Quantification of the effects of nonpoint nutrient sources to coastal marine eutrophication: applications to a semi-enclosed gulf in the Mediterranean Sea

An integrated modelling approach is proposed for the assessment of the nutrient loading of a coastal marine ecosystem from terrestrial sources. The evaluation of the model was based on a data set collected on a monthly basis from a Greek gulf surrounded by an intensively cultivated watershed. It consisted of three interacting components, that is the terrestrial, the hydrodynamic and the biological submodels. A modification of the loading-functions approach formed the basis of the terrestrial submodel, used for the estimation of the nutrient fluxes due to agricultural run-off. Nutrient loads from the atmosphere, the domestic effluents and the industrial activity were also considered. The hydrodynamic submodel, based on the Princeton Ocean Model, was used for the estimation of the spatial transport of nutrients and organic carbon. The third component was the biological submodel that was focused on the interactions of nitrogen, phytoplankton, zooplankton, bacteria and organic carbon. The model has shown good fit to the experimental data and it was further used for the assessment of the role of various exogenous and endogenous sources of nutrients and organic carbon. It was found that nutrient loading from the agricultural run-off was remarkable during winter, the contribution being between 40 and 60% of the total nitrogen stock. The fluxes of nitrogen and organic carbon from sewerage and industrial activity were also quantified and they were estimated to be up to 10% of the total stock whereas, the contribution of the wet and dry deposition from the atmosphere was insignificant. The integrated modelling approach could also be used for the understanding of the energy flow through the marine food web and the testing of various scenaria concerning the development of the coastal zone.