Effect of residence times on River Mondego estuary eutrophication vulnerability

The south arm of the Mondego estuary, located in the central western Atlant ic coast of Portugal, is almost silted up in the upstream area. So, the wat er circulation is mostly driven by tides and the tributary river Pranto dis charges. Eutrophication has been taking place in this ecosystem during last twelve years, where macroalgae reach a luxuriant development covering a si gnificant area of the intertidal muddy flat. A sampling program was carried out from June 1993 to June 1994. Available d ata on salinity profiles and on nutrients loading into the south arm were u sed in order to get a better understanding of the ongoing changes. River Pr anto flow discharges, controlled by a sluice, were also monitored. Integral formulations are typically based on assumptions of steady state and well-m ixed systems and thus cannot take into account the space and time variabili ty of estuarine residence times, due to river discharge flow, tidal coeffic ients, discharge(s) location and time of release during the tidal cycle. Th is work presents the hydrodynamics modelling (2D-H) of this system in order to estimate the residence times variability and to assess their effect on the estuarine eutrophication vulnerability, contributing to better environm ental management strategies selection.