Modelling nutrient mass balance in a temperate meso-tidal estuary: Implications for management

Although mitigation programmes have been implemented in huge eutrophied estuarine systems, satisfactory results are not always immediately achieved. External loadings are reduced but the nutrient pools still existing in the sediments (due to decades of external supply) start to deplete only slowly, to reach the lower steady state level related to the new external loading. In the Mondego Estuary, this situation could be seen from the mass balance calculations including different nutrient fractions (dissolved, SPM and bound in vegetation). Different nutrient fractions quantified over the year showed the retention capacity and supplying capacity of these estuarine systems, and also the seasonal dynamics those fractions can present. External loading of nitrogen was largely dominated by DIN, but after its incorporation by benthic primary producers nitrogen export occurred essentially as SPM-N. Phosphorus loading was dominated by SPM-P, but during warmer periods P-efflux increased DIP concentration inside the system and was afterwards exported through the outer boundary. Although nutrients bound in vegetation were not significant to total mass balance, depending on the occurrence of occasional macroalgal blooms, the vegetation fraction can significantly increase its contribution to the overall balance. A mathematical model was also created for the southern arm of the Mondego (MIKE 11), in the hope that it will become a reliable management tool that could help in future decisions relating to the system. The mass balance calculations and scenario analysis were based on nutrient field measurements and nutrient results from the hydrodynamic simulation (concentration of dissolved inorganic material, adsorbed to suspended particulate matter and bound in vegetation fractions). The importance of biological elements to estuarine dynamics was assessed, comparing simulations including these factors (water quality and eutrophication modules) with others containing only the hydrodynamic and advection-dispersion modules.