lagoon is the control of the submersed vegetation, both in biomass and inventory, to manage a problematical coexistence between macroalgae and macrophytes. While macroalgae are liable to cause dystrophic crises, macrophytes oxygenate and stabilise the sediment and thus control the nutrient flux into the water. A mathematical model was required by the Orbetello Lagoon Managerial Office to predict the development of both groups and test the actions to favour macrophytes over macroalgae, in the context of a decision support system. This model, developed for this need, takes into account the interactions between nutrients and the submersed vegetation in a 2D spatial context including a hydrodynamic model for the water movements and an ecological model describing the interactions between nutrients and the submersed vegetation. In the spatial grid the model operates with two interlocked modules: each cell implements the kinetics of nutrients, vegetation and their interactions, running on an hourly basis to keep track of the circadian cycles, whereas an advection/diffusion mechanism running on a daily basis exchanges information among all the grid cells. The model output consists of daily variations in nutrient concentrations and vegetation biomass showing the relative abundance of either group as a consequence of environmental conditions. After a sensitivity assessment, the model has been calibrated with data from the Orbetello Lagoon, where it can now be used as a forecasting tool to predict the development of vegetation and the relative advantage that macrophytes may have upon macroalgae.