From rooted to floating vegetal species in lagoons as a consequence of the increases of external nutrient load: An analysis by model of the species selection mechanism

In this paper, the substitution mechanism of rooted aquatic plants (as eelgrass) with floating species (as Ulva r.) in lagoons are inquired by using a eutrophication model.The simulations carried out for the specific case of the Lagoon of Tortolì, in Sardinia (Italy), demonstrate the determinant role played by the increase of external phosphorous loads in the vegetal species selection and offer a possible explanation of the selection mechanism.Once the maximum accumulation capability of adsorbed phosphorous in sediments is reached, the rate of external phosphorous loads produces an increment in the dissolved phosphorous in the water column; such an increment favours the growth of floating species which inhibit, mainly due to light competition effect, the growth of rooted plants.In the paper, the serious consequences of such a selection in terms of eutrophication processes and vulnerability of anoxic crises are emphasised.