Exploring the Nitrogen Cycle and Macroalgae Dynamics in the Lagoon of Venice Using a Multibox Model

The paper aims to understand theUlva rigidadynamics in the lagoon of Venice and the contributions to its growth from different loadings, coupling a multibox model to a simple hydrodynamic model. The different pools in the multibox biological model are interconnected by their exchange rates. The latter have been tested by a set of sensitivity experiments to investigate their influence among different nitrogen forms. The steady state of the domain, under summer and winter conditions, was simulated and the different kinds of loading were tested to study the behaviour of the ecosystem due to loading changes. The influence of municipal, industrial, river and Adriatic Sea loadings on the lagoon ecosystem was studied. Time dependent runs made it possible to follow the dynamic processes involved under tidal forcing, to study the N-Ulva seasonal and interannual trends from 1988 to 1993, and validate them by their comparison with experimental data. Model results show that municipal, river, and industrial loading have local effects on macroalgal growth and that an overall macroalgae decrease in the lagoon that occurred from 1988 to 1993 seemed to be supported by the tidal exchange between the northern Adriatic and the lagoon. Even if other causes (for example macroalgae harvest, weather conditions) could have been induced or may have hastened the decrease of macroalgae in the lagoon from 1988, available data show the strong importance of the coastal eutrophic conditions of the northern Adriatic.