Resources competition between Ulva rigida and Zostera marina: a quantitative approach applied to the Lagoon of Venice

The competition between Ulva rigida and the seagrass Zostera marina is described by means of a mathematical model. The two species compete for light and nitrogen dissolved in water. Nitrogen uptake of Z. marina from sediments and the washing out of U. rigida by advective transport are accounted for in the model. Simulation results, in terms of U. rigida biomass and presence of Z. marina, are compared with available data. At 18 sample points out of 80, experimental observations do not agree with the U. rigida biomass calculated by the model. In these cases the model slightly underestimates the standing crop, probably due to an accumulation process not accounted for by the model. At only four stations out of 80 does the predicted presence/absence of U. rigida and Z. marina disagree with the experimental observations. Model behaviour shows that the structure of the primary producers community is strongly influenced by nitrogen availability and water velocity and competition for light is a key point in macroalgae-seagrass interaction.